Standard Laboratory Conditioning Protocols for Coated Folding Boxboard Evaluation
Valid boxboard physical testing requires ISO 187 conditioning at 23°C and 50% RH to prevent hysteresis errors in stiffness, yield, and creasing metrics.
Hysteresis Curve
Moisture Relation
The graphical representation of the difference in moisture content of paper when it is dried from a wet state versus when it absorbs moisture from a dry state is fundamental to understanding paper stability. This hysteresis curve shows that at any given relative humidity, the equilibrium moisture content is higher during desorption than during adsorption. The history of the fiber’s exposure to moisture determines its current physical dimensions and mechanical properties.
Physical Path
Fiber structure changes permanently during the initial drying process at the paper mill. The internal hydrogen bonds between cellulose chains lock into place, reducing the available surface area for future water binding. When the paperboard is exposed to ambient humidity again, it cannot reabsorb as much water as it held originally.
This behavior produces the characteristic loop on the hysteresis curve, which is critical for predicting how the paperboard will behave in varying climates. The curve illustrates that the moisture history of the material must be known to accurately predict its physical stability during overseas transport.
Dimensional Recovery
Print shops and packaging plants use this moisture behavior to prevent curling and misregistration on press. If paperboard undergoes multiple humidity cycles during storage, its dimensions will shift along the hysteresis curve paths. This causes the sheets to expand or contract unevenly, leading to feeding issues or poor ink alignment.
Storing the material in humidity-controlled environments keeps it at a stable point on the curve, ensuring flat sheets and consistent print quality.