External boundary lubrication

Figure 8 Left Schematic graph of the setup for the simulation of rubbing surfaces. Upper and lower walls are separated by a fluid or a boundary lubricant of thickness D. The outermost layers of the walls, represented by a dark color, are often treated as a rigid unit. The bottom most layer is fixed in a laboratory system, and the upper most layer is driven externally, for instance, by a spring of stiffness k. Also shown is a typical, linear velocity profile for a confined fluid with finite velocities at the boundary. The length at which the fluid s drift velocity would extrapolate to the wall s velocity is called the slip length A. Right The top wail atoms in the rigid top layer are set onto their equilibrium sites or coupled elastically to them. The remaining top wall atoms interact through interatomic potentials, which certainly may be chosen to be elastic.

The additive mixtures interact in a variety of ways, both in the bulk oil and on surfaces. Tribochemical interactions of additives in the oil formulation are discussed in Chapter 2. Surfactant molecules, when dissolved in base oil, are capable of self-organization to form aggregates such as soft-core reverse micelles (RMs). The polar or charged head groups of these molecules with the counter ions form the interior of the micelle (core), and the hydrocarbon chains made up its external shell. The most important factor governing the tribochemical reactions under boundary lubrication is connected with the action of soft-core and hard-core reverse micelles discussed in Chapter 3. 

In general, lubricants can be added internally (boundary lubricants) or externally. In the former case, lubricants are added to the powder mixture and blended briefly to coat the particles, whereas in the latter, lubricants are dissolved in an organic solvent and sprayed onto the surface of the punches to form a film [3]. In this case, only the lower punch and die, not the final blend, are lubricated and thus, the external addition is only useful when tablet properties are very sensitive to lubricants. 

Internal and external lubricants are differentiated. Internal lubricants are compatible with the polymer matrix and serve mainly to reduce the friction inside the material and thus the shear heating. To achieve the necessary compatibility with the polar basic polymer PVC, internal lubricants are in most cases polar substances. External lubricants, on the other hand, service mainly to reduce the friction between the polymer and the metallic surface at the boundary of the flow channel. These substances are less compatible with the basic polymer and therefore unfold their effects mainly at the boundary to the metal. This external effect is achieved by means of a specifically adjusted incompatibility with the polar basic polymer. Most external lubricants are therefore chemically nonpolar. Standard lubricants are marketed in fine gradations between purely external and purely internal action, hence also with wide variations in polarity. 

Lubricants are classified to internal and external lubricants. They are described in Fig. 1.24. Internal lubricants are soluble in the polymer matrix and are plasticizers. They are used for lowering melt viscosity and reducing heat generated from shear forces. External lubricants are immiscible in the polymer matrix and are used to produce boundary layer between matrix... 

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