RATE OF VEHICLE REMOVAL FROM OFFSET INKS: a gravimetric determination of the imbibition behaviour of pigmented coating structures

摘要

This work examines experimentally the competitive rate of removal of ink vehicle from a setting commercial offset ink by the absorptive forces of a porous coating network acting against the progressive concentrating boundary ink film. The experiments are made gravimetrically using a compressed tablet of calcium carbonate coating pigment brought into contact with a supersource of ink. The porosities of the model substrates are determined by mercury intrusion porosimetry. Effects such as polymer and ink resin size exclusion at the interface between the ink and the surface of the porous medium are considered. Results are compared with a number of different models. A good correlation is seen with the earlier prediction of fluid loss based on observed ink tack rise on commercial offset paper using the force-time integral recorded by the SeGan Ink Surface Interaction Tester (Schoelkopf, Gane et al, 2000), where a process viscosity was treated as an uniaxial extensional viscosity, whose magnitude is a function of ink oil content. These data confirm the finding that the weight fraction of oil lost from an ink is linear with time during the tack rise period, such that the rate oil mass lost from an ink decays according to an approximate inverse square law with time. The implications for print quality are discussed in relation to the dominance of a preferred network absorption pathway characterising the initial tack rise regime. Control of this pathway dynamic in relation to the total pore volume is shown to be a major factor in controlling the press runnability performance of coated paper in relation to the tack build.