**1. Introduction**

Natural rubber latex (NRL), the viscous liquid from *Hevea brasilensis*, is the main source of commercial natural rubber. Fresh NRL comprises an aqueous colloid with 44–70 wt% water. Generally, latex is processed into high ammonia natural rubber latex (HA-NRL) for convenience of preservation and transportation [1]. The main component of natural rubber is poly(cis-1,4-isoprene). In addition to water and rubber hydrocarbons, natural rubber latex also includes proteins, lipids and other substances [2,3].

Natural rubber latex products can be divided into four categories: impregnated products, molded products, extruded products and foam products. Compared with the former three, natural rubber latex foam (NRLF) is a porous material with a low density [4,5]. NRLF has the characteristics of elastic resilience, absorbency, providing sound insulation, being shockproof and providing ventilation [6–12]. At present, the commercial products made from NRLF mainly include mattresses and pillows.

NRLF, widely used as bedding, is one of the economic mainstays of Southeast Asian countries [13]. However, NRLF is easily attacked by ultraviolet light due to its unsaturated carbon–carbon bonds [14]. NRLF bedding is not suitable for long-term exposure to sunlight, and slow bacterial growth might occur on the cover [15–17]. Therefore, it is necessary to improve the antibacterial activity of NRLF and to choose appropriate antibacterial agents to reduce the cost.

Chitin [(C8H13O5N)n], which widely exists in the shells of crustaceans such as shrimps and crabs, is a renewable, low-cost, antibacterial and well-sourced resource [18–22]. The chemical structure of chitin is shown in Figure 1. Chitin is a cationic natural polymer due to its amide groups. The positively

charged polymer neutralizes the negatively charged functional groups on the surface of bacteria. Once the cell wall is damaged, the cell osmotic pressure would be destroyed and finally the bacteria would die [23,24]. Therefore, chitin could be considered an environmentally friendly antibacterial agent [25–29].

**Figure 1.** Chemical structural formula of chitin.

Most of the research on NRLF composites involves filler loading [30–33]. The focus is on exploring the mechanical properties of composite foams and emphasizing the reuse of natural fibers. Currently, the research on antibacterial NRLF mainly focuses on zinc oxide [34] and silver particles [35–39]. Khemara Mama [40] demonstrated that NRLF treated with silver nanoparticles of only 0.2 per hundred rubber (phr) had improved antibacterial ability by 43.8% against *E. coli* and 25% against *S. aureus* compared to NRLF that was not treated with silver nanoparticles. Few reports have been published on the utilization of natural renewable materials to prepare antibacterial NRLF composites.

Compared with zinc oxide and silver particles, chitin has the advantages of renewability, biodegradability and biocompatibility. Furthermore, the preparation of silver antimicrobials is quite complex due to the synthesis of nanoparticles [39] while blending with chitin is more simple and convenient. The poor solubility of chitin provides the NRLF composite with more durability within its service life. Therefore, chitin–NRLF composites can not only enhance the antibacterial activity of foam, but are also inexpensive and environmentally friendly [19].

In this research, 0, 1, 2, 3, 4 and 5 phr chitin-loaded NRLFs were prepared using the Dunlop method. Morphology, swelling, density, chemical composition, hardness, tensile strength, elongation at break and compressive strength are characterized to verify the antibacterial activity vs. mechanical properties of chitin–NRLF composites. This natural antibacterial composite foam has considerable prospects in commercial applications of natural rubber, which is consistent with the concept of sustainability and the goal of a green economy.

#### **2. Materials and Methods**

#### *2.1. Materials*

The Dunlop method was used to prepare the NRLFs. High ammonia natural rubber latex (HA-NRL) was supplied by Yunnan Natural Rubber Industry Group Jingyang Co., Ltd (Yunnan, China). The chemicals (potassium hydroxide, 2-mercaptobenzimidazole, 2-mercaptobenzothiazole, sulfur, potassium oleate, ammonium sulfate, zinc oxide, sodium fluorosilicone), Luria-Bertani solid medium (deionized water, tryptone, sodium chloride, yeast extract, agar) and chitin were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). *E. coli* BL21 were supplied by Qincheng Biotechnology Co., Ltd. (Shanghai, China). The names, concentrations and proportions of the chemicals used in the synthesis experiment are shown in Table 1.


**Table 1.** Formulation parameters for the synthesis of the chitin–natural rubber latex foam (NRLF) composites.

TSC: total solid content; phr: per hundred rubber.
