Coating Separation
Localized shedding of the pigment-binder matrix from the surface of coated paperboard occurs when adhesive bonds fail under mechanical tension. A flaking failure typically takes place during folding or high-speed printing, where the coating layer cracks and detaches in small, brittle particles. This separation leaves the underlying fibrous sheet exposed, which diminishes both the aesthetic appearance and the barrier properties of the packaging.
It represents a serious quality defect that can contaminate the processing equipment, leading to build-up on printing blankets and subsequent print defects on subsequent sheets. The root of this behavior lies in the poor interfacial adhesion between the clay coating and the base paperboard.
Root Cause
Insufficient binder content or excessive pigment loading in the coating formulation weakens the structural cohesion of the surface layer. During the drying process at the paper mill, rapid water evaporation can cause binder migration, leaving the surface layer starved of adhesive and highly fragile. When the sheet is subjected to the high tack of offset printing inks or the sharp bending angles of creasing, the brittle top layer fractures.
Maintaining the correct ratio of latex to kaolin clay prevents this loss of structural integrity.
Detection Protocol
Adhesive tape tests provide a rapid method for evaluating the surface bonding strength of coated sheets. Technicians apply a standardized tape to the surface and pull it away at a controlled angle to check for dry coating particles. If a flaking failure is observed during this test, the mill must adjust the binder ratio or drying profile to improve coating cohesion.
