Droplet Coalescence

Physical Driver

Thermodynamic forces drive the system to minimize the total interfacial area between the polymer phases. During melt processing, droplet coalescence is promoted by molecular diffusion and flow-induced collisions of the dispersed droplets. The rate of this fusion increases at higher temperatures because the lowered matrix viscosity accelerates droplet mobility.

This mechanism undoes the fine dispersion achieved during compounding and alters the mechanical response. High residency times in the barrel exacerbate the growth. It is particularly active in stagnation zones of the mold runner system.

Molding Consequence

Molded parts with variable wall thicknesses experience uneven cooling and flow rates during the injection cycle. In thick sections where the cooling rate is slow, droplet coalescence has more time to occur, resulting in coarse morphology and reduced ductility. The thin sections freeze rapidly, preserving a fine dispersion.

This spatial variation creates anisotropic mechanical properties within the single moulded component.

Prevention Method

Addition of block copolymers or graft copolymers acting as compatibilizers significantly reduces the rate of phase coarsening. These additives locate at the interface, reducing interfacial tension and forming a physical barrier that prevents droplet coalescence when droplets collide. Stable blends retain their fine dispersion through multiple remolding steps.

This stability permits the use of recycled regrind without compromising quality.