Thermal Phase
Polymer physics defines the critical point at which an amorphous carbohydrate matrix shifts from a rigid glassy state to a flexible rubbery state. Structural cellulose within paperboard fibres exhibits this thermodynamic transition when thermal energy overcomes intermolecular hydrogen bonding. In dry conditions, cellulose glass transition temperature occurs well above ambient processing levels, typically between 200 and 230 degrees Celsius.
Differential scanning calorimetry and dynamic mechanical analysis measure this transition by detecting shifts in heat capacity or mechanical damping under controlled heating rates. The measurement applies exclusively to amorphous regions within the cell wall, as crystalline cellulose domains retain structural rigidity until chemical degradation intervenes.
Softening Behavior
Heated converting operations alter paperboard formability when web temperatures approach structural transition thresholds. Hot embossing and creasing processes exploit local thermal softening to re-form wood fibres without tearing the sheet surface. Below this threshold, rigid cell walls resist permanent deformation and exhibit brittle cracking under mechanical stress.
Moisture Response
Plasticization through moisture sorption shifts the thermal transition point downward toward room temperature. Water molecules disrupt inter-chain hydrogen bonds within amorphous cellulose, acting as an internal plasticizer that lowers energy barriers for segmental polymer motion. At elevated moisture content, cellulose glass transition temperature drops below standard heat-sealing or thermoforming temperatures, allowing lower energy input during carton forming operations.
Creasing matrix efficiency improves when ambient humidity maintains fibre moisture within optimum working ranges. Processing dry board forces converting lines to operate at higher tool temperatures to achieve equivalent crease sharpness.
