Influence of the Compression Force

Mechanical stresses are necessary for the crystallization contacts to be established in hydration structures in the course of hydration hardening. These stresses are either applied externally or originate from the directional growth of the crystals inside the already formed network. Mechanical stresses may also be caused by the weight of powders or result from the action of capillary forces acting in the drying menisci. 

Probability of Bridging, of CaS04 x 2H2O Crystals in Supersaturated CaS04 Solutions as a Function of tbe Compressing Force, f, for a Given Degree of Supersaturation, a, and Time of Contact, t 

Mechanical stresses may eause a decrease in the degree of supersaturation in the contact zone because the stressed regions of the crystals have higher equilibrium solubility. The resulting supersaturation would then be defined as 

The increase in the probability of bridging of crystals with the increase in the applied mechanical stress should thus be related to the increase in the factor 4 in Equation 6.5. This factor 4 should account for the area of the active zone where the formation of the primary crystallization bridge, that is, of the contact nucleus, takes place. 

In the case of solids that undergo only elastic deformation in the contact zone, the dependence of 4on/is different the size of the active zone can in this case be estimated from the Hertz equation (see Section 5.2). 

---

Influence of the compression force on the physical tablet properties (Formulation No. 2)... 

When compressed at subcritical temperatures, the gas condenses, (hat is, macroscopic clusters or droplets are formed under the influence of the attractive forces. During condensation, the pressure remains constant while the volume decreases, giving rise lo an infinite compressibility in the Iwo-phase region. At the criticai point the system is on the verge of condensation and the compressibility is also infinite. 

The ability of the particles to cohere under the influence of a compressing force. This coherence... 

A fundamental property of a solid is the mechanism by which it consolidates under the influence of a compressing force. There are two principal mechanisms— fragmentation and deformation—though most solids will show a mixture of the two with one mechanism predominating. The mechanism can have a major influence on tablet properties. 

As would be expected, there is no detachment of particles under the influence of a compressive force (position 10). When a detaching force is acting at an angle of 30,60,90,120, or 150° to the dust-covered substrate in air (positions 2-6), the detached particles are immediately removed from the surface. 

It is often assumed in the theoretical analysis of liquid-phase sintering that the composition and structure of the grain boundary liquid phase is constant. However, this is not always the case. A wetting liquid layer, as described earlier, leads to the development of a compressive capillary force, which is equivalent to placing the system under a fairly large hydrostatic compression. The evolution of the intergranular liquid layer under the influence of the capillary force as well as other forces has been the subject of recent investigation. 

A consideration often overlooked in BAM studies is the possible influence of the compression rate on the domain structures. In the case of A-acylamino acid monolayers, the associations due to amide-amide hydrogen bonding are very strong and promote rapid domain growth and also make it unlikely that relaxation to an equilibrium domain shape can occur on any realistic experimental timeframe. Domain shape relaxation kinetics are noted to be dependent on the strength of intermolecular forces for example, dendritic condensed-phase domains formed in a phospholipid monolayer required 5 h to relax to equilibrium shapes and, for the phospholipid DMPE, compression rates as slow as 0.2 per molecule per minute were needed to observe equilibrium domain shapes. Examination of the variation of domain structure with time after then-formation or with compression rates are not commonly reported however, it is advisable to consider examining these variables when carrying out BAM experiments. 

In order to prevent or reduce the adhesion of dust, the inner surface of the dust catcher may be wetted. Under the influence of the centrifugal force the wet solid particles and liquid in the form of slurry are thrown toward the inner walls of the apparatus and fall in a thin film into the bimker [414]. In order to clean the inner surface of a cyclone-type dust catcher, the inner tube is sometimes made movable on moving downward it comes into contact with the spiral, which cleans off the adhering dust [415]. The dust catcher may also be cleaned with compressed air after shaking [416]. 

