Greaseproof and Barrier Stocks for Fills That Defeat Plain Kraft
High-density refining and fluorine-free dispersion coatings block lipid migration and moisture breakdown where unrefined kraft papers fail under aggressive fills.
Shear-Thinning Coat Weight
Fluid Rheology
Aqueous coating formulations display reduced viscosity under high speed blade application when the physical structure of the polymer binder temporarily aligns with the direction of the mechanical force. Shear-thinning coat weight denotes the final mass of dry solids per square metre left on a substrate surface after the fluid experiences this specific non-Newtonian reduction in viscosity during the coating station transit. This parameter governs the coverage uniformity and the functional barrier properties of finished packaging materials by dictating how much material stays on the web versus how much returns to the reservoir.
Coatings that lose viscosity too aggressively during high speed transfer leave thinner films than intended, while formulations that fail to thin properly cause streaks or coat weight variability across the width of the sheet.
Application Dynamics
Precise control of these weight variations dictates the operational efficiency of the drying equipment located immediately after the application head. Excessively thin regions create patches of raw paper surface where the barrier film fails to cover the fibre matrix, allowing moisture or oil penetration through the final package. Conversely, heavy deposits slow the evaporation rate because the additional water volume requires more heat energy to drive off, forcing a reduction in machine speed to maintain the required final moisture content.
Engineers calibrate the blade pressure and the nozzle orifice angle to ensure the material distributes evenly across the entire surface area. Operators monitor the coat weight during continuous production runs because small fluctuations in the temperature of the circulating coating broth change the viscosity response even when the shear force remains constant.
Performance Constraints
Variations in raw fibre porosity influence how much coating liquid drains into the sheet before the drying zone locks the structure in place. Higher absorbency substrates demand a compensatory increase in the applied wet layer to hit the target dry coat weight, yet this increases the risk of excessive penetration that degrades the internal bond strength of the paper. This coating property remains the limiting factor for high speed converting lines where standardisation of the surface finish governs the print quality.
Consistent material deposition allows for thinner coatings without compromising the integrity of the barrier layer.