Viscoelastic Property Loss in Aqueous Coated Paperboard Converting
Moisture and thermal stress during converting plasticize aqueous latex coatings, reducing storage modulus and causing fold delamination unless web tension is optimized.
Physical Aging
Structural Evolution
Progressive property changes occur in amorphous polymer coatings and glassy packaging materials during storage below their glass transition temperature without undergoing chemical degradation or cross-linking. In converted paper packaging materials featuring barrier films, physical aging describes the slow, spontaneous rearrangement of polymer chains toward lower thermodynamic energy states over time. The structural evolution reduces free volume, increases density, and alters the mechanical compliance of the coating layer.
These time-dependent changes directly influence the processing performance of coated folding carton stocks.
Performance Drift
Density increases generated by this relaxation mechanism alter barrier, optical, and mechanical properties across coated paperboard surfaces. As physical aging proceeds within an amorphous extrusion coating like polylactic acid or polyvinyl alcohol, the material experiences an increase in elastic modulus alongside a substantial decline in elongation at break. This embrittlement makes aged coated board significantly more susceptible to micro-cracking and barrier loss along score lines during subsequent folding passes.
Gas barrier performance for uncreased areas improves slightly due to the reduced free volume between polymer chains. Sealing performance drifts over time, requiring converting operators to increase heat-sealing temperatures and dwell times to achieve fibre-tearing bonds on aged stock.
Converting Adjustment
Substrates converted immediately after extrusion coating behave differently from inventories stored in climate-controlled warehouses for several months. Calibrating converting parameters, including scoring rule pressure and die clearances, against substrate age prevents unexpected liner fractures and seal failures during commercial carton production.