3.1 Scanning electron microscope image of microcapsules
The surface morphology magnification of the thiol-modified microcapsules was observed by scanning electron microscopy to be 1.00 k and 3.00 k, respectively. The results are shown in Figure.2a and b. Thermochromic microcapsules were successfully synthesized by using the mixture of CVL, BPA, and cetyl alcohol as the core material, TEOS, and KH580 as the wall material. It can be seen that the synthesized thermochromic microcapsules are regular spherical, smooth surfaces, a small number of unreacted particles adhere to the surface of the microcapsules, and the degree of encapsulation is good. The particle size distribution of microcapsules was calculated by the software Imagej as shown in Fig. 2c, and the particle size of microcapsules was concentrated at about 4.05 µm.
3.2 Thermal properties of thiol-modified thermochromic capsules
Phase transition temperature plays a crucial role in thermochromic microcapsule performance. In this experiment, we selected tetracenteol, hexadecyl alcohol, and octadecyl alcohol as the phase change solvent of SH-microcapsules, and analyzed the thermal properties of the phase change solvent by DSC. As shown in Fig. 3, the phase change solvent determines the color-changing temperature of organic thermochromic materials [18]. When the temperature is higher than the melting point of the phase change solvent, the phase change solvent changes from solid to liquid, and the heat absorption peak is different. The curves of tetradecol, hexadecyl alcohol, and octadecol showed obvious melting peaks, and the melting temperature increased gradually (36.91℃, 48.94℃, 57.59℃), and the melting temperature was higher than room temperature.
3.3 Thermochromic behavior of thiol-modified microcapsules
There are generally two ingredients in organic reversible thermochromic materials: an electron donor, an electron acceptor, and phase change solvent, in which the electron donor determines the color, electron acceptor determines chroma, and phase change solvent determines color change temperature [19]. The redox potential of the electron donor (crystal violet lactone) and electron acceptor (BPA) is similar, and the electron transfer between the two causes the structure of the electron acceptor to change when the temperature changes. It was found that when the ratio of crystal violet lactone, bisphenol A, and cetanol was 1:3:40, the composites showed the best discoloration. The discoloration process of thermochromic microcapsules prepared with this ratio is shown in Fig. 4. As shown in Fig. 4a, when the microcapsules are gradually heated at room temperature, they gradually change from blue to white. As can be seen from the figure, without heating, because the melting point of cetanol has not been reached, crystal violet lactone combines with bisphenol A, and crystal violet lactone is in an open ring state, thus the microcapsules appear blue. After heating, the microcapsule solution began to change color about 10 seconds later. As part of cetanol slowly melts with heating, the chemical bond between crystal violet lactone and bisphenol A breaks, crystal violet lactone closes and the color becomes lighter. After 90 s, the microcapsule solution completely changes color, indicating that cetanol has dissolved and CVL completely closes the loop. In the CIE 1931 chroma diagram (Figure. 4b), the color of the thermochromic microcapsule gradually changes from colorless to blue, returning to its original color after removing the heating device.
An infrared thermal imager was used to observe the surface temperature changes of the thermochromic microcapsule solution during heating, as shown in Fig. 5b. At room temperature (30.72℃), the microcapsule solution is blue. When the temperature rose to 39℃, the color of the microcapsule solution began to change, and part of the crystal violet lactone closed loop reaction occurred. The heat released during the Hexadecanol phase transition causes the CVL and BPA to separate, and the CVL returns to a closed-loop state, gradually changing color from blue to white. When the temperature reached 48.34℃, the microcapsule solution turned completely white.
3.4 Microstructure and discoloration properties of wool fabric
The microstructure of raw wool (Fig. 6a), reduced wool (Fig. 6b), and dyed wool (Fig. 6c, d) were observed by scanning electron microscopy. The scale layer on the surface of the original wool is very obvious. After the wool fiber is reduced by TCEP, many small folds and gullies appear on the scale, indicating that the regular scale layer structure on the surface of the wool is destroyed, and a large number of disulfide bonds on the wool fiber are reduced to sulfhydryl groups, which is beneficial to the combination of sulfhydryl modified thermochromic microcapsules and wool. This is because TCEP is an efficient water-soluble reducing agent, which can selectively reduce the disulfide bond in wool fiber, and has the characteristics of high efficiency, fast, and stability [20]. The SEM images of thermochromic wool are shown in Fig. 6c. It can be seen that there are spherical microcapsule particles on the surface of the wool, which are unevenly distributed in the wool scale layer. As shown in the enlarged image in Fig. 6d, many microcapsules accumulate in the place where the wool scale layer is destroyed, and the morphology of the microcapsules is also clearly visible due to the increase in magnification. This also explains well that when heated to a certain temperature, the aggregated microcapsules will change color asynchronously due to uneven heating.
After introducing thermochromic microcapsules into the surface of wool fiber, as shown in Fig. 7, when the temperature rises to 48°C, the color of wool fabric changes from blue to light gray, which cannot be completely whitened. This is different from the color change from blue to white when the thermochromic microcapsule solution is heated to more than 48°C. The reason may be the uneven distribution of microcapsules on the surface of wool. It is speculated that the deeper reason is that the treatment of the wool scale layer is not sufficient, so the microcapsules cannot fully contact the sulfhydryl groups on the wool surface to dye the fibers, which eventually leads to the aggregation of microcapsules in individually areas.
3.5 Fabric color fastness and cycle performance
The thermochromic microcapsules modified by different amounts of KH580 (0%, 5%, and 10% of the amount of TEOS, respectively) were treated on the wool fabric reduced by TCEP, and the color fastness was tested. The test results are shown in Table 1.
Table 1
The color fastness of thermochromic wool fabrics
| KH580/% | Washing fastness(grade) | Wet rubbing fastness(grade) |
| Fading fastness | Staining fastness | Wet | Dry |
| 0 | 1–2 | 4 | 2 | 2 |
| 5 | 2–3 | 5 | 3 | 2 |
| 10 | 3 | 5 | 3 | 4 |
It can be seen that the color fastness of wool fabrics treated with KH580-modified thermochromic microcapsules is significantly better than that of wool fabrics treated with unmodified microcapsules. The washing fastness of wool fabrics treated with KH580-modified microcapsules increases with the amount of KH580. The fading fastness increased from 1–2 to 3, the staining fastness reached 5, the wet rubbing fastness increased from 2 to 3, and the dry rubbing fastness increased from 2 to 4. The excellent color fastness of wool fabric treated with KH580-modified thermochromic microcapsules is attributed to the reaction between the thiol group on the surface of the microcapsules and the thiol group contained in the reduced wool fabric to form a disulfide bond, so the thermochromic wool fabric has excellent color fastness. Subsequently, the thermochromic wool fabric was subjected to multiple heating and cooling experiments to verify the reversibility and stability of the thermochromic behavior of the thermochromic wool fabric, the results are shown in Fig. 8.
After 30 cycles of heating and cooling, the K/S value of the fabric did not change much. The fabric can still undergo cyclic discoloration, because the thermochromic material encapsulated into microcapsules has a stable discoloration environment, and the heating and cooling process does not destroy the structure of the microcapsules. Thermochromic materials can change continuously and reversibly so that the fabric has good fatigue performance. This also ensures the feasibility of the long-term use of thermochromic wool fabrics.
