Plastic-viscoelastic fluid

Now we turn to rubber compounds and specifically to plastic viscous and plastic viscoelastic fluids. 

For a plastic viscoelastic fluid, the dimensionless groups of Eq. (102) should be replaced by... 

Suetsugu, Y. and White, J.L. (1984) A theory of thixotropic plastic viscoelastic fluids with a time-dependent yield surface and its comparison to transient and steady state experiments on small particle filled polymer melts, /. Non-Newtonian Fluid Mech., 14,121-40. 

In 1979 White [49] proposed a three-dimensional theory of a plastic-viscoelastic fluid intended to represent the behavior of small particle-filled compoimds. This was also based on the von Mises stress criterion but contained memory and predicted shear flow normal stresses. This was later generalized to include thixotropy [50, 51]. Leonov [52] developed an alternative three-dimensional tensor theory of this behavior. 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Many of the new plastics, blends, and material systems require special, enhanced processing features or techniques to be successfully injection molded. The associated materials evolution has resulted in new plastics or grades, many of which are more viscoelastic. That is, they exhibit greater melt elasticity. The advanced molding technology has started to address the coupling of viscoelastic material responses with the process parameters. This requires an understanding of plastics as viscoelastic fluids, rather than as purely viscous liquids, as is commonly held... 

In the case of solids it is evident that deformation is either linear elastic - like a Hookean solid (most solids including steel and rubber) - or non-linear elastic or viscoelastic. In the case of liquids, fluids differ between those without yield stress and those with yield stress (so-called plastic materials). Fluids without yield stress will flow if subjected to even slight shear stresses, while fluids with yield stress start to flow only above a material-specific shear stress which is indicated by o0. 

Both polymeric and some biological reactors often contain non-Newtonian liquids in which viscosity is a function of shear rate. Basically, three types of non-Newtonian liquids are encountered power-law fluids, which consist of pseudoplastic and dilatant fluids viscoplastic (Bingham plastic) fluids and viscoelastic fluids with time-dependent viscosity. Viscoelastic fluids are encountered in bread dough and fluids containing long-chain polymers such as polyamide and polyacrylonitrite that exhibit coelastic flow behavior. These... 

A key characteristic of plastic deformations is that they are irreversible. The difference between a viscoelastic fluid and a plastic material is the presence of a yield stress. The yield stress is the stress at which the deformation becomes irreversible and once the yield stress has been exceeded then the deformation is irreversible (Figs. 14 and 15). For example, brittle materials often behave elastically until the yield point has been reached once this point has been exceeded, the material will irreversibly deform or fracture like a piece of chalk (Fig. 15A). The key feature of a brittle material is that there is little deformation after the yield point. In contrast to a brittle material are a ductile materials (Fig. 15B) ductile materials undergo a lot of deformation after the yield point. 

Very low density PE (VLDPE), 52 Vinizene BP 5-5, 449 Vinyl alkoxysilanes, 172 Vinyl silanes, 172 Vinyl trimethoxysilane grafted polyethylene, 172 Vinyltriethoxy-silane, 194 Virgin plastics, 51 Viscoelastic behavior, 225 Viscoelastic fluid, 622 Viscoelastic materials, 631 Viscosity of polyethylene hot melts, 633 Viscosity of polymers, 620 Viscosity of water, 620 Viscosity, 622... 

The plastic at its forming temperature can behave as an elastic solid, a viscoelastic fluid, or a combination of the two. Modeling of thermoforming has been done using all of these models. On a molecular scale, things are just as complex. For an amorphous material, such as polystyrene or PMMA, the forming temperature determines the chain mobility and the ease of flow. For semicrystalline materials, such... 

As with viscoelastic fluids in Eq. (100) and plastic viscous fluids in Eq. (106), dynamic similarity involves systems with the same UIL. Both viscoelastic and plastic flow phenomena are then associated with dimensionless groups involving UIL. 

If the shear rates are constants, the non-Newtonian fluids can also be classified according to their viscosity dependence on time. This classification has been widely applied to describe the rheological characteristics of coatings. For the development of deformation, the time evolution corresponds to the effect of the increase of shear rate. Three typical cases occur with the time evolution the thixotropic fluids exhibit the decrease of viscosity, corresponding to pseudo-plastic fluids the rheopectic fluids exhibit the increase of viscosity, corresponding to dilatant fluids while the viscoelastic fluids exhibit partial recovery of the deformation of pseudo-plastic fluids after the removal of the stress. Since polymers can perform a large scale of elastic deformation, this character appears extremely significant. 

As the use of non-Newtonian fluids in industry (such as in plastics and synthetic fibre manufacture, in polymer processing, in enhanced oil recovery, in biochemistry and biotechnology, and in petrochemicals) is increasing, the interest and research in filtration of such fluids are also growing. Such research is closely linked to further work on the fundamentals of flow through packed beds for non-Newtonian fluids, such as, for example, the recent work of Machac and co-workers on purely viscous and viscoelastic fluids. In future editions of this book or any other in this subject I expect more prominence given to the filtration of non-Newtonian liquids. 

Creep is related to plastics viscoelastic behavior and can be explained with the aid of a Maxwell model such as that shown in Figure 3-55 [12, 287]. When a load is applied to the system, shown diagrammatically, the spring will deform to a certain degree. The dashpot will first remain stationary under the applied load, but if the same load continues to be applied, the viscous fluid in the dashpot will slowly leak past the piston, causing the dashpot to move. Its movement corresponds to the strain or deformation of the plastic material. 

Solomon, J., A. W. Nienow, and G. W. Pace (1981b). Flow patterns in aerated plastic and pseudoplastic viscoelastic fluids, in Fluid Mixing I, Inst. Chem. Eng. Symp. Ser., 64, A1-A13. 

Plastic material processed in the extrusion is regularly regarded as a kind of incompressible viscous or viscoelastic fluid. According to the theory of... 

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