水性轮转凹印油墨在PET和BOPP基材上印刷的对比研究

A Comparative Study on Roll-to-roll Gravure Printing on PET and BOPP Webs with Aqueous Ink

采用凹印机(最大速度500m/min)在PET(聚对苯二甲酸乙二醇酯)和BOPP(双向拉伸聚丙烯)承印物上进行印刷试验,以研究高速凹版印刷下油墨的转移情况和印刷适性。将水性油墨用酒精稀释后,在特定的印刷速度、印刷压力和网穴尺寸等多种印刷设备条件下对柔性承印物进行印刷。采用光学显微镜、UV可见光谱仪和原子力显微镜(AFM)测量印刷图像和油墨转移率。通过对界面张力、黏度以及油墨与承印物之间润湿角的测量与评价,定量分析它们对印刷特性的影响。实验结果表明,影响印刷特性的最重要的因素是油墨黏度、表面张力和界面张力。稀释后的油墨在PET和BOPP薄膜上的铺展系数都小于零,也就是说油墨难以在二者上顺利铺展,因此,扩散可能是由于剪切力导致油墨流动。显微镜或光谱仪测量表明,在各种试验印刷设备条件下,印刷速度不会对PET薄膜上印刷图像的均匀性和色度产生很大影响,而BOPP薄膜上的图像质量则在很大程度上依赖于转速。两者存在差异的主要原因在于BOPP薄膜多孔表面的毛细管堵塞导致油墨的铺展能力降低。此外,网穴尺寸(网点百分比)增大导致色度增大,其原因在于油墨和承印物间的接触面积变大。尤其对于BOPP薄膜来说,其印刷图像的色度会随着网点百分比的增大而线性增大。应该指出,在相同的稀释条件和设备条件下,较之PET薄膜来说,BOPP薄膜上的印刷图像的色度与墨层厚度远远高于PET薄膜。BOPP与PET薄膜在油墨转移率上的较大差异也许可以在一定程度上解释两者在界面张力上的差异,以及BOPP承印物上的吸收现象。

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： 500mmin-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.