Clay platelets swelling

Stresses caused by chemical forces, such as hydration stress, can have a considerable influence on the stability of a wellbore [364]. When the total pressure and the chemical potential of water increase, water is absorbed into the clay platelets, which results either in the platelets moving farther apart (swelling) if they are free to move or in generation of hydrational stress if swelling is constrained [1715]. Hydrational stress results in an increase in pore pressure and a subsequent reduction in effective mud support, which leads to a less stable wellbore condition. 

Use the snperposition principle to calcnlate the electrostatic swelling pressnre generated between parallel clay platelets with surface potentials of -110 mV, at a separation of 35 nm in a 1.5 mM aqneons solntion of NaCl at 25 °C. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

As already indicated, the surface area of the dry clay appeared to be about 50m2g-1. The platy particles (the tactoids) were therefore about 20 layers thick during the first stage of water sorption, the particles were split into smaller tactoids of around six clay platelets (surface area of 105 m2 g-1). The external dimensions at p/p° > 0.25 remained fairly constant, but the interlamellar sorption was accompanied by swelling and the development of an accessible internal area (possibly as high as... 

Clays of the montmorillonite family are lamellar aluminosificates [46] used in many industrial processes and in products such as paints, softeners, and composite materials [47]. They swell when brought into contact with water, which is due to the insertion of water molecules between the sheets. Complete exfoliation can be induced leading to dispersions of disk-like particles of 10 A thickness and 300-3000 A in diameter, depending on the variety of clay used. These clay platelets bear a rather large surface electrical charge so that electrostatic interactions between them must be considered and are actually responsible for the colloidal stability of these dispersions. These suspensions have been widely studied as model colloids and also because they form physical thixotropic gels. 

The swelling of clays in water represents an example in which the internal osmotic pressure is associated with double-layer repulsion between the clay platelets. 

The total aqueous potential (pressure and chemical potential) of the pore fluid increases with the increase in pore pressure and/or chemical potential (decrease in salt concentration). When the total aqueous potential of the pore fluid increases, water will be adsorbed into the clay platelets. The water adsorption will result in either the platelets moving further apart i.e., swelling if they are free to expand, or the generation of hydrational stress if swelling is constrained. The process can be represented by (Tan et al., 1997) ... 

Nanocomposites of thermoset polymers like unsaturated polyester and epoxy can be fabricated by this method. In this process, the monomers/ prepolymers are allowed to intercalate the layer spacing of the clay platelets. Polymerization will then be initiated either by the application of heat or radiation or by introducing suitable organic initiator. The intercalated monomer swells the clay and during polymerization increases the interlayer spacing and results in the formation of intercalated or exfoliated nanocomposites. 

Under appropriate conditions, the gallery spaces can be filled with monomer, oligomer, or polymer. This increases the distance between platelets, swelling the clay. Clay platelets swollen with polymer are said to be intercalated. If the day swells so much that it is no longer organized into stacks, it is said to be exfohated as shown in Figure 9.5. 

The benefit of exchanging polymeric initiators onto the clay surface is that the growing chains are anchored to the clay surface, in a surface initiated polymerization and the growing PS chains may help to push the clay platelets further apart. The clay modification itself however still needs to be compatible with the styrene, in order to allow monomer to swell the platelets. This approach is further discussed in Section 13.4, Figure 13.9. 

Vaccari (66) explains that acid-treatment conditions to maximize catalytic activity depend on the precise reaction of interest. Reactions between polar molecules require mildly treated clays to capitalize on the large number of acid sites available in the internal surface. In contrast, nonpolar molecules react only at the surface-accessible external face and edge sites of the clay platelets. These latter reactions require more severely treated clays, and activity depends on the interplay between surface area and cec. In a swelling solvent, maximum activity can be achieved simply by exchanging with acidic cations such as AF + without any acid treatment. 

Calcined clay and delaminated clay are also used in rubber. The former is used mostly in wire and cable coverings where it provides excellent dielectric and water resistance properties, although poor reinforcement. Delaminated clay is individual clay platelets instead of the platelet stacks characteristic of other clay products. As such it represents the most planar or anisometric form of clay available and imparts particularly high stil ess and low die swell. This is demonstrated in comparison to a hard clay in Table 8. 

Smectite - The smectites are water swellable cl s having a sheet or platelet structure. Smectite is the mineralogical term for this class of clays, which includes montmorillonite, hectorite, and saponite. Montmorillonite clays derive their name from the Montmorillon section of France where this material was first observed and later classified. Most smectites are more commonly known under the geological term bentonite. By convention, a bentonite is understood to be an ore or product with a substantial smectite content. The name bentonite derives from Fort Benton, Wyoming, the site of an important deposit. Lattice substitutions within the smectite clays creates a charge imbalance which is compensated by exchangeable alkali and alkaline earth cations. This contributes to the ability of these clay to swell and impart considerable plasticity in ceramic formulations. When the exchangeable cations are predominately sodium, the individual platelets can separate to produce a colloidal structure in water. 

Swelling clay minerals (e.g., smectites) consist of multiple layers of electron-donating, negatively charged layers, with an interlayer of charge compensating cations (e.g., Na+). Immersed in water and in a number of other polar liquids (e.g., formamide, ethylene glycol, dimethylsulfoxide) smectite clay particles swell, i.e., the distance of about 0.32 nm between two platelets can increase to, e.g., 1.2 nm, or in some cases (with Na+ and Li+) the individual... 

The process of separating the nanoclay platelets is referred to as the intercalation process. Without this separation, the nanoclay would not be capable of allowing the polymer to penetrate the platelet layers. There are two techniques for intercalating the matrix polymer molecule between clay platelets melt intercalation and solution intercalation. In case of melt intercalation, a layered silicate is mixed with the polymer matrix material in the molten state. In case of solution intercalation, the modified nanoclay is swelled in monomer, allowing it to enter the clay gallery. Subsequently, the monomer is polymerized with the result that polymer is formed inside the clay gallery. In either case, once mixed the clay platelet material swells in the polymer matrix and forms a very strong interaction with the polymer chains to produce a composite matrix with enhanced performance. 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

Platelets are held together by cations. They impart a positive charge to the edge of the particles. These interlayer cations play a key role in the physicochemical properties of bentonite and in the stability of aqueous dispersions. Normally calcium is predominant and the clay swells to a moderate extent when dispersed in water. When Ca ions are replaced by Na, e.g., by reacting with Na2CC>3, the bentonite is said to be activated. This activation makes the clay much more swellable. 

The swelling of clays is a two-step process. First, hydration of the platelet surface occurs, leading to a slight volume increase. Second, repulsion takes place between the electric double layers, leading to the complete separation of the platelets this is so-called osmotic swelling. 

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