Physical and chemical properties of the microcapsules
The structure of the microcapsules was determined by SEM. As shown in Fig. 3a, the PM comprised microspheres with diameters ranging from 1 to 5 μm with relatively smooth globular surfaces. The PM was completely covered with the cured UMF resin shell to form the MP. As expected, most of the visible MP was polypore, with highly heterogeneous particle sizes and shapes (Fig. 3b).
The FTIR spectra of the PM and MP are presented in Fig. 3c. The typical absorption peak of the PM at 3023 cm−1 was attributed to the aromatic C–H stretching vibration (Kim et al. 2008). The absorption peaks at 2962 and 2930 cm−1 were attributable to alkane C-H stretching vibrations. The strong and wide absorption peak at 3346 cm−1 was attributable to the ring vibration of melamine in the UMF resin (Pakdel et al. 2007). The band at 1652 cm−1 was associated with the C = O stretching vibrations of the amide group of methyl urea and the C = N stretching vibrations of the triazine ring (Chai et al. 2018). The peak at 1560 cm−1 was attributable to a combination of the vibrations of C–N and –NH2 in the amide and melamine (Benson 2003). The band at 1440 cm−1 was indicative of amide and triazine stretching vibrations (Marvel et al. 1946). The surface elemental compositions of carbon, oxygen, and nitrogen in the PM and MP were determined by XPS (Fig. 3f). The peaks located at 294, 408, and 541 eV were attributed to C 1 s, O 1 s, and N 1 s, respectively. The MP showed a significant increase in the nitrogen concentration compared with the PM, whereas the oxygen content remained nearly constant. The MP N/C ratio determined by elemental analysis was 0.863, which was higher than the PM N/C ratio of 0.123. This indicated that that the PM was well-coated by the resin (Coullerez et al. 2000).
The TG and differential thermogravimetric (DTG) curves of the PM and MP shown in Fig. 3d, e reveal that the PM began to decompose at approximately 310 °C and decomposed almost completely at approximately 440 °C. The residual weight of the PM was 11.6% at 800 °C. Compared with PM, MP has two main decomposition processes. The initial decomposition stage of the MP occurred more quickly than the decomposition of the PM owing to water evaporation and slow free formaldehyde emission (Wu et al. 2008). During the second decomposition stage (above 425 °C), the MP was more stable than the PM because the UMF resin produced incombustible gases, such as NH3 and CO2, which formed a thermally stable honeycomb char (Siimer et al. 2009). The Tmax values for the two MP decomposition stages were 305 and 410 °C (Németh et al. 2018). After decomposition at 800 °C, the MP left approximately 23.5% residue, which was much more than the PM (Li et al. 2007). Therefore, the MP was more effective at improving the thermal stability than the PM. As mentioned above, the results further indicated that the PM was completely covered with the cured UMF resin shell to form the MP.
Morphology of the photochromic wood surface
The surface morphologies of the pristine wood and the MP-PDMS nanocomposite-coated wood are presented in Fig. 4a, d. The original wood surface comprised a heterogeneous porous material with a micro-grooved surface (Fig. 4a). After dipping in the MP-PDMS coating solution, a continuous polymer film was formed on the surface of the wood (Fig. 4d). Micro-scale protrusions comprising the MP were distributed throughout the film. The MP played an important role in the photochromism, and the texture of the wood remained visible because the film was transparent.
The samples were also investigated by non-contact surface profilometry and AFM to determine changes in the surface morphology. Figure 4b, e presents 3D images of the as-prepared samples. As shown in Fig. 4b, the original wood surface was almost completely covered with a clear vertical gradient due to the complex surface texture. Compared with the original wood surface, the MP-PDMS nanocomposite-coated wood surface (Fig. 4e) was smooth and flat. The surface roughness (Ra) values were 4.85 μm (original wood) and 0.432 μm (MP-PDMS wood).
Tapping mode AFM images of the surface topography of the original wood and the MP-PDMS wood are shown in Fig. 4c, f. The different surface roughness values are reflected in the Rrms values (rms = root mean square = the standard deviation of the Z value; Z is the total height range analyzed) of the two surfaces, which were 60.3 nm (original wood) and 27.9 nm (MP-PDMS wood).
