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Printing Properties of Water-based Pigmented Inkjet Inks on the Oxygen-plasma Modified Polylactide Sheets 232 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 Figure 2 Surface roughnesses of PLA sheets: (a) the untreated surface, and the treated surfaces with the oxygen gas plasma of 30 W at (b) 30 s, (c) 20 min and (d) 40 min. Effect of exposure time on surface physical property According to the results presented in Tables 1 and 2, a limit amount of increased polar contribution component of surface energy was found at 30 W at various exposure times. Interac- tion of the etching oxygen plasma and the PLA surface can be observed via the 3-D images shown in Figure 2. Figure 2 shows the 3-D surface structure of PLA sheets in which Figure 2(a) is the untreated sheet in Figure 2 with a flat and smooth layer (Ra value = 7.1 nm). When exposing to the oxygen plasma of 30 W for (b) 30 s, some small roughness was seen on the surface (Ra = 10.4 nm). With the longer exposure times from 20 min exposure in Figure 2(c) to 40 min exposure in Figure 2(d), the heights of surface roughness increased markedly from 53.8 nm to 85.5 nm, respectively. Likewise, the root-mean-square roughness (RMS) was increased from 10, 13, 67, and 105 nm for the untreated sheet and the treated sheets at 30 s, 20 min and 40 nm exposures, respectively.