Fracture Classification
Internal rupture within the adhesive layer during mechanical separation indicates that the adhesive itself is the weakest component of the bonded structure. A cohesive failure mode refers to a split that occurs entirely inside the bulk of the bonding agent, leaving a visible coating on both the substrate surfaces. This behavior differs from adhesive failure, which takes place cleanly at the boundary between the adhesive and the paperboard surface.
Analyzing the split provides valuable information regarding the internal polymer strength and curing level of the laminating resin.
Diagnostic Significance
Observing a cohesive failure mode in a rejected batch of laminated board points toward an incomplete chemical reaction or incorrect mix ratios of the adhesive. When a two-part polyurethane is not fully cured, its under-developed molecular network cannot withstand shear or peel forces. This failure can also arise from excessive plasticization or solvent retention during the drying phase of the conversion line.
Adjusting the oven temperatures or curing times helps to rebuild the internal strength of the polymer matrix.
Bond Optimization
Modifying the coating weight or changing the adhesive formula can shift the rupture behavior toward substrate tearing. Higher coat weights can sometimes exacerbate the issue if drying is incomplete, whereas thin, evenly distributed films maximize the efficiency of the cure.