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

Some tablet presses are equipped with two sets of rollers a smaller roller located between the feed frame, and the main compression roller which allows a small degree of compression (e.g., precompression) to take place. Precompression can remove air from the powder bed in order to minimize tablet capping and lamination. Good com-pactibility of the powder blend is essential to tablet formation under the influence of compression forces and ensures that tablets will remain intact compact when the compression force is removed. 

Our discussion has centered on a single nonflow process, the expansion of a gas in a cylinder. The opposite process, compression of a gas in a cylinder, is described in exactly the same way. There are, however, many processes which are driven by other-than-mechanical forces. For example, heat flow occurs when a temperature difference exists, electricity flows under the influence of an electromotive force, and chemical reactions occur because a chemical potential exists. In general, a process is reversible when the net force driving it is only differential in size. Thus heat is transferred reversibly when it flows from a finite object at temperature T to another such object at temperature T - dT. 

The structure of a foam, which deforms in a specific manner also influences physical properties. Upon initial compression there is a deformation of the structure that requires an increase in the amount of force applied. Once the foam has been deformed significantly the sides of the cell walls buckle leading to cell collapse and the production of elliptically deformed structures. A picture of such cells deformed under extreme compression is shown in Figure 14. During this phase of the compression the rate of increase in the force applied is significantly decreased. 

Compressed solids, tablets, or caplets, are prepared by placing the blend of component additives in a cylinder or die, above a moveable piston or punch. An upper punch is brought into the top of the piston, and pressure applied to the distal ends of the punches forces the powder into a compact (Fig. 7). The quality of the product depends upon the cohesive forces acting on the powder upon compression. These cohesive forces are influenced by the selection of additives in the dosage formulation. One method of evaluating... 

Fig. 111. Influence of the type of dry binder on the hardness of acetaminophen tablets 500 mg obtained by direct compression (3.5% dry binder, tablet weight 700 mg, compression force 25 kN)...

The effect of compression force on drug release from hydrophilic matrices is minimal when tablets are made with sufficient strength and optimum levels of polymers are used [48]. One could relate variation in compression forces to a change in the porosity of the tablets. However, as the porosity of the hydrated matrix is independent of the initial porosity, the compression force is expected to have little influence on drug release rate [79], Once a sufficient tablet hardness suitable for processing and handling is achieved, tablet hardness would have little further effect on drug release profile. To ensure consistent porosity and avoid entrapment of air within the dry tablet core, a pre-compression step may have to be considered in the manufacture of matrices. 

Elevated pressures can induce functional and structural alterations of proteins. The effects of pressure are governed by Le Chatelier s principle. According to this principle, an increase in pressure favours processes which reduce the overall volume of the system, and conversely increases in pressure inhibit processes which increase the volume. The effects of pressure on proteins depend on the relative contribution of the intramolecular forces which determine their stability and functions. Ionic interactions and hydrophobic interactions are disrupted by pressure. On the other hand, stacking interactions between aromatic rings and charge-transfer interactions are reinforced by pressure. Hydrogen bonds are almost insensitive to pressure. Thus, pressure acts on the secondary, tertiary, and quaternary structure of proteins. The extent and the reversibility, or irreversibility, of pressure effects depend on the pressure range, the rate of compression, and the duration of exposure to increased pressures. These effects are also influenced by other environmental parameters, such as the temperature, the pH, the solvent, and the composition of the medium. 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion is the resultant of the surface dilata-tional viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear or dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is rather difficult. The interface is connected to the substrate on either side of it, and so are the interfacial viscosities coupled to the bulk viscosities. Therefore, it becomes laborious to determine purely interfacial viscosities without the influence of the surroundings. 

The lag period before back diffusion starts is just like the whole diffusion time course it is practically identical for all the prepulse amplitudes. The lag period is also independent of the viscosity of the aqueous solution. It is possible that the photosystems aggregate when compressed under the influence of the electrical field, in which case the lag time could be identified with disaggregation time. Because contact between particles is a necessary condition for aggregation, elastic as well as electrostatic forces are probably invoked to stop electrophoretic motion. 

---