Rheology of clay particle suspensions

However, the basic principles of the interaction of day with water, and the rheology of clay particle suspensions, are discussed in the following section. 

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In some cases, an extrudable and injectable paste may consist of 65% vol. ceramic powder and 35% vol. polymeric binder. In others, an extrudable paste may consist of a highly loaded aqueous suspension of clay particles such that its rheology is plastic. Hie low shear (i.e., <100 sec ) viscosity of such a paste is between 2000 and 5000 poise at ambient temperature. Highly nonlinear stress strain curves are typical of ceramic pastes, as well as time dependent thixotropy. In many cases, pastes behave like visco-elastic fluids. This complex rheological behavior of ceramic pastes has made theoretical approadies to these problems difficult. For this reason, the discussion in this chapter is limited to Newtonian fluids where analytical solutions are possible, with obvious consequences as to accuracy of these equations for non-Newtonian ceramic pastes. 

For conventional wet processing of sheet steel, the porcelain enamel frit is ball-milled using clay, certain electrolytes, and water to form a stable suspension. This clay-supported slurry of small particles is called the sHp and has the consistency of a heavy cream. The ingredients of the mill batch are carefully controlled. The amount and purity of all materials in the mill, including the clay and water, affect the rheological character of the sHp as well as a number of the properties of the fined enamel such as chemical resistance, reflectance, gloss, color, and abrasion resistance. 

Information on interparticle forces can be obtained from rheological studies. As the shear rate is increased, the particles rearrange, or floes distort in a flocculated system, and eventually the system starts to flow. The magnitude of the Bingham yield stress, Tg, reflects maximum rearrangement of the particles in an unflocculated system deformation of the floes in a flocculated system before flow. The shear moduli, G, G and G effectively probe the unperturbed system at very low shear rates and reflect the particle-particle interactions in a pure clay suspension the intraparticle structure of the floes in a flocculated suspension. 

The sedimentation results obtained with the structured suspensions, are consistent with the predictions from rheological investigations. In the absence of any bentonite clay, the pesticidal suspension exhibits Newtonian behaviour with unmeasurable yield value, modulus or residual viscosity. In this case the particles are free to settle individually under gravity forming a dilatant sediment or clay. On the other hand, at bentonite concentrations above the gel point (> 30 g dm the non-Newtonian behaviour of the suspensions and in particular their viscoelastic behaviour results from the formation of a "three-dimensional" network, which elastically supports the total mass. After 21 weeks standing in 100 ml measuring cylinders, no separation was observed when the bentonite concentration was >37.5 g dm corresponding to a modulus > 60 Nm. Clearly the modulus value required to support the mass of the suspension depends on the density difference between particle and medium. 

The clay component is the main factor controlling the rheological properties and the slip stability. Some part in this respect, hovewer, can also be played by the other very fine components. From the frit particles, water extracts alkalis which affect the colloidal and rheological properties of the slip. Since the extraction is gradual, the suspension is allowed to stand for a certain time before application to stabilize its properties. 

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