Figure 2 shows the FTIR spectra of uncoated (0 wt.%) and MP-coated (0.3, 0.5, 1, 3, 5 wt.%) PLA/PBS nonwoven fabrics, vis-à-vis the MP extract powder. The absorption peaks of the MP extract powder occurred at 1639, 1460, and 1278 cm− 1, which corresponded to the carbonyl (C = O), aromatic C = C, and ether C-O groups, respectively [22, 23]. The absorption peaks of the PLA/PBS nonwoven fabric were observed at 1749 and 1180 cm− 1, which corresponded to C = O stretching and C − O−C stretching [24]. The presence of MP extract (0.3 − 5 wt.%) on the PLA/PBS nonwoven fabric was evidenced by the absorption peaks at 1639, 1460, and 1278 cm− 1, indicating that the MP extract was well coated onto the PLA/PBS nonwoven fabric.
Figure 3 shows the TGA results of uncoated (0 wt.%) and MP-coated (0.3, 0.5, 1, 3, 5 wt.%) PLA/PBS nonwoven fabrics. The thermal degradation temperatures at 10% (T10%), 50% (T50%), and 80% (T80%) of uncoated PLA/PBS nonwoven fabric (0 wt.%) were 324.6, 352.7, and 366.4°C. The corresponding thermal degradation temperatures of MP-coated PLA/PBS nonwoven fabrics (0.3, 0.5, 1, 3, 5 wt.%) were 318.3–329.1°C; 353.9–358.5°C, and 369.4–377.4°C. The TGA results indicated that the MP extract coating had minimal effect on the thermal stability of the PLA/PBS nonwoven fabric.
Table 1 tabulates the thermal degradation temperatures at 10%, 50% and 80% of uncoated and MP-coated nonwoven PLA/PBS fabrics. In comparison, the residual weight (%) of the MP-coated PLA/PBS nonwoven fabrics (0.3, 0.5, 1, 3, 5 wt.%) were higher than that of the uncoated nonwoven fabric (0 wt.%; control). Besides, the residual weight was positively correlated with the MP extract concentrations. The finding could be attributed to the phenolic compounds in the mangosteen pericarp (e.g., tannins, flavonoids, xanthones). These phenolic compounds have aromatic rings in the backbone, resulting in the rigid structure with high mechanical strength and thermal stability [25].
Table 1
Thermal degradation temperatures of uncoated and MP-coated nonwoven PLA/PBS fabrics.
Mangosteen pericarp extract (wt.%) | T 10% ( ̊C ) | T 50% ( ̊C ) | T 80% ( ̊C ) | Residual weight at 550°C (%) |
0 | 324.6 | 352.7 | 366.4 | 1.3 |
0.3 | 323.6 | 353.9 | 369.4 | 1.5 |
0.5 | 329.1 | 356.7 | 371.6 | 1.9 |
1 | 326.5 | 358.5 | 375.1 | 2.2 |
3 | 318.3 | 356.2 | 375.3 | 2.6 |
5 | 319.7 | 357.4 | 377.4 | 4.5 |
Figure 4 shows the FE-SEM images of uncoated (0 wt.%) and MP-coated (0.3, 0.5, 1, 3, 5 wt.%) PLA/PBS nonwoven fabrics. Given 0.3–1 wt.% MP extract, the fiber surface appeared smooth, as evidenced by the absence of tiny droplets on the fibers. The smooth fiber surface could be attributed to low viscosity of the MP coating solution. Meanwhile, the tiny droplets appeared on the fiber surface as the MP extract concentrations increased to 3–5 wt.%. The presence of droplets was attributable to high viscosity of the coating solution [26].
