Meaning
Interfacial instability phenomena in multiphase polymer blends drive dispersed melt droplets to collide and join into larger domains during quiescent or low-shear thermal processing. Driven by the thermodynamic minimization of interfacial surface energy, domain coallescence increases the average diameter of dispersed rubber or minor polymer phases when molten compounds traverse hot runners, transfer lines, transfer valves or stagnant barrel zones. This morphological coarsening reduces interfacial contact area between immiscible phases, undoing the fine sub-micron dispersion generated during twin-screw compounding.
The mechanism terminates when solid crystallisation freezes polymer chains, when shear stress exceeds droplet break-up thresholds, or when interfacial saturation by block copolymers provides steric hindrance against droplet film drainage.
Interfacial Mobility
Film drainage kinetics between approaching liquid droplets control the rate at which phase structures coarsen. During quiescent processing stages, Brownian motion and gentle shear fields press adjacent dispersed domains together, thinning the continuous polymer film separating them. Uncompatibilized polymer blends experience rapid domain coallescence because bare phase interfaces lack the steric resistance required to prevent film rupture.
Adding block or graft copolymers slows this interfacial mobility, as anchored copolymer segments generate repulsive Marangoni stresses that stabilize droplet boundaries. Extended residence times inside injection moulding barrels accelerate drainage kinetics, transforming desirable sub-micron morphology into coarse droplet clusters that undermine finished part performance.
Mechanical Degradation
Toughness properties in rubber-toughened polymers depend directly on maintaining a sub-micron dispersed phase architecture. Excessive domain coallescence enlarges elastomeric particles beyond their optimal cavitation diameter, diminishing the compound notched Izod impact strength and low-temperature ductility. Moulded articles exhibit irregular stress whitening and brittle failure paths when phase coarsening occurs within the runner system.
Processing Mitigation
Moulding process parameters directly influence the thermal and kinetic conditions that permit droplet growth. Restricting barrel residence times and lowering melt temperatures minimize domain coallescence before the melt solidifies in the cavity. Faster injection speeds reintroduce elongational stresses that stretch and break coarse domains before freezing occurs.
Controlling thermal exposure throughout conversion stages preserves compound phase morphology, guaranteeing that mechanical performance matches raw resin datasheet specifications.
