Shear properties foams

Those stmctural variables most important to the tensile properties are polymer composition, density, and cell shape. Variation with use temperature has also been characterized (157). Flexural strength and modulus of rigid foams both increase with increasing density in the same manner as the compressive and tensile properties. More specific data on particular foams are available from manufacturers Hterature and in References 22,59,60,131 and 156. Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. The shear properties increase with increasing density and with decreasing temperature (157). 

Shear strength and modulus of rigid foams depend on the polymer composition and state, density, and cell shape. Shear properties increase with increasing density and decreasing temperature [30]. 

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. 

One simple rheological model that is often used to describe the behavior of foams is that of a Bingham plastic. This appHes for flows over length scales sufficiently large that the foam can be reasonably considered as a continuous medium. The Bingham plastic model combines the properties of a yield stress like that of a soHd with the viscous flow of a Hquid. In simple Newtonian fluids, the shear stress T is proportional to the strain rate y, with the constant of proportionaHty being the fluid viscosity. In Bingham plastics, by contrast, the relation between stress and strain rate is r = where is... 

Throne has reported that the relationship between foam modulus and density can be generalised to other properties such as tensile strength, fatigue strength, creep properties as well as shear and compression modulus. Thus if X is the general material property then... 

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. 

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. 

The viscous properties of HIPEs and high gas fraction foams have also been studied extensively, using a two dimensional, monodisperse, hexagonal cell model. Khan and Armstrong [52] showed that, under steady shear flow (i.e. beyond the yield point of the system), the foam viscosity was inversely proportional to shear rate. At high rates of shear, a constant viscosity value was approached. Gas fraction, <)>, was assumed to be very close to unity. 

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 -... 

The properties of syntactic materials are influenced by several factors including the binder/filler ratio, the process and hardening conditions, and the physicochemical processes at the binder/filler interface 12,76,99). The best syntactic foams, at given apparent densities of 680-700 kg/m3, have a compression strength of 10 MPa, shear and tension elastic moduli of 2500—3000 MPa ultimate bending strengths of 40 to... 

As previously demonstrated, the shear rheological properties are an important factor relevant for the processing and foaming. In addition, morphological features of the blend system can be detected at low shear rates [95], The shear viscosity and the storage modulus of the present blends are highlighted in Fig. 32. An in-... 

The categorizations just presented are helpful in describing the rheological properties of many emulsions, foams, and suspensions. It should be remembered, however, that some dispersions are not well described by any single category. Some dispersions exhibit Newtonian behaviour at low shear rates, shear thinning at moderate shear rates,... 

In addition to suspensions, pharmaceutical products may be emulsions or foams. In any case the rheological properties have to be tailored to suit the nature of the application [215], Therapeutic ointments are usually not very viscous and encounter only moderate shear rates upon application, about 125 s-1 when gently smeared on with fingers, and about 210 s-1 when smeared on with a spatula [215], An opthalmic ointment is usually very soft, with a viscosity of about 20-30 mPas, whereas a medicated ointment needs to be soft enough to apply easily but stiff enough to remain on the area to which it was applied, with a viscosity of about 30-40 mPas [215], A protective ointment like zinc oxide paste needs to be hard and stiff enough to stay in place where applied, even when moist. 

The foam retains some of the properties of the phases that is formed from. For example, its compressibility is determined mainly by the ability of the gas to compress, and its wetting power by the properties of the foaming solution. At the same time, being a disperse system, the foam acquires the properties of a solid body maintains its shape, possesses a shear modulus, etc. 

The yield stress of a foam depends to a considerable extent on the character of foam interaction with the tube walls or the cylindrical surface of the viscometer, used in the study of its rheological properties. At low flow rates and smooth tube walls the maximum shear stress of the foam layers contacting the wall can be less than the shear stress of the foam matrix (shear of bubble layers). Hence, the foam flow will occur as a movement of a continuous medium in a cylinder covered with a thin lubricating layer of thickness 2-10 pm [9,16], In this case t0 is ca. 1 Pa, that is, much less than its theoretical value. 

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