Quantifying Regrind Degradation through Gel Permeation Chromatography and Rheology
High-temperature GPC and oscillatory shear rheology reveal chain scission and elasticity losses in regrind that standard melt flow testing consistently misses.
Cross over Frequency
Meaning
Dynamic mechanical rheology identifies the angular frequency at which the storage shear modulus equals the loss shear modulus in a molten polymer. Evaluating cross over frequency provides direct insight into the average molecular weight of thermoplastic resins. The variable governs viscoelastic transitions during high-shear moulding and low-shear cooling stages.
It applies to molten viscoelastic polymer fluids and loses physical relevance once the resin solidifies below its glass transition or crystallization temperature.
Molecular Weight Sensitivity
Frequency sweeps performed on rotational rheometers map the elastic and viscous behavior of polymer melts across shear spectrums. A shift in cross over frequency toward lower frequency values indicates a higher average molecular weight in the tested resin batch. Moulders monitor this rheological shift to detect resin lot variation before charging material into injection hoppers.
Processing Window
Rheological crossover behaviour dictates how easily a polymer melt flows through runner systems into mould cavities. Resins exhibiting low cross over frequency demand higher injection pressures and higher barrel temperatures to fill thin wall sections. Incorrect temperature settings cause shear burning or incomplete cavity filling in fast cycle moulding processes.
Elastic Recovery
Melt elasticity near the crossover threshold influences post-die swell during profile and pipe extrusion. Extruders adjust die land length based on cross over frequency readings to control dimensional tolerances on extruded profiles. Unstable melt elasticity leads to severe wall thickness variation and warpage in cooled extrudates.