E. Shotton

THE STRENGTH OF COMPRESSED TABLETS

Examination of the fracture of tablets in a strength test has revealed two types of failure. Where the interparticulate bond is strong, fracture occurs across the grains and the strength of tablet is a simple function of the particle size. If the interparticulate bond is weak, fracture occurs around the grain and particle size has little effect. A thin coating of stearic acid was used to weaken the interparticulate bond when the effect of grain size on strength disappeared or was reversed. The relation of bonding and capping is discussed and it is shown that capping can be reduced by weakening the interparticulate bond.

THE STRENGTH OF COMPRESSED TABLETS: Part I. The Measurement of Tablet Strength and its Relation to Compression Forces

A motorised single‐punch eccentric tablet machine has been fitted with instruments to measure the forces operating during the compression cycle. Apparatus has also been designed to measure the resistance to crushing of the final compact. Sodium chloride was compressed at varying pressures and the relationship between compaction forces, ejection force, strength and voidage investigated.

The effect of waxes, hydrolysed gelatin and moisture on the compression characteristics of paracetamol and phenacetin

Stearic acid or hard paraffin added to crystals of paracetamol and phenacetin reduced capping of tablets prepared by direct compression but did not produce acceptable tablets because the inter‐particular bonds were very weak. The pressure cycle that can be constructed from the measurement of the axial pressure and the corresponding die wall pressures offers information that is useful in the formulation of tablets. The behaviour of paracetamol or phenacetin and their mixtures with gelatin hydrolysate or water or both shows a similarity to a Mohr body and it appears that the maximum die wall pressure is affected by the particle size of the material compressed and also by the additives present. Good transmission of radial force implies that the material can be initially consolidated, but alone it does not indicate that the tablet formed is physically stable. When the tablet formed remains coherent after the axial pressure is removed the residual die wall pressure remains high. Measurement of the residual die wall pressure might therefore be a useful indicator for identifying satisfactory formulations of substances that cap readily. Hydrolysed gelatin or water or both together produced paracetamol and phenacetin mixtures with satisfactory compression characteristics.

The effect of humidity and temperature on the cohesion of powders.

The cohesion of wheat, maize and potato starch, acacia, tragacanth, alginic acid, lactose, dextrose and sucrose powders has been determined at different relative humidities and temperatures. For maize, wheat and potato starches, cohesion was a maximum at an intermediate moisture content, whilst for acacia, tragacanth and alginic acid it was independent of the moisture content over the range considered. Lactose, dextrose and sucrose remained free flowing except at high relative humidities.

The effect of some binding agents on the mechanical properties of granules and their compression characteristics

The modulus of elasticity and the crushing strength of cylindrical granules prepared by a template method, and the friability of conventional granules were measured and the effect of the binding agents, PVP, methyl cellulose and maize starch examined. These properties were related to the compression characteristics of the granules and to the crushing force of the subsequent tablets. According to the base material two cases were observed. With dicalcium phosphate little plastic flow was evident and low radial stress transmission was noticed, the softest granules led to the weakest compacts. With paracetamol more plastic flow occurred giving a greater axial to radial conversion of force and the softest granules produced the strongest tablets. The extent of further generated contact area played the major role. Generally, due to the better bonding capacity, dicalcium phosphate gave stronger compacts than paracetamol, although lower radial transmission ratios were recorded. A good conversion of axial to radial force was thus proved not to be the main factor in obtaining good tablets, as was also confirmed by the paracetamol powder alone. With paracetamol the residual pressure on the die wall was greater with the formulations producing satisfactory tablets than with paracetamol alone which gave rise to capping.

The influence of 1-hexadecanol on the acid-catalysed hydrolysis of sodium dodecyl sulphate.

The kinetics at 60° of the acid‐catalysed hydrolysis of sodium dodecyl sulphate in the presence of 1‐hexadecanol above the critical micelle concentration showed the hydrolysis to be first order and the rate to depend on the molar ratio of alcohol to alkyl sulphate. Up to a ratio of 0.75 the rate increases; above this there is a steady decrease. The changes in rate have been correlated with phase changes and explained on the basis of the effect on the charge density on the micelle, and on the dielectric constant of the associated medium, in passing from a mixed spherical micelle to the lamellar structure and then to the expanded lamellar structure prevailing in liquid crystals.

SOME OBSERVATIONS ON THE EFFECT OF LUBRICATION ON THE CRUSHING STRENGTH OF TABLETS.

The effect of mean compaction pressure, base particle size, lubricant particle size, and lubricant concentration, on crushing strength of tablets has been determined for crystalline aspirin, hexamine, sucrose and sodium chloride, and simple granulations of sucrose and of hexamine. Magnesium stearate powder was used as a lubricant. The lubricant decreased the strength of all tablets, the reduction being greatest for the crystalline materials. Variation of lubricant particle size had no effect on strength, and the behaviour of the granulations was not predictable from the characteristics of the crystalline materials. The results are compared with those of other workers.

The compaction properties of sodium chloride in the presence of moisture

The effect of moisture on the behaviour of sodium chloride under compression was investigated by measurement of compaction forces and ejection forces. Saturated solution present at the crystal surfaces effected a reduction in friction at the inter‐particulate boundary and between the particles and the die wall. Dry material formed compacts of higher strength than wet material except at low pressures. Subsequent drying of compacts prepared in the presence of moisture resulted in an increase in strength due to interparticulate recrystallisation.

The effect of humidity and temperature on the equilibrium moisture content of powders

The equilibrium moisture content of maize, wheat and potato starches, alginic acid, tragacanth, acacia, lactose, dextrose, sucrose and hexamine has been determined at different relative humidities and temperatures. Samples were exposed to an atmosphere of controlled temperature and humidity until equilibrium was attained. The moisture content was determined by drying. The moisture content of each powder at equilibrium was dependent on the relative humidity.

Some observations on the ageing of sodium chloride compacts

The diametrical crushing strength of an ideal system of compacted particulate material, and the relation between strength and mean compaction pressure depend on the time at which the strength is determined. Strength increases of over 100% during the first hour after compression are attributed to stress relief of the sodium chloride crystals and interparticulate bonds. The effects of elevated temperature, and also humidity, on the strength and physical structure of compacts are evaluated. Relaxation behaviour of compacts containing light liquid paraffin is also investigated.