Meaning
Polymer blends comprising polyethylene and polypropylene exhibit positive free energy of mixing and separate into distinct microscopic phases upon cooling. Found extensively in mixed post-consumer recycling streams, immiscible polyolefins lack thermodynamic compatibility and form weak mechanical interfaces between domain boundaries. Unmodified blends suffer from low elongation at break and phase delamination under shear.
Analysis governs binary and multi-component polyolefin mixtures lacking chemical compatibilizers.
Phase Separation
Thermodynamic driving forces push polyolefin molecules of differing structures to segregate during melt solidification. In uncompatibilized blend processing, immiscible polyolefins form spherical domains of the minor phase within the matrix phase. Coarsening of these domains during cooling weakens interfacial stress transfer.
Poor interfacial adhesion allows cracks to propagate along phase boundaries under mechanical shock.
Melt Rheology
Viscosity ratios between matrix and dispersed phases determine domain morphology during extrusion and injection moulding. Processing immiscible polyolefins under high shear elongates dispersed droplets into thread-like fibrillar structures. Unstable fibrils break up into non-uniform droplets when shear drops in the mould cavity.
Melt fracture and irregular surface textures appear when phase viscosities differ significantly.
Recyclate Upgrading
Blending post-consumer rigid packaging streams yields mixed polyolefin fractions with inconsistent performance. Converting immiscible polyolefins into structural materials requires block copolymer additions or reactive extrusion to tie polymer chains across phase boundaries.
