Meaning
Spatial distribution and shape of distinct polymeric phases within a multi-phase polymer blend or copolymer. In rubber-toughened plastics such as acrylonitrile butadiene styrene, the domain morphology determines the ability of the material to absorb mechanical energy without catastrophic failure. This microstructure consists of elastomeric rubber particles dispersed throughout a rigid thermoplastic matrix.
The size and dispersion of these domains are established during the melt compounding step and must be preserved during subsequent injection moulding to achieve the expected impact performance.
Blend Performance
Mechanical toughness depends on the creation of a stable and finely dispersed rubber phase within the rigid matrix. When the rubber domains are too large or poorly distributed, they act as defect sites that initiate cracks rather than arresting them. In contrast, sub-micron domains can trigger localized shear yielding or crazing, which effectively dissipates the energy of an impact.
This relationship explains why datasheet impact values from virgin resins can be difficult to replicate when using regrind, as repeated processing can alter the size and stability of these elastomeric phases.
Flow Influence
High shear rates during injection moulding can deform and orient the dispersed domains in the direction of the melt flow. This deformation creates an anisotropic domain morphology, where the part exhibits different mechanical properties along the flow direction compared to the transverse direction. This anisotropy can cause the part to split easily under load along flow lines, which represents a severe defect in high-stress applications.
Control of melt temperature and gate design is necessary to minimize excessive shear and reduce the degree of domain orientation. This adjustment requires the use of mold-flow analysis tools to predict high-shear regions and optimize the gate locations before steel is cut. By keeping the shear rate below critical limits, moulders can ensure a more isotropic dispersion of the rubber phase, which translates to uniform toughness throughout the molded component.
Thermal Stability
Elevated processing temperatures can cause the dispersed domains to coalesce, coarsening the microstructure and reducing the performance of the blend. This coalescence is driven by thermodynamic forces that seek to minimize the interfacial area between the immiscible polymer phases. To prevent this deterioration, compounders often add compatibilizers that lower the interfacial tension and stabilize the domain structure against thermal degradation.
This stabilization ensures that the finished molded parts maintain their required mechanical integrity even when exposed to demanding thermal environments.
