Low Frequency Rheological Testing for Hydrolyzed Fraction Identification in Resins
Low frequency oscillatory rheology unmask hydrolyzed resin fractions by measuring zero-shear viscosity drop and storage modulus slope collapse near zero frequency.
Phase Angle Shift
Meaning
The change in the angular delay between an applied sinusoidal stress and the resulting strain in a viscoelastic material represents a core parameter in polymer melt rheology. This phenomenon is known as the phase angle shift and it shows the balance between elastic and viscous behavior in the resin. In a purely elastic solid, the stress and strain are in phase, whereas a purely viscous liquid has a shift of ninety degrees.
Polymer melts exhibit a shift between these two extremes, depending on their molecular structure and temperature.
Viscoelastic Behavior
Temperature and frequency influence the transition of a polymer from elastic to viscous behavior. At low temperatures or high frequencies, the phase angle shift is small, indicating that the polymer behaves like a solid. As the temperature rises, the shift increases as the viscous flow behavior begins to dominate.
This transition is essential for understanding how the polymer will flow through the gates and runners during the injection phase of moulding.
Processing Implication
Processors use this parameter to predict how a material will respond to the high shear rates encountered in the gate. A smaller phase angle shift indicates high melt elasticity, which can lead to flow defects like melt fracture or surface sharkskin during extrusion or injection. Conversely, a larger shift means the material is more viscous, which improves surface finish but may increase the risk of flash or jetting if the melt viscosity is too low.
Rheological Measurement
Dynamic mechanical analysis is used to measure this parameter across a range of temperatures and frequencies. By plotting the phase angle shift against frequency, the laboratory can determine the polydispersity and average molecular weight of the polymer. Polymer melts with a broad molecular weight distribution show a gradual increase in the shift over a wide frequency range.
This testing is necessary for comparing different resin batches to ensure they will perform consistently on the production line, especially when transitioning from virgin to recycled materials which often exhibit different viscoelastic profiles. In particular, the onset of melt instability can be identified by looking for sudden changes in the shift during high-strain tests.