A study of lithographic ink and water interactions

• Main Author: Chambers, Dennet
• Published: Open University 1994
• Subjects: 660; Printing
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.241461

Achieving and maintaining the correct ink/water balance is crucial for acceptable print quality; this involves factors affecting the emulsification of water and ink. This process was investigated with two designed model ink varnishes (one being 'hydrophilic' relative to the other 'hydrophobic' varnish) and two commercial varnishes. The Surland test, widely used to characterise ink/water balance, showed differences in water uptake between inks and corresponding varnishes; however it did not distinguish between the 'hydrophilic' and 'hydrophobic' systems. Thus, the rheology of the systems are considered together with implications to the lithographic process. Rheological studies of creep and flow showed that the hydrophilic and hydrophobic systems behaved differently in the presence of water. The rheology of the ink varnish plays a major role in emulsification and the nature of processes at the interface affecting droplet fragmentation, droplet coalescence and time-dependency. Slippage occurred in the hydrophobic systems to a greater extent compared with the hydrophilic systems increasing with water concentrations from 0 to 25% water. Droplet size distributions of the emulsion depend on temperature and agitation speeds. As emulsions approached dynamic equilibria, mean droplet diameters in the hydrophobic varnish at 70°C and 40°C were found to be ca. 2S and 12 I'm respectively; corresponding mean droplet diameten in the hydrophilic varnish were ca. 20 I'm and 5 I'm. Increase of interfacial tension for both varnishes (ca. 6 mNm-l) between 40°C and 70°C, may not be significant to droplet breakdown. A reduction in viscosity i.e. from 100 to 5 Pas in the hydrophobic varnish and from 300 to 5 Pas for the hydrophilic varnish was the determining factor. Droplet size depends on temperature: e.g. temperature increases led to larger droplets. Viscosity ratios of the continuous and disperse phases account for these effects. Investigating the time-dependency of the system showed that allowing 'rest periods' between episodes of mixing altered the rate and nature of emulsification. Thus using a modified varnish, rest periods of 1 and 3 minutes produced mean droplet diameters in the range of 1.5-1.8 um whereas rest period of 5 minutes produced diameters of 2.3-2.4 I'm indicating that thixotropic recovery which occurs up to 3 minutes, restores the rate of droplet breakdown; whereas, after 5 minutes, coalescence is dominant. Extending the 'rest period' allows time for drainage of a film of continuous phase between two colliding droplets. These time-dependent effects varied with different varnish modifications. The effect of IP A on emulsification was also time-dependent. During 3 minutes of stirring at 900 rpm, the number of droplets below 2 I'm was greater in an emulsion containing IP A than in one containing water only; the situation was reversed after 4 minutes of stirring where the number of droplets below 2 um was lower. A decrease in interfacial viscosity at the varnish/water interface is considered; such a decrease may reduce the effective shear stress at the interface and thus the extent of droplet breakdown.