Time Deformation
Ratio of time dependent strain to a constant applied stress identifies the ease with which a material deforms over a long period. Unlike pure elasticity, viscoelastic compliance accounts for both the immediate recovery and the permanent flow of the substance. This value is a function of time and temperature.
It describes how much a paper or plastic component will stretch or sag while under a load.
Molecular Mechanism
Internal friction between polymer chains or cellulose fibres dissipates energy and leads to a delayed response. When a weight is first applied, the viscoelastic compliance starts at a low value and increases as the material relaxes. This process occurs because the molecular segments need time to reorient themselves into a more stable configuration.
In a paperboard sheet, the presence of lignin and hemicellulose affects this rate of movement. Increasing the temperature speeds up the molecular motion and raises the compliance.
Packaging Consequence
Stacked boxes in a warehouse undergo a slow increase in deformation that can lead to leaning pallets. A high level of viscoelastic compliance means the material is prone to excessive sagging under its own weight. This property is critical for engineers designing secondary packaging for long term storage.
