Polymer Plasticity
Transition of non-crystalline wood polymers from a rigid to a plastic state happens when paper is exposed to heat and moisture. The hemicellulose softening allows the fibres to deform and bond more effectively during the pressing and calendering stages. This process occurs at a lower temperature than the lignin transition, making it the primary driver for surface finishing.
Surface Refinement
Softened polymers act as a natural adhesive that increases the internal bond strength of the sheet. When the paper passes through a heated nip, these molecules flow into the gaps between fibres, creating a smoother and more closed surface. This effect is necessary for achieving high gloss in coated grades.
If the paper is too dry, the softening will not occur, resulting in a rough and porous sheet. Moisture acts as a plasticizer that lowers the required temperature for this phase change. Controlled heating ensures that the surface becomes dense enough to resist ink penetration.
Structural Pliability
Controlled plasticity prevents the fracturing of fibres when the board is folded into a carton. A mill uses steam showers to precisely trigger this state before the web enters the calender stack. Excessive heat can lead to permanent loss of bulk, so the window for successful softening is narrow.
