Physical Mechanism
Fluid transport through the narrow exposed cross-section of a paperboard barrier board determines the resistance of a sealed package to liquid penetration. When paper is cut, the internal fibre network is exposed directly to the environment, allowing liquids to enter through capillary action. This phenomenon, known as edge permeability, is highly dependent on the degree of sizing in the board and the orientation of the wood fibres.
Machine direction alignment often leads to faster fluid migration than cross-machine direction alignment.
Measurement Standard
Laboratory testing of this property involves sealing the flat faces of a paperboard strip with water-resistant adhesive or metal plates to isolate the cut edges. The sample is then submerged in a liquid of specified temperature and pressure for a set duration. The mass of the liquid absorbed through the cut borders is determined by weighing the specimen before and after exposure.
This technique provides a precise value in grams per square metre of edge area, which helps converters select the correct board grades for liquid packaging.
Barrier Application
Reducing liquid penetration at the margins is essential for dairy cartons and beverage containers. High edge permeability can cause the package walls to soften and bulge, resulting in structural failures on the retail shelf. Manufacturers utilize edge-wicking inhibitors and edge-singeing techniques to prevent the liquid from contacting the exposed fibres.
