Internal Rupture
A separation occurring within the bulk of an adhesive layer or the thickness of a substrate indicates that the material itself has reached its mechanical limit. This cohesive bond failure differs from adhesive failure because it leaves a visible layer of the bonding agent on both surfaces of the joint. In the context of paper packaging, it often appears as the delamination of the paper fibers themselves when the glue is stronger than the internal bond of the sheet.
Testing for this condition requires a pull test where the fracture path is observed under magnification.
Stress Distribution
The stress applied to a laminate must be distributed across the entire interface to avoid localized overloading. When cohesive bond failure occurs in the adhesive, it suggests that the polymer chains have not formed a sufficiently strong network or that the curing process was incomplete. High temperatures during the shipping of a carton can soften a hot melt adhesive, leading to this type of split under the weight of the contents.
Formulators adjust the wax content or the resin type to increase the internal toughness of the material.
Material Limitation
Selecting the correct grade of adhesive involves matching the internal strength of the glue to the expected load and the nature of the substrate. If a cohesive bond failure happens in a plastic film laminate, it might indicate that the ink layer between the film and the adhesive is the weakest point. This distinction helps technicians decide between increasing surface energy or changing the chemical grade of the adhesive.
