Viscoelastic Rheology and Creep Response in Cellulosic Fiber Matrices
Viscoelastic creep in cellulosic matrices accelerates under humidity cycling, reducing long-term compression strength and requiring calibrated safety margins.
Permanent Set
Plastic Deformation
The irreversible strain that remains in a material after it has been subjected to stress and then fully unloaded defines the limit of elastic performance. In paper and paperboard products, permanent set occurs when the applied mechanical force exceeds the yield point of the fibre network, causing non-recoverable structural changes. This residual deformation indicates that the internal bonding of the sheet has been altered at a microscopic level.
Fibrous Displacement
Fibre-to-fibre bond disruption and cell wall damage are the primary drivers of this irreversible elongation or compression. When paperboard is bent or stretched beyond its elastic limit, the hydrogen bonds between cellulose fibrils break and reform in new, displaced positions. This reorganization prevents the network from returning to its original dimensions, leaving a permanent distortion in the sheet.
Performance Impact
During the creasing and scoring of folding cartons, the formation of a localized permanent set is necessary to create a clean, functional fold that does not spring back. However, when permanent set occurs unintentionally during tension control on web-fed printing presses, it leads to register errors and dimensional instability. Press operators must maintain web tension below the critical yield threshold to prevent this permanent elongation.
Controlling this property ensures that the paperboard retains its flat profile and structural integrity throughout the conversion process. Uncontrolled deformation can cause feeding jams in high-speed sheet-fed presses, leading to costly downtime and wasted stock.