Meaning
Interfacial tension describes the physical boundary force existing between two immiscible phases, such as molten polymer blends or a liquid polymer and a gas during processing. Thermodynamics governs this phenomenon by defining the excess free energy present at the boundary area. Extrusion and injection moulding operations rely on this metric to control droplet breakup and morphology development in polymer blends.
Boundary forces cease to dictate phase behaviour once the system reaches thermodynamic equilibrium through compatibilisation.
Phase Morphology
Immiscible polymer blends depend directly on interfacial tension to determine droplet size distribution and domain dispersion during compounding. High boundary forces prevent fine dispersion unless shear stresses from the screw overcome the resistance. Co-continuous structures form when phase ratios balance the thermodynamic forces operating at the boundary.
Poor control over this property leads to phase separation and weak mechanical performance in the finished part.
Resin Economics
Virgin polymer blends require specific additive packages to reduce interfacial tension and lower the energy barrier for uniform mixing. Regrind materials often exhibit altered surface energies due to thermal degradation during prior processing cycles, which shifts the required processing window. Compounders balance additive costs against the risk of delamination in multi-layer film coextrusion.
Virgin resin specifications assume stable boundary interactions that lower-grade recycled streams rarely deliver without chemical modification.
Moulding Defects
Surface delamination and weld line weakness appear when interfacial tension prevents proper molecular entanglement across adjacent flow fronts during mould filling. Datasheet values measured under laboratory conditions rarely match the dynamic values experienced inside high-speed injection tools. Moulders adjust melt temperatures to lower the boundary resistance and eliminate peeling defects in engineering thermoplastic components.
High shear rates inside the gate alter local boundary conditions and cause streaks on the moulded part surface.