Photochromic properties of samples with various MP concentrations
Figure 5 shows the color parameters of the pristine wood and the wood coated with MP-PDMS with MP concentrations of 2, 4, 6 and 8% in weight. As the MP concentration increased, the lightness factor L* of the samples under UV irradiation decreased from 60.1 to 12.2, indicating that the surface color darkened. The a* value increased markedly from 5.2 to 10.3, implying the surface color turned a deeper shade of red. The b* value decreased from 13.0 to − 22.3, indicating that the surface color changed from yellow to blue. Furthermore, the total color change (△E*) increased from 3.7 to 80.2 as the MP concentration increased from 0 to 8%, confirming the photoresponsivity of the MP-PDMS-coated wood.
As shown in Fig. 6a, the lightness and chroma of the wood coated with MP-PDMS with MP concentrations of 2% in weight was very similar compared to the pristine wood. With an increase in the MP concentrations from 4 to 8% in weight, the lightness of the treated wood surface is gradually increased due to the microcapsule shell (UMF) is white (Fig. 6b–d). However, the natural color texture of the wood surface is not affected by the composite coating. After UV light irradiation, surface color slightly darker (Fig. 6e). With increasing MP concentrations, the surface color turned dark red (Fig. 6f–h). Color changes of the photochromic coatings were related to the increasing number of chromophores. To further explore the relationship between the color change of the photochromic coating and the intensity of the UV irradiation, MP-wood samples with 8% concentration in weight under different UV intensity irradiation the digital images (Fig. 6a–d) were utilized. As a rule, high intensity UV irradiation leads to further increase of color intensity due to the accumulation of the open MC forms.
The practical applications of photochromic wood samples are directly affected by the response time, which includes the chromogenic time and the fading time. Figure 7 shows the response time curves of 2%–8% MP-wood. At a constant temperature, the response time of the sample gradually increased as the MP concentration increased, which is attributed to the fact that an increase in the MP concentration results in strong electrostatic interaction between the MP molecules and an increase in the degree of aggregation, as shown in Fig. 7a. As the concentration of the spiropyran increases, a transformation of the chemical structure is less likely because the closed form of the spiropyran increases steric hindrances; hence, there is a lag in the response time of MP-wood formed with higher MP concentrations. As a result, at high concentrations, the photochromic microcapsules samples are more stable under changing temperature conditions. As the temperature increased, the chromogenic time decreased. When the temperature exceeded 30 °C, the MP concentration exerted a minimal influence on the fading time (Fig. 7b). The result for the response time is attributed to the fact that an increase in the temperature favors the breakup and recombination of the C–O–C bond in the spiropyrane, thereby increasing the conversion rate of spiropyrane to merocyanine.
Color stability and adhesion properties of samples
Figure 8a illustrates the △E cycle characterization of samples with various MP concentrations. The results indicate that the photoresponsivity of the MP-wood was not affected by 100 UV irradiation cycles. To assess the mechanical resistance of the MP-wood, samples were abraded with sand and sandpaper. The surfaces of the MP-wood samples were subjected to impingement with 15 g of sand from a height of 25 cm, the sand grains had diameters in the range 200–250 µm. Figure 8b shows that an as-prepared sample with a 100 g weight on its top was placed face-down on the sandpaper and moved 50 cm. After 100 sand impact tests, the surfaces of the films appeared smooth, with no signs of damage (Fig. 8e). In contrast, the SEM images showed that the film was broken and severely worn because the external load of the sandpaper exceeded the strength of the film (Fig. 8f). Figure 8d shows the ΔE* values of the samples made with various MP concentrations (2, 4, 6 and 8%) before and after UV aging. The sample with an MP concentration of 8% had the highest ΔE* value during the aging test, indicating that UV aging resistance was proportional to the MP concentration. ΔE* decreased significantly from 100 to 150 h. The photochromic performances of the samples degraded with UV exposure time, but the samples with high MP concentrations still exhibited satisfactory photochromic performances after 250-h UV aging.