Fiber Softening
Moisture absorption by cellulose fibers reduces their glass transition temperature and temporarily increases their flexibility. This physical process, known as hydroplasticization, occurs during the creasing and folding of paperboard when water or steam is applied. Softening the fiber network makes the board more pliable and decreases the force required to create a clean fold.
Mechanical Application
The presence of moisture relaxes the internal stresses within the fiber network, allowing the cell walls to slide past each other during deformation. This temporary softening prevents the outer plies of the board from cracking when subjected to sharp bends. Once the moisture evaporates, the board regains its dry stiffness and locks the fold into its permanent shape.
Process Consequence
Applying this principle on the converting line requires precise control over the amount of water used to avoid weakening the adhesive bonds or causing dimensional instability. If hydroplasticization is carried out excessively, it can lead to carton warp or poor registration during die-cutting. However, when properly managed, it enables the high-speed folding of thick, high-density paperboards that would otherwise crack under dry conditions.
This balance between moisture addition and drying time is essential for optimizing the runnability of heavy packaging grades.
