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Printing Properties of Water-based Pigmented Inkjet Inks on the Oxygen-plasma Modified Polylactide Sheets 230 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 Figure 1 structure of PLA The above data indicate that the PLA sheets were not degraded after the PLA pellets were processed to become the sheets. Surface tension of the ink and surface energy of PLA sheets Surface tensions of 26±1, 32±0.5, 30±0.5 and 28±0.5 mN m -1 were found for the yellow, cyan, magenta and black pigmented inks, respectively. The standard devia- tion values are within ± 0.5. The surface tensions of all inks are within the standard range and it is acceptable for ink performance, i.e., droplet formation and spreading on printing substrates. Therefore, surface energy of the printing substrate is very important. The contact angles and surface energies of the PLA sheets for the untreated and treated sheets with the set condition are presented in Table 1. Table 1 indicates that the untreated PLA sheet (0 W and 0 s) has a relatively low polar contribution component of surface energy. With a fixed exposure time of 30 s and varied power, one can see that the polar contribution component of surface energy increased when compared with that of the untreated sheet. However, increasing the treater power did not increase the polar contribution component because the exposure time was too short to allow the reaction to take place although there were abundant amount of electron to functionalize the surface. One can also see that the dispersion contribution component of the exposed area remained constant as that of the untreated sheet. The polar/dispersion contribution ratio at various plasma powers and a constant exposure time is found approximately 1:1 and the total surface energy of the sheets is between 47 - 50 mN m -1 . Table 2 exhibits the effects of increasing exposure time on the extent of polar contribution component. The similar trend as those exhibited in Table 1 was found. Increases in exposure time to expose the oxygen gas plasma cannot increase the extent of polar contribution component. An increase of 11-14 mN m -1 of polar contribution component was ob- tained when subjecting the PLA sheets to the RF treater at 30 W for 30 s or longer exposure times.