Thermoplastic polymers molten state rheology

Rheological Properties Thermoplastics are processed and shaped in the molten state. This can be loosely defined as a state in which a polymer flows under the action of heat and pressure. Molten polymers are non-Newtonian fluids, as opposed to the simpler Newtonian fluids. In the latter, the stress a (force per unit area) is proportional to the shear rate y (velocity per unit length) with a proportionality factor [I (viscosity) which is constant at a given temperature. Newtonian fluids follow the law... 

Rheology is the science of deformation and flow of matter. Essentially, all thermoplastic resins (and many thermosetting resins) are required to undergo flow in the molten state during the course of product manufacture. Important fabrication processes such as injection, extrusion, and calendering all involve the flow of molten polymers. In plastics fabrication, it is important to understand the effect, on melt viscosity, of such factors as temperature, pressure, rate of shear, molecular weight, and structure. It is also equally important to have reliable means of measuring viscous properties of materials. 

The analysis indicates that presently quite adequate phenomenological models are available for description of the straining of commercial (polydisperse) polymers in the liquid state. A comparatively clear understanding of the mechanics of the processes of manufacturing of sleeve-type and flat films of molten thermoplastics also has been developed. So far, physical approaches have provided rheological models only for monodisperse polymers (the properties of which differ significantly from those of the ones used in industry). 

---