Polymer Film Micro Fracture Mechanisms under High Strain Shear in Folding Boxboard Creases
High strain shear during boxboard creasing causes polymer film micro fractures when local elongation exceeds film yield capacity along the outer bend radius.
Barrier Integrity
Functional Metric
Polymeric coatings and inorganic films applied to paperboard substrates must form a continuous layer to prevent liquid, grease, or gas penetration. Maintaining barrier integrity ensures that paper-based packaging retains structural rigidity and protects sensitive products from atmospheric exposure or chemical migration. Functional coatings, including aqueous dispersions and extruded polyolefins, must resist pinhole formation and micro-cracking during converting operations.
Structural Evaluation
Laboratory testing measures continuity across flat sheets and folded packaging edges under stress. Evaluating barrier integrity involves grease resistance testing via ASTM F119, water vapor transmission rate measurements, and dye penetration assays to locate micro-voids in the polymer layer. Creasing, scoring, and deep drawing introduce localized mechanical stresses that fracture brittle barrier materials, creating pathways for moisture or mineral oil migration.
Converting parameters, such as crease depth and sealing temperature, must remain within narrow operational windows to avoid damaging fragile functional coatings during high-speed carton forming. Microscopic inspection reveals whether coating defects originate from poor wet-out during aqueous application or mechanical strain during box folding.
Performance Limit
Complete barrier performance degrades when package integrity encounters mechanical damage, severe relative humidity, or elevated storage temperatures. Chemical exposure to aggressive fats or essential oils can soften functional polymers, accelerating permeation across the substrate over extended shelf lives.