The expansion in the Z-direction of individual layers in a wet sheet after leaving the nip of a double-felted press nip has been studied. In this investigation, pads of up to five wet sheets of CTMP, bleached kraft or mixtures of the two pulps, were used to simulate multilayer high-grammage sheets such as carton board. Using different lay-ups it was thus possible to study the expansion of both homogeneous and heterogeneous sheets.
The expansion of 350 g/m2 sheets is illustrated by the air fraction in the wet sheet immediately after the nip. The results show that a sheet of CTMP can expand up to ten times as much as a sheet of bleached kraft. It is also shown that CTMP has the ability to expand even when sandwiched between bleached kraft as middle layer in a stratified sheet.