Rheological Determination of Ultra High Molecular Weight Tails in Bimodal Resins
Low-frequency storage modulus and creep recovery accurately quantify ultra-high molecular weight tails in bimodal resins, ensuring stress crack resistance.
Relaxation Time Spectrum
Meaning
Distribution of the various timescales at which polymer chains return to their equilibrium state characterizes the molecular complexity of a resin. The relaxation time spectrum is calculated from dynamic mechanical data and provides a detailed view of the material than a single average relaxation time. This information is used to predict how a polymer will behave under the rapid deformations of injection moulding or the slow stresses of gravity-induced sag.
Molecular Structure
Chains of different lengths and architectures respond to stress at different rates. In a relaxation time spectrum, the presence of long chains or branches appears as a shift toward longer times on the horizontal axis.
Flow Characterization
Engineers use this distribution to model the non-linear flow behavior of the melt during processing. By understanding the relaxation time spectrum, the lab can predict when a material will exhibit shear thinning or when it will experience melt fracture at a die exit. This predictive capability is used to optimize the screw design and die geometry for specific resin grades.
Tooling Design
Mould dimensions must account for the fact that different parts of the polymer distribution relax at different rates. Residual stresses in a finished part are often the result of the longest relaxation times in the relaxation time spectrum not being fully satisfied before the material solidifies. Controlling the cooling rate based on this data ensures that parts remain dimensionally stable and free from internal cracks.