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.
Strain Sweep Limit
Meaning
The maximum amplitude of oscillation that a polymer melt can withstand before its internal structure begins to break down defines the boundary of linear viscoelastic behavior. This value is the strain sweep limit and it marks the transition to non-linear response. Measuring this limit is a standard step in rheological characterization to ensure that subsequent tests are run without damaging the sample.
It is a critical parameter for understanding how a polymer behaves under the high-stress conditions found in industrial tooling.
Dynamic Testing
During a dynamic strain sweep, the sample is subjected to increasing strain amplitudes at a constant frequency. Below the strain sweep limit, the storage and loss moduli remain constant and independent of the strain. Once this limit is exceeded, the storage modulus begins to drop, indicating that the polymer network or entanglements are being disrupted.
This test is necessary for defining the safe operating window for all subsequent dynamic rheological measurements.
Molecular Structure
Materials with high fillers or strong intermolecular networks show a very low threshold. The strain sweep limit of a polymer composite is typically lower than that of the unfilled resin. This is because the filler-filler network is easily broken by small deformations.
In unfilled polymers, the limit is governed by chain entanglement and molecular weight. Highly entangled or long-chain branched polymers have a broader linear range than linear polymers.
Processing Correlate
Processors use this limit to understand how a polymer will behave in high-shear regions such as the screw and gate. In these zones, the strain rates are far beyond the strain sweep limit, meaning the material is in the non-linear regime where its viscosity decreases rapidly. This shear-thinning behavior is beneficial because it reduces the injection pressure needed to fill the mould.
However, if the transition is too abrupt, it can lead to localized shear heating and material degradation. Designers must balance the shear rates in the runner system to stay within the optimal flow region of the polymer. This analysis is especially important when using recycled resins, which often have a narrower processing window due to prior thermal cycles.