*3.4. Chemical Composition*

The FTIR transmittance spectra of the NRLFs are shown in Figure 6. FTIR spectra are used to analyze the chemical compositions and to identify the structural transformation that occurs during blending. The peak at 2900 cm−<sup>1</sup> is due to the stretching vibration of the C-H bonds in methyl, the peak at 1660 cm−<sup>1</sup> is due to the stretching vibration of C = C bonds, the peak at 2850 cm−<sup>1</sup> is due to the stretching vibration of C-H bonds in methylene, the peak at 1460 cm−<sup>1</sup> is due to the bending vibration of C-H bonds in methylene, and the peak at 842 cm−<sup>1</sup> is due to the out-of-plane bending of C-H bonds [43–45]. These peaks are the key features in the spectra for the rubber hydrocarbons. Peaks at 3280 cm−<sup>1</sup> are probably connected to moisture in the samples.

Natural rubber latex originally contains proteins, carbohydrates, lipids and other substances, hence pure NRL possesses hydroxyl and amide peaks as well. From 1200 cm−<sup>1</sup> to 1000 cm−<sup>1</sup> , there are ether bond peaks [46,47]. This characteristic peak differentiates the chitin from the polyisoprene. As shown in the second local graph, the transmittance peaks of ether bonds are enhanced compared with pure NRLF. This demonstrates that chitin blends with the NRLF and does not significantly change the chemical structure of the natural rubber. It can be seen that with an increase in the chitin content, the natural rubber peaks in the transmittance spectra do not change. This indicates that there is no reaction between the chitin and natural rubber latex during blending and only physical loading occurs. In other words, the chitin has no significant impact on the spatial structure of the natural rubber.

**Figure 6.** FTIR spectra of chitin–NRLF.

#### *3.5. Hardness*

The effects of the chitin on the hardness are presented in Figure 7. The hardness (Shore C) mainly represents the ability of rubber to resist the pressing or intrusion of hard objects. The NRLF samples with an increased chitin loading have an increased hardness, according to the trend in Figure 7. The increase in chitin content leads to the collapse and partial adhesion of the NR, so the rubber clots provide additional resistance to the external intrusion. Chitin is a linear polymer composed of (1–4) linked 2-acetamido-2-deoxy-D-glucosamine. The hardness of chitin is 7–7.5 (Mohs) due to its regularly arranged structure. The Mohs hardness is used to describe the hardness of minerals, which is absolutely higher than that of rubber. Therefore, chitin could obviously enhance the hardness of NRLF composites.

**Figure 7.** Hardness of chitin–NRLF composites.

#### *3.6. Tensile Strength and Elongation at Break*

As shown in Figure 8, the tensile strength of the chitin–NRLF composite decreases with increasing chitin content. For NRLF–5 phr chitin, the tensile strength decreases to approximately half that of the pure NRLF. This decline proves that the structure of the NRLF is destroyed during the foaming process and that additional cells appear to burst before solidification. The low compatibility between the chitin and NR leads to a large surface tension on the foam, resulting in an easy bursting of the bubbles. With increasing pore size, the cells gradually become more fragile. The increase in the filler loading could cause poor dispersion and agglomeration of the chitin. The precipitation of the chitin on the surface would form stress concentrations during tensile testing.

**Figure 8.** Tensile strength of chitin–NRLF.

The elongation at break of NRLFs normally depends on the crosslinking density and the foam structure. Given that the NRLF composites herein were vulcanized under the same conditions [48–50], the difference in elongation at break would mostly be attributed to the size and quantity of the pores. As shown in Figure 9, the elongation at break of the NRLF decreases with increasing chitin loading. Similar to the trend for the tensile strength, the elongation at break of NRLF decreases remarkably when the chitin reaches 3 phr or more. The trend of these results is analogous to other literature [30]. Stress concentrations would suddenly develop at the agglomerations of the chitin, and the thinned foam walls could accelerate the breaking. Moreover, the increased hardness of the NRLF composite might contribute to the brittleness that appears during the tensile testing, which means a decreased amount of deformation before a break occurs.

**Figure 9.** Elongation at break of chitin–NRLF.

#### *3.7. Compression Strength*

As shown in Figure 10, the compressive strength of the NRLF increases substantially with increasing chitin loading. Chitin has a much higher compression strength than natural rubber, and a small amount of chitin could dramatically enhance the compressive strength of NRLF composites. In addition, when the chitin content continues to increase, the fracture of the cell might lead to adhesion of the natural rubber. For the same volume, the NRLFs with a high loading allocate an increased proportion to rubber but a decreased proportion to air. During compression, NRLFs with a high loading would have a decreased amount of inner space available to shrink. Therefore, when the samples are compressed at 50% during the compression testing, the additional rubber and chitin particles in the NRLFs with a high loading provide an increased resistance to the pressure and increased compressive strength as well.

**Figure 10.** Compression strength of chitin–NRLF.
