Meaning
Flow properties represent the range of shear rates where the viscosity of a polymer melt remains constant regardless of the flow speed. The Newtonian plateau is found at very low shear rates where the polymer chains have enough time to relax and return to their equilibrium state. This value provides a baseline for the zero shear viscosity which is a direct indicator of the average molecular weight of the resin.
Shear Stability
Molecular alignment occurs only when the force applied to the melt exceeds the internal forces holding the chains in a random coil. Within the Newtonian plateau, the internal structure of the polymer remains undisturbed and the flow is simple and predictable. As the shear rate increases beyond this point, the chains begin to orient themselves in the direction of the flow and the viscosity starts to drop.
Relaxation Time
Internal dynamics of the polymer melt determine the width of this stable viscosity region. A material with a long relaxation time will have a narrow Newtonian plateau because the chains are easily disturbed by even low levels of shear. This behaviour is typical of high molecular weight polymers or those with a high degree of long chain branching.
Polymer Structure
Design of the molecular architecture influences how the melt behaves at the limit of zero shear. A resin with a broad molecular weight distribution will show a less defined Newtonian plateau than a resin with a narrow distribution. This happens because the different chain lengths respond to shear at different rates, causing the transition to shear thinning to be more gradual.
By understanding where the flow becomes non-Newtonian, a technician can set the processing parameters to avoid excessive pressure build up during the start of the cycle.
