Structural Property
Physical behaviour and strength of paper or board in the thickness direction, perpendicular to the plane of the sheet. Understanding z-direction mechanics is necessary for predicting how the material will respond to the internal stresses of multi ply construction and the external forces of printing. Adhesion is the driver.
This property is governed by the bonding between the fibre layers and the distribution of additives through the sheet.
Internal Bond Strength
Resistance of the board to delamination during high tack offset printing or lamination is a primary measure of its performance. If the z-direction mechanics are weak, the layers of the board can pull apart, leading to surface picking or a complete failure of the structure. Factors such as the type of pulp used, the degree of refining, the wet end chemistry and the moisture content during the pressing stage all influence the final bond strength.
Chemical bonding agents are often added to enhance this internal integrity.
Testing and Control
Laboratory tests like the Scott Bond or the z-directional tensile test provide quantitative data on the internal integrity of the substrate. Monitoring the z-direction mechanics allows mill operators to adjust the refining and chemical dosage to meet the demanding requirements of high speed converting. A consistent profile in this dimension ensures that the board remains intact during the complex folding and gluing steps of carton manufacturing.
