Rheology internal stresses

As is known, a highly elastic deformation is accompanied by redistribution of internal stresses and is always associated with viscous deformation. Because of this, for full deformation of a viscous polymer we will have a rheological model (Figure 2.42c), which is governed by the equation ... 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. Critical velocity of most fluoropolymers is usually much lower than most thermoplastics. Parts molded in a process where critical velocity is exceeded exhibit typical symptoms of melt fracture that is, they may have a frosty or cloudy surface. In some cases, a part may have a smooth and shiny surface but is internally fractured. 

Malik TM, Carreau PJ, Qiapleau N (1989) Qiaracterization of liquid crystalline polyester polycarbonate blends. Polym Eng Sci 29(9) 600-608 Manson JAE, Seferis JC (1992) Process simulated laminate (PSL) a methodology to internal stress characterization in advanced composite materials. J Compos Mater 26(3) 405 31 Meng YZ, Tjong SC, Hay AS (1998) Morphology, rheological and thermal properties of the melt blends of poly (phthalazinone ether ketone sulfone) with liquid crystalline copolyester. Polymer 39(10) 1845-1850... 

Foams have a wide variety of appHcations that exploit their different physical properties. The low density, or high volume fraction of gas, enable foams to float on top of other fluids and to fiU large volumes with relatively Httle fluid material. These features are of particular importance in their use for fire fighting. The very high internal surface area of foams makes them useful in many separation processes. The unique rheology of foams also results in a wide variety of uses, as a foam can behave as a soHd, while stiH being able to flow once its yield stress is exceeded. 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

VISCOSITY. The internal resistance to flow exhibited by a fluid the ratio of shearing stress to rate of shear. A liquid has a viscosity of one poise if a force of 1 dyne/sqnare centimeter causes two parallel liquid surfaces one square centimeter in area and one centimeter apart to move past one another at a velocity of 1 cm/second. One poise equals 100 centipoises divided by the liquid density at the same temperature gives kinematic viscosity in centistokes (cs). One hundred centistokes equal on e stoke. To determine kinematic viscosity, the time is measured tor an exact quantity of liquid to flow by gravity ilirough a standard capillary. See also Rheology. 

In Eqs. (6) and (7) e represents the internal energy per unit mas, q the heat flux vector due to molecular transport, Sh the volumetric heat production rate, ta, the mass fraction of species i, Ji the mass flux vector of species i due to molecular transport, and 5, the net production rate of species i per unit volume. In many chemical engineering applications the viscous dissipation term (—t Vm) appearing in Eq. (6) can safely be neglected. For closure of the above set of equations, an equation of state for the density p and constitutive equations for the viscous stress tensor r, the heat flux vector q, and the mass flux vector 7, are required. In the absence of detailed knowledge on the true rheology of the fluid, Newtonian behavior is often assumed. Thus, for t the following expression is used ... 

From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

Murayama, S., and Shibata, T. 1964. Flow and stress relaxation of days (Theoretical studies on the rheological properties of day—part 1). Rheology and Soil Mechanics. Symposium of the International Union of Theoretical and Applied Mechanics. Grenoble, France. 

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