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.
Dynamic Shear Rheology
Meaning
Viscoelastic characterisation evaluates polymer melt response under oscillatory sinusoidal deformation across varying temperatures and frequencies. Industrial laboratories perform dynamic shear rheology to determine the storage modulus and loss modulus of virgin resins and regrind blends. The technique reveals molecular weight distribution and long-chain branching in multi-component polymer systems.
It governs melt strength during extrusion and cavity filling behavior during injection moulding, ending at shear rates where polymer slip or elastic melt fracture disrupts linear viscoelastic strain.
Viscoelastic Response
Oscillatory shear testing subjects molten resin to controlled strain waves between parallel plates or cone-plate geometries. The phase angle between applied stress and measured strain separates elastic energy storage from viscous energy dissipation. Molders rely on these parameters to predict die swell and structural sag during profile extrusion.
Thermal Stability
Extended dwell times at processing temperatures degrade heat-sensitive resins like polyvinyl chloride or post-consumer polyolefins. Time-sweep oscillatory testing detects viscosity decay or gel formation over specified time frames. Processors use thermal stability limits to establish safe barrel residence times.
Frequency Sweep
Variable frequency testing simulates the wide shear rate spectrum experienced inside injection nozzles and runner channels. Low frequency measurements yield zero-shear viscosity related to average molecular weight. High frequency measurements indicate shear thinning behavior critical for fast cavity filling.