Non-Linear Melt Viscosity Calibration in Polymer Processing Operations
Non-linear viscosity calibration requires multi-point shear rheometry and Cross model fitting to prevent extreme die sizing and molding pressure errors.
Relaxation Time Constant
Meaning
Characteristic timescale measuring the rate at which a viscoelastic material releases internal stresses after a sudden deformation has ceased governs the transition between solid-like and liquid-like behaviour. The relaxation time constant defines the memory of the material, showing how long polymer chains remain oriented under flow. In highly elastic materials, this value is large, meaning the fluid retains its deformed structure for longer.
Flow Dynamics
Processing polymer melts requires balancing the deformation rate with this relaxation time constant to prevent structural defects in the final product. If the processing timescale is shorter than this constant, the material behaves like an elastic solid, leading to melt fracture, surface distortion, or internal stresses. When the processing timescale is much longer, the polymer behaves like a viscous liquid, allowing the molecules to relax and flow smoothly.
Experimental Measurement
Determining this constant involves using dynamic mechanical analysis or stress relaxation tests where a step strain is applied and the decay of stress is monitored over time. This decay is usually represented by a Maxwell model or a multi-mode spectrum of relaxation times.
Polymer Architecture
Highly branched polymer chains increase this relaxation time because the branches restrict the movement of the backbone. Such branch points prevent the long backbones from slipping past one another efficiently during elongation.