A review on the angle of repose of granular materials

Tabletability, the ability to make a tablet of adequate mechanical strength by powder compaction, is of paramount importance in the successful manufacture of tablet products. Poor tabletability is a persistent problem in the pharmaceutical industry. Tablet strength can be understood based on a qualitative model where contributions of bonding area and bonding strength are simultaneously considered. Formation and elimination of bonding area is related to compaction conditions, mechanical properties and particulate properties (such as particle size and shape). Plastic deformation emerges as the most important mechanism for creating a large bonding area among deformation mechanisms. Interfacial adhesion defines bonding strength and is dependent on the chemical nature of the materials involved. An analysis of how the chemical, mechanical and physical properties of a powder impact bonding strength and bonding areas leads to an understanding of their effects on powder tabletability. Appropriate use of this model...

Decoding Powder Tabletability: Roles of Particle Adhesion and Plasticity

The compressibility and compactibility of powder systems

The effect of particle size variation on tablet tensile strength for spray-dried lactose, Sta-Rx 1500 and Avicel PH-101 was investigated. Decreasing the particle size of spray-dried lactose and Sta-Rx 1500 resulted in stronger compacts whereas the tablet tensile strength of Avicel PH-101 was unaffected by particle size variation. The Heckel relationship, at two different contact times, was used to examine the predominant compaction mechanism. This was independent of the size fractions studied for all three materials. Angle of repose and Hausner ratio measurements indicated a correlation between the internal forces of friction and cohesion of the sized powders and the tensile strength of compacts formed from them.

Effect of particle size on the compaction mechanism and tensile strength of tablets

THE mechanisms by which a mass of powder is compressed into a coherent tablet, compact or pellet are not yet fully understood. One subject requiring clarification is the knowledge of the transmission of forces through the compacting mass and of the effects of these forces within the mass. Several workers have touched on this problem1-6 but the most useful contributions have been made from the field of powder metal1urgy7-12. However, there is conflicting evidence about several aspects of the p ~ o c e s s ~ ~ ~ , and it is possible that some of the theories which have been advanced to explain the mechanism of pressing, based on the behaviour of metals, will not necessarily hold when applied to nonmetals. In the work described in this paper, compacts have been prepared from a suitable material under controlled conditions, partly in order to obtain additional evidence on the sequence of events in the compaction of powders and partly to provide information for use in determining operating conditions for further work13.

An investigation into the compaction of powders

The relationship of the upper and lower punch forces and the ejection force during the process of tablet compression has been studied. Large differences may exist between the maximum force exerted by the upper punch and the maximum force obtained at the lower punch. When such differences exist, substantial forces are needed to eject tablets from the die. The magnitude of the difference between the maximum upper and lower punch force and the magnitude of the ejection force appear to be linearly dependent on the area of the tablet in contact with the die wall. The value of the ejection force and the difference between the upper and lower punch forces are both markedly affected b the presence of lubricants. With a well-lubricated granulation the force exerted by the upper punch is essentially the same as that transmitted to the lower punch and a negligible force is required to eject the tablets from the die. A quantitative method for evaluating lubricants on the basis of these observations is suggested.

The physics of tablet compression: IV. Relationship of ejection, and upper and lower punch forces during compressional process: Application of measurements to comparison of tablet lubricants

A study of lubricants during compression and in finished tablets has shown that, (1) lubricants are confined to the surface and intergranular regions of the finished tablets and do not mix intimately with the contents of the granules, (2) approximately 1 per cent lubricant appeared to give nearly o timum granulation lubricity when applied to the test granulations used in this stucfy, (3) fluid lubricants appeared to flow into tablet voids and unlike solid lubricants did not usually cause significant increase in punch forces when added in increasing amounts if the machine settings and weights of die fill were constant, (4) the addition of lubricants, e. g., ma nesium stearate, to granulations tended to roduce slightly softer tablets and at t8e same time significantly prolonged tablet $isintegration time in the usual concentrations.

The Physics of Tablet Compression IX: Fundamental Aspects of Tablet Lubrication

Energy expended in the tablet compression process was calculated by consideration of the magnitude and relation between the several compression variables. In the compression of about 0.4 Gm. of unlubricated sulfathiazole granulation to a maximum force of 1200 Kg, on the lower punch with flat‐faced 3/8‐inch punches, about 8.6 calories were expended. For about the same weight of lubricated granulation the corresponding value was about 2.0 calories. By considering that only energy expended in the process of eliminating void space to form the tablet caused a temperature rise in the tablet, the temperature rise expected in the lubricated or unlubricated tablets mentioned above would be about 5°. When the energies expended in the processes of tablet ejection, overcoming wall friction and removing the upper punch from the die, were summed for a single punch machine making these tablets at the rate of 100 per minute, it was found that 43 kilocalories would be expended in each hour of operation if the tablets were unlubricated. If lubricated granulation were used the value would be 3 kilocalories. The bulk of these expenditures would probably cause heating of the punches and die and if the granulation was either poorly lubricated or unlubricated the heating might be severe.

The physics of tablet compression. VII. Determination of energy expenditure in the tablet compression process

The design and performance of an instrumented tableting machine are resented. A Stokes Model A-3 single punch machine has been fitted with strain and displacement gauges to permit simultaneous recording of several variables including the force exerted by the upper punch, the force exerted on the lower punch, the movement of the upper punch, and the ejection force. Several fields of tablet research which can benefit by the use of this instrument are discussed.

The physics of tablet compression. III. Design and construction of an instrumented tableting machine

In order to study the problem of granulation flow in tableting as it is related to interparticle friction, the response angle of a sulfathiazole granulation was studied as a function of average particle size, presence of lubricants, and admixture of fines.In general, the repose angle increased with decreasing particle size.Talc added to granules in small concentrations decreased the repose angle but tended to increase the angle at higher concentrations.Magnesium stearate had little or no effect on the repose angle of the granulation samples studied.Addition of fines to coarse granules caused a striking increase in repose angle.

Eino Nelson

Papers

The physics of tablet compression: IV. Relationship of ejection, and upper and lower punch forces during compressional process: Application of measurements to comparison of tablet lubricants

The Physics of Tablet Compression IX: Fundamental Aspects of Tablet Lubrication

The physics of tablet compression. VII. Determination of energy expenditure in the tablet compression process

The physics of tablet compression. III. Design and construction of an instrumented tableting machine

Measurement of the Repose Angle of aTablet Granulation