Transport of Immiscible Vapor Pressure Contrasting Liquids in Multiple Nip Impressions: Application to Heatset Offset Printing of Paper

A multistation heatset offset printing press was studied here to exemplify the transport of immiscible liquids into a porous substrate under dynamic rolling nip conditions. In offset printing, oil-based ink and a water-based fountain solution are used to define the image and nonimage areas according to the respective surface energy of the printing plate. The fountain solution and ink inevitably form an emulsion after the press has run sufficiently to reach equilibrium. It is this emulsion that is transported via a rubber blanket to the paper and forms the printed image. The emulsification degree depends on the ink properties and the printing process at the equilibrium ink–fountain solution balance, which, in turn, supports stable continuous running of the press in order to achieve a good print quality. Focus is given to the transfer of fountain solution to both nonimage areas of paper as well as to the inked areas carried in the emulsion. In order to monitor the amount transferred, each of two tracing elements, lithium and cesium in this case, was added separately to the fountain solution circulation. Printed paper samples as well as ink samples were taken from different positions in the press at various time intervals to follow the evolution of the process. The amount of lithium and cesium tracers was analyzed using time-of-flight secondary-ion mass spectrometry and inductively coupled plasma mass spectrometry, respectively. It was found that nonimage areas take up a constant amount of liquid per printing unit following the first unit, with the uptake being limited by the coated paper properties, while the tracer concentration in the printed image areas increased as a function of time because of progressive evaporation of the fountain solution in the system over time.