*3.8. Antibacterial Activity*

As shown in Figure 11, the antibacterial activity of the NRLFs on *E. coli* was characterized by inhibition zone testing against *E. coli*. Figure 11a–f represents the NRLF composites filled with chitin from 0 to 5 phr.

**Figure 11.** Inhibition zone against *E. coli* of chitin–NRLF composites. (**a**) pure NRLF; (**b**) NRLF–1 phr chitin; (**c**) NRLF–2 phr chitin; (**d**) NRLF–3 phr chitin; (**e**) NRLF–4 phr chitin; (**f**) NRLF–5 phr chitin.

From Figure 11a, pure NRLF without chitin already has antibacterial activity against *E. coli*. This result is consistent with the previous study [40]. There are two possible explanations for this phenomenon. First, the proteins in natural rubber latex might be bacteriostatic [51]. Second, 3 phr of zinc oxide was added, as listed in Table 1, and zinc oxide is a proven antibacterial [52,53].

The increase in the inhibition zone of each of the NRLF composites can be seen in Figure 12. When the chitin loading increases, the antibacterial activity of the NRLF gradually increases. The samples were tightly embedded in the solid medium so they contacted the *E. coli* completely. The antibacterial activity of NRLF–3 phr chitin appears to be twice that of pure NRLF. When loaded with 5 phr, the inhibition zone is obviously enlarged and is quite clear compared with that of the former

samples. Furthermore, the NRLF–5 phr chitin composite improved the antibacterial activity by 181.3% against *E. coli* compared to that of the pure NRLF.

**Figure 12.** Increase in the inhibition zone of chitin–NRLF.

#### **4. Conclusions**

In this research, chitin was chosen as an antibacterial agent to enhance the antibacterial activity of NRLF. NRLF composites were prepared by the Dunlop method, and chitin was blended with foam during the foaming process. In addition to the antibacterial activity, morphology, swelling, density, chemical composition, hardness, tensile strength, elongation at break and compression strength were characterized as well.

Compared with that of pure NRLF, the performance of the NRLF composite is related to the loading of chitin. When the chitin content increases, the cells expand and deform with the chitin. Overexpansion could lead to bursting of the bubbles and fracture of the rubber walls. The tensile strength, elongation at break and water swelling decrease gradually, resulting in uneven force and stress concentrations.

The chitin is inclined to aggregate due to the poor interaction between the chitin and natural rubber latex. The cracked cells and broken walls stick to the central chitin agglomerations. A decreased number of bubbles contribute to the collapse and contraction of the foam materials. With a decreased proportion of air and additional space for chitin, when the chitin loading increases, the compressive strength, density, hardness and antibacterial activity increase.

Using the natural antibacterial agent chitin as a loading filler, environmentally friendly and antibacterial NRLFs were prepared herein. The antibacterial activity of NRLF composites could be increased by up to 181.3% compared to pure NRLF. When loaded with 3 phr chitin, the microstructure and mechanical properties of the chitin–NRLF composite change moderately and the antibacterial activity reaches more than double that of pure NRLF. Therefore, chitin–NRLF composites could be widely applied to household products, such as pillows, mattresses or cushions, and possess good practical value for future applications.

**Author Contributions:** Conceptualization, N.Z.; Data curation, N.Z.; Formal analysis, N.Z.; Methodology, N.Z.; Supervision, H.C.; Writing—Original draft, N.Z.; Writing—Review and editing, H.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (21865026); Xinjiang Production and Construction Corps Project (2018BC005); Hebei key research and development project (18273401D); Hebei Province's Innovation Capacity Improvement Project (18952814D).

**Conflicts of Interest:** The authors declare no conflict of interest.
