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Printing Properties of Water-based Pigmented Inkjet Inks on the Oxygen-plasma Modified Polylactide Sheets 236 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 However, the better quality prints are not observed with the exposure times longer than 25 min; i.e., an optimum roughness can better accommodate the applied ink film with leveling and fuller toner reproduction range. Adhesion impediments can be resulted from surface abnormalities such as roughness, one general property for good adhesion. Improved adhesion of the interfaces between the PLA surface and the ink film could be due to physical interaction and chemical bonds. It is suspected that the long-lived oxygen ions or radicals may etch the pigment density due to its etching power and ion interaction between the PLA surface and the pigmented ink. Conclusion The oxygen plasma can be used a pretreatment gas for increasing surface energy of the PLA surface by increasing the polar contribution component by 10 _ 12 mNm -1 which exceeds the surface tension of the inks used by more than 10 mN m -1 . Increases in the polar contribution component of the surface energy reached a maximum limit depending on the RF power and exposure time from which the longer exposure time of the oxygen plasma imposed the effective wetting, leveling, surface roughness and better adhesion of the printing ink. It is found that primary adhesion impediments depended on low surface tension of the printing ink, a higher surface polarity, better wettability and medium RMS of the printing substrate. Acknowledgements The technical support from the ThEp Center, the Commission of Higher Education is very grateful. We thank the Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, Thailand for providing research facilities. PLA sheet donation from the Biogreen World is also appreciated.