Characterization on the Ozone Oxidation of Raw Natural Rubber Thin Film using Image and FT-IR Analysis

Abstract

The characterization of the ozone oxidation for raw natural rubber (NR) was investigated under controlled conditions through image and fourier transform infrared (FT-IR) analysis. The ozone oxidation was performed on a transparent thin film of raw NR coated on a KBr window in a dark chamber at $40^{\circ}C$ under low humidity conditions to completely exclude thermal, moisture, or light oxidation. The ozone concentration was set at 40 parts per hundred million (pphm). Before or after exposure to ozone, the image of the thin film for raw NR was observed at a right or tilted angle. FT-IR absorption spectra were measured in the transmission mode according to ozone exposure time. The ozone oxidation of NR was determined by the changes in the absorption peaks at 1736, 1715, 1697, and $833cm^{-1}$, which were assigned to an aldehyde group (-CHO), a ketone group (-COR), an inter-hydrogen bond between carbonyl group (-C=O) from an aldehyde or a ketone and an amide group (-CONH-) of protein, and a cis-methine group ($is-CCH_3=CH-$, respectively. During ozone exposure period, the results indicated that the formation of the carbonyl group of aldehyde or ketone was directly related to the decrement of the double bond of cis-1,4-polyisoprene. Only carbonyl compounds such as aldehydes or ketones seemed to be formed through chain scission by ozone. Long thin cracks with one orientation at regular intervals, which resulted in consecutive chain scission, were observed by image analysis. Therefore, one possible two-step mechanism for the formation of aldehyde and ketone was suggested.

Figure 1. Images for a transparent raw NR thin film coated on a KBr window (a) at a rectangular angle before ozone exposure and (b) at a tilted angle after ozone exposure of 753 hours.

Figure 2. Magnification image of the ozone oxidation at a tilted angle for the same sample shown in Figure 1(b).

Figure 3. The regular rearrangement with one orientation for a raw NR thin film coated on a KBr window.

Figure 4. FT-IR absorption spectra for a transparent thin film of raw NR with the ozone exposure time ranging (a) from 2600 to 3600 cm^-1, (b) from 1600 to 1800 cm^-1, and (c) from 700 to 1400 cm^-1 in a dark at 40^oC.

Figure 5. The inter-hydrogen bond between an aldehyde or a ketone and protein of NR.

Figure 6. The change curves of the increment and reduction in each absorbtion frequency with the ozone exposure time based on Table 4.

Figure 7. 1^st step : The formation of a zwitterion and an aldehyde.

Figure 8. 2^nd step : The formation of ketone from two adjacent zwitterions.

Table 1. Standard Malaysian Rubber Specifications of SMR 20.

Table 3. The Peak Height in each FT-IR Absorption according to the Ozone Exposure Time based on Figure 4.

Table 2. Characteristic FT-IR Peaks for Un-oxidized and Oxidized Natural Rubber.

Table 4. The Relative Abundance Calculated from the Absorption Peak Ratio of A_x to A₂₉₅₉ based on the Maximum Peak.