A comparative study on roll-to-roll gravure printing on PET and BOPP webs with aqueous ink

Abstract

Ink transfer and printing characteristics in high-speed gravure printing were investigated on poly(ethylene terephthalate) (PET) and biaxial-oriented polypropylene (BOPP) substrates employing a pilot-scale gravure printing machine (maximum speed: 500 m min −1). The flexible substrates were printed in an aqueous ink diluted with ethanol under various machine factors such as printing speed, nip force, and groove size. The printed images and ink transfer efficiency were examined using an optical microscope, UV–visible spectroscopy, and an atomic force microscope (AFM). A quantitative study was performed on the measurements and estimation of interfacial tension, the work of adhesion, and the contact angle between the ink and the substrates to correlate with the printed features. According to the experimental results, the most critical factors affecting the print features were interpreted to be the viscosity and surface tension of the ink, and interfacial tension. The spreading coefficients of the diluted inks were below zero on both the PET and BOPP films, indicating that spreading hardly occurs spontaneously, and thus limited spreading may be associated with a shear-induced flow of ink. Among tested machine factors, the printing speed did not significantly affect the uniformity and chromaticity of the image on the PET web as observed under microscopy or spectrometry. On the other hand, those on the BOPP film were found to heavily depend upon roller speed and thus dwell time. This difference was attributed to low spreadability of the ink due to capillary immobilization effect on the porous BOPP surface. In addition, the increasing groove size (dot%) resulted in a higher chromaticity due to a larger contact area between ink and substrate. Especially in the case of BOPP, the chromaticity of the printed image increased linearly with increasing dot%. It should be also noted that the images printed on BOPP exhibited enormously higher chromaticity and thickness compared with those on PET at the same dilution and machine factors. The large difference in ink transfer efficiency between the BOPP and PET substrates may be explained partly by the difference in interfacial tension and absorption of the BOPP substrate.