2.3.2. Swelling

The NRLFs were made into 25 mm × 25 mm × 25 mm samples. The dried samples were weighed and then hermetically immersed in deionized water for 72 h. The surfaces of the samples were wiped with kitchen paper before the swollen samples were weighed. The testing temperature was 25 ◦C. The swelling is indicated as S (%).

S = (mass of swollen sample−mass of initial sample)/mass of initial sample (1)

### 2.3.3. Density

The dry NRLFs were made into 50 mm × 50 mm × 25 mm (length × width × height) samples for measurement. The testing temperature was 25 ◦C. Five samples were tested under each loading condition, and the density is indicated as ρ (kg/m<sup>3</sup> ).

#### 2.3.4. Chemical Composition

The chemical composition was characterized by FTIR (Nicolet iS50, Madison, WI, America). The transmittance of the samples to infrared light was measured. The chemical composition of the NRLFs with chitin loading (0, 1, 2, 3, 4 and 5 phr) was investigated in a mid-infrared region (500–4000 cm−<sup>1</sup> ).

#### 2.3.5. Hardness

The hardness was tested according to the Shore C standard. A Shore C hardness tester (LX-C, Jiangdu, China) was used to measure the hardness of the soft foam materials. The dry NRLFs were made into 10 mm × 10 mm × 6 mm (length × width × height) samples for measurement. The testing temperature was 25 ◦C. Five samples were tested under each loading condition.

#### 2.3.6. Tensile Strength and Elongation at Break

Tensile strength and elongation at break were tested according to the Chinese National Standard GB/T 6344-2008. The NRLFs were made into 13 mm × 152 mm dumbbell-shaped samples with a gauge length of 50 mm. The speed of the universal material testing machine (XWW-20A, Beijing, China) used herein was 500 mm/min. The testing temperature was 25 ◦C. Five samples were tested under each loading condition.

#### 2.3.7. Compressive Strength

The NRLFs were made into 50 mm × 50 mm × 25 mm (length × width × height) samples. The compression ratio was 50%. The speed of the universal material testing machine (XWW-20A, Beijing, China) was 50 mm/min. The testing temperature was 25 ◦C. Five samples were tested under each loading condition.

#### 2.3.8. Antibacterial Activity

The antibacterial activity was characterized by the inhibition zone against *E. coli*. The NRLFs were made into cylindrical samples with a diameter of 8 mm and a height of 2 mm. The LB solid medium was poured into the culture dishes at 60 ◦C, and the samples were embedded in the medium. After the culture medium was completely solidified, bacterial liquid was added. All the dishes were incubated at 37 ◦C for 12 h.

#### **3. Results**

#### *3.1. Morphology*

Figure 2 shows the micrographs of the NRLFs in different compositions. The microstructure, shape of the cells and dispersion of the chitin can be observed. Figure 2a–f shows the NRLFs loaded with 0–5 phr chitin. From Figure 2a–c, it is obvious that the shape of the cells changes little with a low chitin content. When the loading is 3 phr, the chitin begins to agglomerate on the walls. At the same time, the walls attached to the chitin would gain traction on the surrounding foam. Due to the influence of the chitin, the cells are no longer circular in shape as the loading increases.

The deformation and fracture of the cells were detected, as shown in Figure 2e. When the loading reaches 4 phr, the rubber walls fracture due to the over-expansion of the bubbles and the obstacles provided by the chitin. In Figure 2f, the number of cell cracks and the pore size increase. The broken foam wall adheres to the chitin. The pores are not stable enough, and chitin becomes aggregated, so the rubber begins to break under an uneven force. Macroscopically, small particles can be seen on the surface of the NRLF. The pores become large and weak with high filler contents.

**Figure 2.** Micrographs 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.

The cell diameters of the NRLFs are shown in Figure 3. The relationship between pore size and filler loading can be seen in the diagram. First, as the loading of chitin increases, the pore size of the NRLF increases gradually. The large surface tension of the foam around the chitin results in the bursting and merging of the cells. However, the cell diameter in the NRLF–5 phr chitin case decreases. Under this loading, many bubbles become large enough to burst, and the remaining cells are weak and easily deformed. Therefore, only small cells could be measured completely. Second, with the increase in chitin loading, the standard deviation of the cell diameters increases as well. This indicates that the random dispersion of the chitin in NRLF leads to an uneven pore size. Compared with the cells in the

NRLFs with a low chitin load, the difference among the cells in the NRLFs with a high chitin load is too large to be accurately estimated.

**Figure 3.** Cell diameters of chitin–NRLF composites.

#### *3.2. Swelling*

The swelling ability could directly explain the porous structure of the NRLFs according to Equation (1). The NRLF composites were immersed in deionized water for 72 h, and the mass of the NRLF composites was measured. As shown in Figure 4, with increasing chitin content, the swelling percentage decreases. The reason might be the collapse and adhesion of the rubber foam when the loading increases. For NRLFs with a higher loading, the number of pores decreases sharply even though the cells merge and expand. Therefore, the low porosity of the NRLF composites diminishes the absorption capacity.

**Figure 4.** Swelling of chitin–NRLF.

#### *3.3. Density*

As shown in Figure 5, the density of the NRLF increases with increasing chitin loading. The density of chitin particles is 1370 kg/m<sup>3</sup> , which is much higher than that of the pure NRLF. In addition, agglomerates destroy the original bubble structure in the NRLF and cause collapse of the cells. A decrease in bubble volume leads to a decreased proportion of air, so the NRLFs with a high loading contain additional rubber and chitin. Although the density of the NRLF composite increases as the filler content increases, it is still lower than that of the other materials. This is attributed to the unique porous structure of the NRLF. The density of the NRLF composite is still less than 0.2 kg/m<sup>3</sup> .

**Figure 5.** Density of chitin–NRLF composites.
