Residual Quantity
Trace quantities of volatile organic solvents remaining in printed paperboard or flexible packaging layers after drying operations indicate residual process chemicals. Converting plants track solvent retention to ensure packaging materials comply with sensory odor and food safety regulations. Testing follows standardized protocols detailed in EN 13628 using static headspace gas chromatography.
The concept ceases to apply when converting operations use solvent-free extrusion coatings or water-based inks containing zero volatile organic solvents.
Evaporation Process
During flexographic or gravure printing, liquid inks dissolved in organic solvents such as ethyl acetate or ethanol apply to paperboard webs. Hot air drying tunnels evaporate the carrier solvent, but complete removal is hindered by polymer binder trapping and substrate pore absorption. Residual solvents migrate into paperboard fibres and synthetic barrier coatings, remaining trapped inside rolled or stacked sheets.
High press speeds, insufficient drying tunnel air volume, or low web drying temperatures increase residual solvent amounts. Retained solvents cause web blocking in rolls and off-odor contamination in packaged food products. Regular laboratory testing verifies drying tunnel efficiency and confirms compliance with brand owner residual solvent limits.
Safety Threshold
High absorbency un-coated recycled paperboards retain higher solvent fractions than dense solid bleached sulphate boards under identical drying settings. Barrier coatings with low gas permeability slow down solvent release during post-print curing periods.