For this master thesis, two methods to apply a pattern with a controlled amount of moisture to hand sheets were evaluated. The two methods evaluated were spraying and pressing. Spraying moisture onto the sheets was deemed the easiest method to control and was chosen for further studies. The sheets were sprayed with four spray times and patterns to create different moisture content variations (4.2, 8.0, 14.2 and 26.9 pp moisture content difference). The moisture patterns were designed so the sheets had either moist spots with drier surroundings or reversibly, drier spots with moist surroundings. The sprayed sheets were dried unrestrained or fully restrained to study how in-plane moisture variations could affect paper properties and out-of-plane deformation. Unrestrained drying resulted in out-of-plane deformation around the areas where moisture had been applied. Restrained drying resulted in no out-of-plane deformation but instead changes in opacity, permeance, grammage and thickness occurred. The severity of cockling and difference in paper properties after drying were found to increase with greater moisture content variation before drying. However all effects could be obtained even at the lowest moisture content variation. The resulting out-of-plane deformation varied between ±100 μm for 4.2 pp moisture content difference and ±250 μm for 14.2 pp moisture content difference

A hygroexpansimeter, cockling tester and a novel x-ray microtomography technique were employed to characterize hygroexpansivity as well as macro- and micro-scale cockling of wood-free paper sheets containing varying amounts of aspen high yield pulp (HYP) in order to validate its potential applicability in inkjet printing. Results from the hygroexpansimeter confirm previous findings that substitution levels of 30% and above lead to a slight deterioration of hygroexpansivity; however, contrary to the expected outcome, aspen HYP did not necessarily result in a higher tendency to form cockles. In fact, a comparison of the cockling behaviour between various paper samples and commercial paper substantiates that up to 40% of aspen HYP can replace hardwood bleached Kraft pulp. The preservation of out-of-plane dimensional stability in non-coated samples is most likely due to the increase in bending stiffness of the paper as it became bulkier with more aspen HYP.

A hygroexpansimeter, cockling tester and a novel x-ray microtomography technique were employed to characterize hygroexpansivity as well as macro- and micro-scale cockling of wood-free paper sheets containing varying amounts of aspen high yield pulp (HYP) in order to validate its potential applicability in inkjet printing. Results from the hygroexpansimeter confirm previous findings that substitution levels of 30% and above lead to a slight deterioration of hygroexpansivity; however, contrary to the expected outcome, aspen HYP did not necessarily result in a higher tendency to form cockles. In fact, a comparison of the cockling behaviour between various paper samples and commercial paper substantiates that up to 40% of aspen HYP can replace hardwood bleached Kraft pulp. The preservation of out-of-plane dimensional stability in non-coated samples is most likely due to the increase in bending stiffness of the paper as it became bulkier with more aspen HYP.