Plastic Barrier
A thermoplastic polymer is used as a moisture barrier on paper and board packaging materials. In liquid packaging boards and paper cups, low density polyethylene provides the heat-sealable and liquid-resistant layer that prevents the paper fibers from absorbing water. This synthetic layer is typically applied as a thin coating on one or both sides of the board.
Extrusion Coating
The coating process involves melting the plastic resin and extruding it through a slot die directly onto the moving paperboard web. To ensure good adhesion, the low density polyethylene must be applied at a high temperature and pressed onto the board by a cooling roll. This synthetic barrier prevents the liquid product from weakening the carton structure, while also providing a surface that can be heat sealed to form a leak-proof seam.
This combination of properties makes it the most common polymer used in beverage carton manufacturing.
Recyclability Challenge
While this plastic layer is highly effective for containment, it complicates the recycling process of the used packaging. Because low density polyethylene does not dissolve in water, it must be separated from the paper fibers in specialized pulpers during recycling. This separation requires mechanical agitation, which can reduce the recovery yield of the high quality fibers.
Many mills now use enzymes or alternative dispersion techniques to improve the efficiency of this process and recover more clean fiber.