Figure 5 illustrates the tensile properties (i.e., tensile strength and elongation at break) of the uncoated (0 wt.%) and MP-coated PLA/PBS nonwoven fabrics (0.3, 0.5, 1, 3, 5 wt.%). The tensile strength of the uncoated and MP-coated nonwoven fabrics were 0.41, 0.57, 0.91, 0.97, 1.14, and 0.87 MPa, respectively, and the corresponding elongation at break were 21, 19, 14, 10, 9, and 10%. The tensile strength and MP extract concentration were positively correlated, while the elongation at break was inversely correlated with the MP extract concentration. However, the tensile strength of MP-coated PLA/PBS nonwoven fabric decreased when the MP extract concentration exceeded 3 wt.%. The finding could be attributed to poor adhesion of MP extract solution on the PLA/PBS nonwoven fabric [27].
Figure 6 compares the antibacterial activity of the uncoated (0 wt.%) and MP-coated (0.3, 0.5, 1, 3, 5 wt.%) PLA/PBS nonwoven fabrics against E.coli and S. aureus after 24 h. The figures on the left show the Petri dishes with uncoated and MP-coated nonwoven fabrics, while those on the right depict the bacterial inhibition zone underneath the uncoated and MP-coated nonwoven fabrics (after removal of the nonwoven fabrics).
The experimental results showed the growth of the bacteria around the uncoated (0 wt.%) and MP-coated nonwoven fabrics (0.3, 0.5, 1, 3, 5 wt.%), indicating the absence of diffusible action. The uncoated PLA/PBS nonwoven fabric (i.e., underneath the nonwoven fabric) exhibited no antibacterial activity, while the antibacterial activity of the MP-coated nonwoven fabrics varied, depending on the MP extract concentrations. In comparison, the MP extract was more effective against S. aureus than E.coli. The phenomenon could be attributed to the double membrane structure of E.coli, which was more complex than the single membrane structure of S. aureus [28].
The antibacterial activity of the 3–5 wt.% MP-coated PLA/PBS nonwoven fabrics resembled that of the nonwoven fabrics coated with 0.3–1 wt.% MP extract. The finding could be attributed to the haphazard structure of PLA/PBS nonwoven fabric (Fig. 4). The haphazard and loosely packed structure limits the contact areas between the MP-coated nonwoven fabric and the bacteria [29].
In this research, the disintegration tests of the uncoated and MP-coated PLA/PBS nonwoven fabrics were carried out under composting condition in accordance with the ISO 20200 standard. Figure 7 depicts the disintegration of the uncoated (0 wt.%) and MP-coated (0.3, 0.5, 1, 3, 5 wt.%) PLA/PBS nonwoven fabrics at days 0, 1, 4, 10, 16, 23, and 28. The disintegration was clearly visible after 10 days under composting condition.
Figure 8 shows the degree of disintegration of uncoated and MP-coated PLA/PBS nonwoven fabrics under composting condition. The uncoated PLA/PBS nonwoven fabric (0 wt.%) was decomposed by 93% after 28 days. In comparison, the disintegration of the MP-coated PLA/PBS nonwoven fabrics were noticeably slower, depending on the MP extract concentrations. The delayed disintegration of the MP-coated PLA/PBS nonwoven fabrics could be attributed to the nonpolar compounds such as α-, β, and γ-Mangostins in xanthones derived from MP [30]. The degradation of polyester is determined not only by the properties of the specimen (crystallinity, molecular weight, chemical structure, and so on) but also by the presence of water or moisture, which is crucial for the growth of microorganisms in the degradation environment and an increasing the rate of hydrolytic degradation [31]. The coating xanthones on those fabrics hindered the diffusivity of water inside the matrix due to its hydrophobic nature leading to the delayed degradation of those MP-coated fabrics. In addition, the presence of aromatic rings and side groups in xanthones might provide a sterical hindrance for an enzymatic attack to those fabrics resulting in resistance to enzymatic hydrolysis in aerobic bacterial degradation. Furthermore, carbonyl group content in xanthones has been reported to have an antibacterial ability, which can react with the amino acid residues on the extracellular enzymes leading to damage of proteins of biofilm matrix polymer [32, 33]. Therefore, the degradation of those PLA/PBS nonwoven fabrics was decreased with MP.