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.
Mechanochemical Degradation
Polymer Scission
Chain cleavage describes the irreversible breaking of primary covalent bonds within high molecular weight cellulose and synthetic sizing agents during intense mechanical processing. Polymer scission reduces average degree of polymerization values inside kraft pulp refining and high shear coating preparation mixers. Excessive mechanical action forces macromolecular backbones beyond their elastic limits before hydrodynamic relaxation occurs.
Shear fields generated within twin screw extruders and ultrafine dispersion equipment break molecular chains through localized stress concentration. Cellulose degradation lowers tear resistance across paper webs because shorter polymer segments transfer tensile loads less efficiently through fiber networks.
Molecular Weight
Intrinsic viscosity measurements quantify molecular weight reduction by tracking hydrodynamic volume changes in cupriethylenediamine solutions. Molecular weight distributions shift toward shorter chains when refiners operate at high specific edge loads during secondary stock preparation. High molecular weight fractions govern wet web strength and ultimate folding endurance in specialty packaging substrates.
Shorter fragments increase solubility fractions within alkaline extraction tests and accelerate aging yellowing rates in bleached chemical pulps.
Stress Threshold
Critical energy release rates define the exact stress threshold where mechanical forces transition from elastic fiber deformation to destructive chain scission. Boundary conditions depend heavily on suspension consistency, rotor tip speed and local temperature profiles inside the refining zone. Operating below this specific energy input preserves fiber length distribution while achieving necessary surface fibrillation for bonding.
Thermal drying steps following mechanical action can exacerbate existing chain scission if dryer surface temperatures exceed degradation limits set for modified starches.