Mechanical retention

Amalgam restorations are prepared by mixing a powdered alloy with mercury to form a plastic moldable mass that is packed or condensed iato the prepared cavity. The cavity is designed to provide mechanical retention, maximum marginal mass, support to absorb the functional stresses transmitted through the restoration, and maximum protection to the remaining tooth stmcture. The restoration reestabUshes the normal tooth anatomical form and function. 

The mechanism of these alkylations involves a tetracoordinate boron intermediate formed by addition of the enolate of the a-bromo ester to the organoborane. The migration then occurs with displacement of bromide ion. In agreement with this mechanism, retention of configuration of the migrating group is observed.23... 

In surface filtration the solids retained are those that do not pass through the smallest cross-section of the capillary flow channels of the filtering layer. Many particles are trapped by adsorption in the labyrinthine three-dimensional sieve of the filter aid. This means that substances can be retained that are smaller than the mesh size of the filter aid. In depth filtration, on the other hand, the solids are trapped in the interior of the layer this is due to the mechanical retention capacity (inertia and size of the particles, sedimentation, diffusion) and to the composition of the juice. 

Water-insoluble pigments have a somewhat limited, but nonetheless essential, part to play in the colouration of certain papers. They are normally applied as anionic dispersions. They have virtnaUy no affinity for ceUulosic fibres and instead rely upon either mechanical retention from the paper machine itself or flocculation via a cationic product (fixing agent) to produce small floes which can be retained on the fibre snrface. 

The predominant bonding mechanism is mechanical retention and the most common bonding systems are either a composite, generally light-cured, or polyalkenoate cements similar to those used in restorative dentistry (see Adhesion in dentistry restorative materiais). In a recent development to minimize operator errors, the bracket base is pre-coated with the adhesive so that there is no mixing, no contamination and there is a controlled amount of adhesive. The adhesive is typically a lightly filled combination of bisphenol A diglycidyl dimethacrylate and bisphenol A bis(2-hydroxyethyl ether)... 

Ceramic brackets are used for aesthetic reasons, and they are commonly treated with Silane adhesion promoters, either a vinyl silane, typically methacryloxypropy-Itrimethoxysilane, coupling agent or a micro-crystalline roughened base to enhance mechanical retention. 

Again, mechanical retention, either macro or micro, is the predominant mechanism. The metal structure can be perforated and the bonding system, a structural adhesive, flows through the holes to provide the retention, for example Rochette Bridge. The metal can be etched either chemically or elecirochemically to produce a surface suitable for micro-mechanical retention. 

The smear layer is often removed with phosphoric acid, which again deminerahzes the superficial dentine and exposes collagen. The amphiphilic monomer creates the hybrid layer and diffuses into the dentinal tubules where it forms tags and provides additional micro-mechanical retention. If the collagen layer becomes desiccated, then it collapses, monomer infiltration is inhibited and the hybrid zone is not successfully formed. 

The phenomena observed are explained by the actual properties and structure of random coils and solution and by molecular and molecule-wall interactions. On account of simultaneity of the adsorption and the mechanical retention, an exact mathematical formulation of the adsorption phenomena in consolidated porous media is difficult. 

As a result of point 2, the adsorption is accompanied by dynamic and irreversible entrapment of the molecules when a natural porous core or model is flooded by polymer solutions. This makes the application of a compatible polymer-model system imperative in adsorption studies. In the course of our investigations, this requirement was fulfilled as far as possible. Despite all our efforts, however, it should be emphasized that the mechanical retention can have a share in the adsorbed amount if the polymer concentration is over the critical value. 

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