Rheological properties foaming

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of soHds, Hquids, and gases. Like an ordinary soHd, foams have a finite shear modulus and respond elastically to a small shear stress. However, if the appHed stress is increased beyond the yield stress, the foam flows like a viscous Hquid. In addition, because they contain a large volume fraction of gas, foams are quite compressible, like gases. Thus foams defy classification as soHd, Hquid, or vapor, and their mechanical response to external forces can be very complex. 

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. 

The numerous previous studies of the flow of foam in porous media and of its application for. improving the displacement of oil from such media, have almost always been conducted under ambient conditions of temperature and pressure there have been very few reports of laboratory studies under reservoir conditions. Although many interfacial properties are known to be temperature dependant, little attention has been paid to the influence of temperature upon the properties of foam. Furthermore, the rheological properties of foams, and their effectiveness for the displacement of oil are strongly dependant upon foam quality, which is in turn... 

Rheological properties, See also Rheology of dry foams, 12 16 of embedding materials, 10 10 of encapsulants, 10 12-13 of linear low density polyethylene,... 

H.M. Princen Rheology of Foams and Highly Concentrated Emulsions I. Elastic Properties and Yield Stress of a Cyhndrical Model System. J. Colloid Interface Sci. 91, 160 (1983). 

Journal of Polymer Science Polymer Physics Edition 39, No.18, 15th Sept.2001, p.2159-67 RHEOLOGICAL PROPERTIES AND FOAM PROCESSABILITY FOR BLENDS OF LINEAR AND CROSSLINKED POLYETHYLENES Yamaguchi M Susuki K-I Tosoh Corp. 

Blends of poly (ethylene terephthalate) (PETP) and polypropylene (PP) with different rheological properties were dry blended or compounded, and extrusion foamed using both physical blowing and chemical agents, and the foam properties compared with those of foam produced from the individual components in the absence of compatibilisers and rheology modifiers. The foams were characterised by measurement of density, cell size and thermal properties. Low density foam with a fine cell size was obtained by addition of a compatibiliser and a co-agent, and foamed using carbon dioxide. The presence of PP or a polyolefin-based compatibiliser did not effect... 

A study was made of relationships between compound rheological properties, microwave vulcanisation parameters and accelerators on the quality of extruded EPDM foam seals for the automotive industry. The influence of these factors on cell size and structure, density and mechanical properties was investigated. Correlations were found between the chemical composition of the compound, variations in processing parameters and the quality of the finished product. 12 refs. 

MC is used as an adhesive in ceramics to provide water retention and lubricity in cosmetics to control rheological properties and in the stabilization of foams in foods as a binder, emulsifier, stabilizer, thickener, and suspending agent in paints, paper products, plywood as a rheology control for the adhesive in inks, and in textiles as a binder, and for coatings. 

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. 

In much of the work on rheology, foams and HIPEs have been considered as analogous. The expressions derived are applicable to both systems, only the actual values are different. Consequently, workers in this area choose to study either emulsions or foams (or both) and so, in this section, the rheological properties of HIPEs and high gas-fraction (or dry ) foams will be discussed jointly. 

Another important rheological property of dry foams and highly concentrated emulsions is G, the shear modulus. Princen and Kiss [57] demonstrated that this property was dependent on < >, the volume fraction of the system. Previously, Stamenovic et al. [58] and, much earlier, Derjaguin and coworker [59], had derived an expression for the shear modulus of foams of volume fraction very close to unity. The value was found to depend on the surface tension of the liquid phase (in foams), for the particular case of (Jja 1. However, Princen demonstrated that the values of G obtained were overestimated by a factor of two. This error was attributed to the model used by Stamenovic and coworker, which failed to maintain the equilibrium condition that three films always meet at angles of 120° during deformation. 

In a later investigation, Kraynik and Hansen [62] demonstrated that the shear rate and liquid film viscosity greatly affect the rheological properties of foams. They studied the effect on foam properties and structure with variation of capillary number, Ca, which is the ratio of viscous to surface tension forces in the liquid films, and is given by -... 

Thus, it is highly evident that wall-slip must be taken into account when investigating the rheological properties of HIPEs and foams [87], Failure to do so will result in false and irreproducible results. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of solids, liquids, and gases, and their mechanical response to external forces can he very complex. 

While the Bingham plastic model is an adequate approximate description of foam rheology, it is by no means exact, especially at low strain rates. More detailed models attempl to relate the rheological properties of foams to the structure and behavior of the bubbles. 

Irrespective of the method used, the quality of a syntactic foam depends substantially on the rheological properties of the initial mixture with the microspheres68 70). The investigation of the rheology of syntactic compositions by Petrilaenkova et al. 71)... 

In the present case, the foam density relates perfectly with the previously observed rheological properties, as a transition in the flow behavior was detected at approximately 20 wt% of PPE (Fig. 13). In the viscoelastic case (below the percolation limit), the PPE content neither significantly influences the foamability nor the blend rheology. At elevated contents (beyond percolation), however, the PPE content strongly affects the rheological response of the blend and, subsequently, degrades the foaming behavior, which is verified by a reduced expandability. 

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