Viscoelastic Shear Thresholds for Polymer Matrix Degradation under High Contact Pressures
Exceeding polymer viscoelastic shear thresholds under high nip pressure causes molecular chain scission, reducing adhesion and triggering score cracking.
Chain Scission
Molecular Cleavage
Polymer degradation occurs when high energy radiation or thermal stress breaks the covalent bonds along a long chain molecule. Chain scission represents the primary mechanism for reducing the average molar mass of a substrate during these reactions. The process severs the backbone of a polymer string, which leads to a direct loss of physical strength and a shift in the viscosity of the material.
This event limits the lifespan of cellulose stocks and synthetic films exposed to ultraviolet light or heat during long term storage.
Structural Impact
Mechanical properties of paper and board depend heavily upon the length of the internal fibre chains. A high degree of chain scission generates shorter segments that no longer bridge across the crystalline regions of the sheet. Such shortening reduces the tensile strength and the burst resistance of the finished packaging material.
Converting lines detect this degradation as an increase in dust or brittle edges during high speed folding and die cutting operations. Coating performance also suffers because the fragmented polymer chains alter the rheology of adhesives and barrier layers.
Measurement Technique
Quantitative analysis of the degradation usually relies on viscosity testing of a dissolved sample. Chemists measure the intrinsic viscosity of the material to derive an estimation of the molecular weight distribution. A decrease in viscosity relative to the virgin material marks the progress of the bond breakage.
Gel permeation chromatography offers a more detailed view by separating the molecules by their hydrodynamic volume as they elute from a column. These analytical methods provide the data required to predict the remaining service life of a substrate stored under extreme environmental conditions.