Meaning
The elongation and distortion of dispersed polymer phases during melt blending determine the final morphology of co-continuous and dispersed polymer blends. This physical phenomenon, known as droplet deformation, occurs when viscous forces overcome the interfacial tension holding the droplets in a spherical shape. The process is critical for establishing the mechanical properties of blended resins.
Understanding this behavior allows compounders to design tougher materials with consistent performance across varying shear environments.
Shear Response
Flow fields within the compounding extruder generate the mechanical stress needed to deform the droplets. When the capillary number of the mixture exceeds a critical value, the dispersed droplets stretch and eventually break into smaller particles. This action reduces the average domain size of the secondary polymer.
Morphology Control
Interfacial tension between the incompatible polymers opposes the deformation process. High interfacial tension keeps the droplets spherical, while lower tension facilitates extension and subsequent break up into a fine dispersion. Viscosity ratio between the dispersed phase and the matrix also dictates the efficiency of this phase change.
Processing Impact
Inadequate deformation leads to large and non-uniform domains in the final product. These large domains act as stress concentrators, reducing the impact strength of the injection moulded part. Careful control of extruder screw speed and temperature ensures the necessary shear rate to achieve uniform blending.
