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Summary:
The interaction of water with paper plays a role in web handling characteristics during commercial heat set web offset printing. Water absorption leads to lower web tension for a given strain. The purpose of this work was to determine the papermaking reasons for lateral shift of the moving paper web made from a particular paper machine on a specific printing press during heat-set web offset (HSWO) printing. The lateral movement is seen as a steady state shift to the gear side of the printing press, which can be measured at the exit of the chill section. In upset conditions, such as start up or blanket wash the web may shift so far that it runs off the paper guiding rollers and/or jams the folder section of the printing press. The paper in question is a coated mechanical paper made on a Fourdrinier paper machine (PM) in the northern United States using pressurized groundwood (PGW) mechanical pulp and northern bleached softwood kraft (NBSK) pulp. The paper is coated with a blend of kaolin and ground calcium carbonate pigments along with starch and latex binders, and supercalendered to 64% gloss. Absorption rate and air permeability testing showed that supercalendered (SC) papers absorb water faster than the target PM and air permeability of SC paper is greater than that that from target PM yet they suffer no lateral movement during printing. The reason for lower web tension in the target paper machine web is most likely due to lower fiber orientation. Fibers oriented in the direction of stress have reinforcement from the cellulose microfibrils. The stiff microfibrils serve to reinforce the matrix and prevent movement. Fibers oriented in the cross direction do not have this reinforcement. Therefore, a less oriented sheet has less reinforcement from the microfibrils and is more susceptible to lateral movement.