Evaluating Pulp Yield Losses from Extrusion Coated Board in Industrial Hydro-Pulpers
Polyethylene-coated board yields drop 8 to 22 percent in industrial pulpers as unseparated fiber bundles exit screens bound to rejected plastic film.
Polyethylene Film
Plastic Membrane
Thin layers of thermoplastic resin applied to paperboard create a flexible and waterproof barrier suitable for food and industrial packaging. This polyethylene film is the most common material used for liquid board because it provides an effective seal against water and grease. It is typically applied through an extrusion process where the heated polymer is stretched and pressed onto the paper surface.
The resulting composite maintains the rigidity of the board while gaining the protective properties of the plastic.
Bonding Requirement
Bonding potential of the base sheet must be high enough to allow the molten polyethylene film to wet the surface and form a mechanical bond. Converting mills often use corona discharge or flame treatment to activate the paper surface before the plastic meets the web. If the bond is weak, the film can peel away during the folding of a carton or the heat sealing of a bag.
Consistent thickness across the width of the roll is necessary to ensure that the material behaves predictably on high speed packaging lines. Maintaining the correct melt temperature prevents the formation of pinholes that would compromise the oxygen barrier of the finished container.
Barrier Performance
Resistance to moisture vapor transmission is the primary technical metric used to evaluate this coating. A thicker polyethylene film provides a more robust barrier but also increases the difficulty of recycling the finished product. In most pulping systems, the plastic must be separated from the fibers through intense mechanical agitation and screening.
The recovered plastic is then usually diverted to energy recovery or low grade plastic manufacturing.