Mechanical Refining Impact on Multi Ply Boxboard Z Direction Tensile Development
Controlled low energy refining of middle ply mechanical pulp yields superior Z direction tensile development without caliper loss in multi ply boxboard.
Moisture Blistering
Coating Failure
Rapid thermal expansion of entrapped liquid creates localized separation between a paper substrate and its laminated film or varnish layer. Moisture blistering occurs when water vapor pressure exceeds the internal bond strength of the material surface during high temperature processing or drying cycles. This phenomenon manifests as small circular dome formations that disrupt the surface finish and compromise the integrity of the protective barrier.
The defect often arises when residual solvent or ambient humidity trapped within the fibre network undergoes a phase change into steam under the influence of heat from infrared dryers or heated rollers. High density boards with low porosity exhibit higher susceptibility because the rigid surface prevents the release of gaseous buildup.
Substrate Dynamics
Proper conditioning of base materials before the application of finishing layers minimizes the risk of internal expansion. Paper stocks absorb atmospheric humidity according to the ambient conditions of the warehouse or press room. Equilibrium moisture content represents the state where the substrate neither gains nor loses water to the surrounding air.
When a mill supplies paper at a level far from the target production environment, the fibres undergo significant stress upon entering the converting line. Correct acclimation allows the sheet structure to adjust before the application of adhesives or coatings seals the surface. High humidity environments force the inclusion of pre-drying stages to remove excess water molecules from the cellulose matrix before the final sealing occurs.
Thermal Tolerance
Converters must calibrate drying systems to account for the evaporation rate of the specific chemical additives used in the top coat. Excessive heat flux causes immediate vaporization of liquid components before the coating develops sufficient cross-linked structure to contain the pressure. This physical breach results in permanent damage to the visual appearance of the printed sheet.
Maintaining the temperature below the threshold of water phase change inside the substrate remains the most effective control method. Careful adjustment of belt speed or cooling air flow allows the material to reach a finished state without forcing trapped moisture to expand through the surface layer.