Printing Properties of Water-based Pigmented Inkjet Inks on the Oxygen-plasma Modified Polylactide Sheets 226 The Journal of the Royal Institute of Thailand Vol. 37 No. 1 Jan.-Mar. 2012 Bioplastic has slowly replaced the functions of petroleum-based plastics in packaging sec- tors which led to a demanding growth at a rate of 30% while the production capacity of bioplastic is only at 360,000 tons per year with a market share of only 1% . Bioplastic is a green product obtained from fermentation of naturally derived feed stock which needs low energy to run its pro- duction . More importantly, it can be degraded by heat, sunlight and soil compostable to carbon dioxide and water . Therefore, better quality printing on the bioplastic surface should conform to those high qualities so obtained from the petroleum-based plastics. One way to improve good printed image qualities is to give corona discharge, flame treatment or plasma treatment on bioplas - tics surface before printing. These treatments increase the surface energy of plastic sheets, foils, paper and polymer objects to improve wettability and adhesion of inks, coatings and adhesives. Plasma treatment only modifies the outermost thin layer of the surface, while the bulk properties will be kept and do not generate waste water [4, 5]. The treatment produces new functional groups on the surface. Like many gas pretreatments, plasma has an ageing effect. Surface roughness is another parameter which can be correlated with the amount of coating adhered to surface and can be altered by the plasma treatment . Junkar et al.  investigated the effects of oxygen and nitrogen gas plasma on PET [poly(ethylene terephthalate)] surface for fucoidan coating. They investigated the extent of plasma treatment with surface roughness and fucoidan coating. The oxygen plasma treatment produced the rougher surface (Ra = 9.9 nm) and fucoidan could wet the treated PET surface much better than that of nitrogen plasma treated surface. Kim and Masuoka  studied the carbon dioxide plasma treatment on degradation of PLA and PHBV sheets. The plasma treatment increased hydrophilicity of the sheets as the contact angles decreased from 80º to 45º. XPS (X-ray photoelectron microscopy) results indicated the presence of C and O as a ratio of O/C; and PLA sheet had a high ratio than that of the PHBV sheet. Both treated sheets degraded at a higher rate in acidic, neutral and basic conditions. Chen et al.  used oxygen plasma to treat PBO [poly(p-phenyl benzobisoxazole)] for characterization of surface functional groups and roughness as a function of the treater power. In the present research, a commercially available polylactide plastic was treated with oxygen plasma. The freshly treated plastics were characterized for surface wettability via contact angle measurement, infrared spectroscopic and AFMmicroscopic analyses. The sheets were printed with a set of commercially available pigmented inkjet inks. The printed sheets were analyzed for the effect of oxygen plasma treatment on color reproduction and ink adhesion.