DETERMINATION OF ELONGATIONAL STRAIN DURING INK-ON-PAPER TACK MEASUREMENT: a novel technique to measure work of cohesion and adhesion of thin films

In a previous paper we proposed a model of uniaxial extension to describe the viscous deformation of an offset-ink film on paper during ink-on-paper tack measurement as analysed on the Ink-Surface Interaction Tester (ISIT) (Gane et al., Proceedings Tappi IP&GA 2000, Savannah). This now forms the subject for direct experimental confirmation. An optical reflection device based on a diode laser beam is used as a motion sensor to obtain data for the extensional rate of strain, ε& . The application of the sensor requires that it have a resolution of approximately 1 μm and be able to record movement over a millisecond timescale. The motion of the tack wheel contacting a pre-printed surface is transferred via its extended axis to a thin connected mirror, pivoting around a fixed base-point. The laser beam acts as a pointer monitoring this displacement at a distant plane, where the position of the laser dot is recorded by means of a high speed digital camera. The collected data show a motion of quasi-linear acceleration, and hence strong extensional flow. The discontinuity at the start of the motion, during which any inertial effects arising from the initial short timescale acceleration, is limited to less than ~ 1 % of the overall motion. This verifies the assumption made in the previous work, and, as a result, it is proposed that the experimental design encompassed by the ISIT can be used to evaluate the uniaxial extensional viscosity of relatively viscous fluids and pastes which show the necessary adhesional properties to a given substrate. Furthermore, the evolution of viscosity of such fluids can be monitored as a function of time, for example during concentration on contact with an absorbing porous medium. The case of ink-on-paper tack is used to exemplify the possibility to determine the work of extension during cohesive failure, i.e. the work of tack. The implications for boundary contact 1 ISIT is a product name of SeGan Ltd., Perrose, Lostwithiel, Cornwall PL22 0JJ, U.K. adhesion are also discussed in terms of the measurement of the work of adhesion failure, if it occurs, during concentration of the ink at maximum tack levels.