Surface Resistance
Aqueous barrier performance on paper substrates relies heavily on hydrophobicity to prevent liquid penetration during converting operations. Liquid water droplets sitting on a coated board encounter contact angles exceeding ninety degrees when sizing agents successfully lower surface energy. Converting machinery processing untreated kraft paper often experiences fiber swelling because moisture enters open capillary networks between cellulose fibrils.
Applying fluorochemical alternatives or dispersion coatings creates a non-polar boundary layer that repels moisture during high speed flexographic printing runs. Water drop absorption tests measure this repellency by recording the seconds required for a specific droplet volume to sink into the sheet.
Moisture Migration
Ambient humidity fluctuations cause untreated cellulose packaging to absorb ambient water vapor through capillary condensation inside microscopic pores. Dimensional stability degrades rapidly when relative humidity levels exceed ambient thresholds because hydrogen bonds between adjacent fibers weaken under high moisture loads. Hydrophobicity limits this dimensional distortion by restricting water vapor transmission through corrugated containerboard walls during long distance maritime transit.
Starch sizing applied at the wet end of a Fourdrinier paper machine establishes baseline resistance against water vapor ingress before secondary barrier coatings are applied.
Coating Formulation
Extrusion polyethylene layers and aqueous barrier dispersions require precise surface energy matching to achieve defect free adhesion on paperboard substrates. Aqueous dispersions demand specialized defoamers and rheology modifiers to prevent crawling or pinholing over hydrophobic base sheets during gravure application. Converter operations monitor surface tension dynes using calibrated inks to verify that corona treatment levels provide adequate wetting without destroying underlying sizing chemistries.
Polymer crystallization kinetics determine how effectively a barrier coating maintains its non-polar character after thermal drying cylinders evaporate residual water from the web.
