Meaning
Thermodynamic transitions in polymer blends or solutions can be triggered by rapid changes in the applied force. Pressure induced phase separation occurs when a previously homogeneous mixture splits into two or more distinct phases due to a shift in pressure. This phenomenon is often used to create porous membranes or to control the morphology of multi-component materials.
It is distinct from separation caused by temperature changes, although the two effects are often coupled.
Morphology Control
Size and distribution of the resulting phases are determined by the rate at which the pressure is released. During pressure induced phase separation, the polymer can form a bicontinuous network or a discrete droplet structure. This allows for the engineering of materials with specific filtration properties or mechanical strengths.
The process is highly repeatable, making it suitable for high-volume industrial manufacturing of specialized films.
Density Gradient
Variations in the local pressure within a moulding cavity can lead to uneven separation of the blend components. This results in a part with a density gradient where the surface has a different composition than the core. While sometimes a defect, this effect can be used to create parts with a hard skin and a tough, flexible interior.
Controlling the pack pressure in the injection cycle is the primary way to manage this outcome.
Processing Pressure
Selecting the correct operating window is essential to avoid unintended demixing of a polymer system. If the pressure induced phase separation occurs at the wrong time, it can lead to poor mechanical properties or surface imperfections. Moulders must understand the pressure sensitivity of their resin blends to ensure a stable process.
High-pressure injection moulding requires careful monitoring to prevent these phase transitions from occurring during the filling stage. Pressure sensitivity is a critical factor in the stabilization of advanced polymer blends.
