Adhesive Interlocking
Depth to which molten synthetic resin migrates into the surface pore structure of a paper substrate determines the mechanical adhesion of the extruded barrier layer. Polymer penetration occurs when the heated polymer web is pressed against the porous paperboard sheet in the nip between a chill roll and a pressure roll. This action forces the viscous polymer into the voids between the cellulose fibres, creating a strong mechanical interlock upon cooling.
Achieving the correct depth is necessary to preventing the polymer film from peeling off during subsequent converting operations.
Process Variable
High melt temperatures and elevated nip pressures increase the fluidity and force the resin deeper into the fibrous network. If the polymer penetration is too shallow, the barrier film will delaminate easily under the shear forces of die-cutting or heat-sealing. However, excessive migration of the resin into the sheet can deplete the surface film thickness, which reduces the barrier performance of the coating against grease or moisture.
Controlling the extruder temperature and line speed allows operators to balance these competing effects.
Testing Method
Microscopic analysis of polished cross-sections allows laboratory technicians to measure the depth of the resin-fibre interface. By staining the polymer or utilizing fluorescent lighting, they can clearly distinguish the boundary where the plastic meets the cellulose fibres. This measurement helps optimize the corona treatment and pressure settings on the coating line.
