In most situations, when the essence oil is directly released in the air, it shows a common diffusion mass transfer and follows Fick’s law. However, with respect to the fragrance microcapsules, as the core material (essence oil) was protected by the shell material and its release from inside the core to the outside environment was controlled, the release rate slowed down, and the release process was more complicated. Generally, the fragrance microcapsule release forms are classified into sustained release and broken release.
According to Poucher’s theory [
24,
25], the scent of perfume is divided into three parts: top note, middle note, and base note (
Figure 3), and they describe the scent type and intensity of the aroma compositions volatilized at different times. In this work, we selected nine characteristic substances with different notes (
Table 3) to study the odor release behavior of the different samples.
3.2.1. Sustained Release
For the sustained release of essence oil from the prepared microcapsules, it was considered that the essence oil firstly permeated from the inside core to the shell material, further migrated to the surface, and then showed similar volatilization and diffusion states to those of liquid essence oil under normal circumstances (
Figure 4). However, it should be noted that the essence oil release from the microcapsules was restricted by the surface morphology and porosity of microcapsules [
16,
26]. The surface morphology of the prepared microcapsules was analyzed by SEM, and the results are shown in
Figure 5. It was found that the prepared microcapsules had very rough surface structures, and it made them possess large surface areas. Additionally, the special surface structure was beneficial to the volatilization of the essence oil migrating from the inside core of the microcapsules. Meanwhile, the migration rate of the core material essence oil to the outside shell surface was slow, and thus, from the viewpoint of dynamics, the sustained release rate of the essence oil from the microcapsules was mainly determined by its migration rate in the shell material.
The release properties of essence oil from the microcapsules over a short time (10 min, Sample 1) and a long time (12 h, Sample 2) were tested, and the results are shown in
Figure 6. It was found that over a short time, negligible essence oil was detected (
Figure 6a); however, over a long time a series of essence oil compounds were detected obviously (
Figure 6b).
By comparing with the database, the peak intensity of the selected substrates was shown in
Table 4. For Sample 1, almost no top note substrates were detected, and only the middle note substrate 1, 8-cineole and some base note substrates were detected, suggesting that the fabric samples finished with the prepared microcapsules had a very faint scent. However, as shown in Sample 2, all the note substrates were detected after a long release time, which illustrated that the prepared microcapsules had a seal protection effect and controlled-release performance on the core material.
The seal protection effect and controlled-release performance could be analyzed by SPME-GC-MS technology, especially transient or short-term changes (such as at initial time t
0 or at a specific time t
n), but it is difficult to describe the dynamic process. Therefore, in order to investigate the controlled-release performance of the prepared microcapsules, a weighing calculation method was used to test the release rate of essence oil from the prepared microcapsules over 2400 h, and the results are shown in
Figure 7. It was found that the mass percentage of the essence oil continuously decreased, and the decrease rate gradually became slower as the release time increased. After fitting and comparing with various kinetic models, it was found that the release rate was consistent with both the first-order kinetic (
Figure 7a) and the Peppas model (
Figure 7b), and they were in accordance with Equation (4) as follows:
According to the fitting results, the R2 of the first-order kinetic model was higher, indicating that this model was more correlated with the experimental data. At the same time, the parameter a = 75.56 in Equation (4) represented the release limitation of 75.56%, meaning that the microcapsules can only release 24.44% of the total essence oil. However, in the experiment, it was found that after 2400 h of sustained release, the microcapsule samples still showed a light but distinct odor, which suggested that the release limitation might be lower than the fitting parameter a.
In view of this point and the overall trend of the weight loss rate, the Peppas model might be more suitable for the further description of release dynamic processes, and the parameter
k = 0.3213 indicated that the sustained release of the prepared microcapsules basically followed the Fickian diffusion character. Furthermore, the sustained release half-life was predicted under the Peppas model, and the results are shown in
Figure 7c. The calculated result was 1.65 years, and it implied that the prepared microcapsules possessed a long application and storage time.
3.2.3. Sustained–Broken Release Comparison
(1) Broken Release Properties after Different Sustained Release Times
In order to further investigate the airtightness and broken release property of the prepared microcapsules, the essence oil composition released from the impregnated fabric samples with and without 2400 h sustained release and further hitting o 5 times (Samples 4 and 7) was analyzed, and the results are shown in
Figure 10 and
Table 6 for the impregnated fabric samples. It could be seen that although Sample 7 had undergone a 2400 h sustained release time, Sample 4 without sustained release showed only a 1.02–3.78 times higher peak intensity. The obtained experimental results further verified that the prepared microcapsules showed a very slow sustained release rate and maintained good broken release properties under external force even after a long, sustained release time.
Additionally, it was found that the essence oil compositions in different notes showed different release rates. In the top note, the ratio of the characteristic peak intensities between Sample 4 and Sample 7 was 3.01–3.78. However, in the base note, this ratio was only 1.02–1.57. The difference suggested that for the essence oil compositions inside the microcapsule core, their volatile characteristics could also affect the release rate, and the higher volatility of the substances, the greater decrease in the release rate.
Moreover, Sample 7 showed good broken release properties under external force after sustained release for 2400 h, and the top note and middle note substances had a large amount of residue. This provided some support to the model analysis results shown in
Figure 7 and, indeed, illustrated that the release limitation was far more than the fitting parameter
a in Equation (4).
(2) Comparison between Sustained Release and Broken Release
The different release properties of the essence oil from the impregnated fabric samples in the sustained release state (Sample 2) and broken release state (Sample 4) were also investigated, and the results are shown in
Figure 11 and
Table 7. It can be seen that in different release states, the samples showed obviously different behaviors. The characteristic peak intensity for composition A–C in curve a (sustained release) was lower than that in curve b (broken release); however, the characteristic peak intensity E–I showed the opposite trend. The results illustrate that the middle and base note odor showed advantages in the sustained release state, and the top note odor was more outstanding in the broken release state. This conclusion also could be verified by the data comparison list in
Table 7.
3.2.4. Release Property of Fabrics Finished by Essence Oil and Microcapsules
The essence oil compositions released from the pure essence oil (Sample 6), fabric samples finished with essence oil (Sample 5), and those finished with microcapsule suspension (Sample 4) were analyzed by SPME-GC-MS. The results are shown in
Figure 12. By comparison with the pure essence oil (curve c), it was found that in the fabric sample finished with the essence oil suspension (curve b), the characteristic peak species, numbers, and intensities decreased, which could be seen from the c/b ratio shown in
Table 8. This change illustrated that in the fabric samples finished with pure essence oil, the top note odor almost vanished and the middle note lost potency remarkably, which resulted in significant difference between the released odor and the designed odor (such as in
Figure 3a). The reason for this was that the fabric could not reconcile the essence oil mixture due to their different physicochemical properties, such as solubility, polarity, and characteristic groups. Furthermore, the finishing and drying process resulted in a certain loss of and chemical changes in the essence oil. By comparison, in the fabric sample finished with the prepared microcapsule suspension (curve c), the characteristic peak species and intensities were similar to those of the pure essence oil, suggesting that the prepared microcapsule showed an excellent odor recovery and explosiveness (such as in
Figure 3c). In particular, most of the middle note and base note essence oil compositions were the same as those of the pure essence oil, which could be expressed via the c/a ratio shown in
Table 8.