Hardness Test

Hardness is also so called crushing strength. It is the load required to crush the tablet when placed on its edge. There are a variety of presentations for tablets as delivery systems for pharmaceutical agents, such as rapidly disintegrating, slowly disintegrating, eroding, chewable, and lozenge. Each of these presentations places a certain demand on the bonding, structure, and integrity of the compressed matrix. Tablets must be able to withstand the rigors of handling and transportation experienced in the manufacturing plant, in the drug distribution system, and in the field at the hands of the end users (patients/consumers). Manufacturing processes such as coating, packaging, and printing can involve considerable stresses, which the tablets must be able to withstand. For these reasons, the mechanical strength of tablets is of considerable importance and is routinely measured. Tablet strength serves both as a criterion by which to guide product development and as a quality control specification.

Measure of the mechanical integrity of tablets is their breaking force, which is the force required to cause them to fail (i.e., break) in a specific plane. The tablets are generally placed between two platens, one of which moves to apply sufficient force to the tablet to cause fracture. For conventional, round (circular cross-section) tablets, loading occurs across their diameter (sometimes referred to as diametral loading), and fracture occurs in that plane.

The breaking force of tablets is commonly called hardness in the pharmaceutical literature (“hardness is defined as the resistance of the tablet against the applied force till it breaks”); however, the use of this term is misleading. In material science, the term hardness refers to the resistance of a surface to penetration or indentation by a small probe. The term crushing strength is also frequently used to describe the resistance of tablets to the application of a compressive load. Although this term describes the true nature of the test more accurately than does hardness, it implies that tablets are actually crushed during the test, which often is not the case. Moreover, the term strength in this application can be questioned, because in the physical sciences that term is often used to describe a stress (e.g., tensile strength). Thus, the term breaking force is preferred.



Why do we measure hardness?
To determine the need for pressure adjustments on the tableting machine.
Hardness can affect the disintegration. So if the tablet is too hard, it may not disintegrate in the required period of time. And if the tablet is too soft, it will not withstand the handling during subsequent.

processing such as coating or packaging.

In general, if the tablet hardness is too high, we first check its disintegration before rejecting the batch . And if the disintegration is within limit, we accept the batch.

If H. is high + disintegration is within time è accept the batch.

APPARATUS FOR MEASURING HARDNESS

1. Monsanto Hardness Tester or Stokes Hardness tester (1930).

2. Pfizer Hardness Tester (1950).

3. Strong cob Hardness Tester.

4. Heberlain Hardness Tester or Schleeniger Hardness tester.

Early measuring devices were typically hand operated. For example, the Monsanto (or Stokes) hardness tester was based on compressing tablets between two jaws via a spring gauge and screw. In the Pfizer hardness tester, the vertically mounted tablet was squeezed in a device that resembled a pair of pliers. In the Strong Cobb hardness tester, the breaking load was applied through the action of a small hydraulic pump that was first operated manually but was later motorized. Problems associated with these devices were related to operator variability in rates of loading and difficulties in proper setup and calibration. Modern testers employ mechanical drives, strain gauge–based load cells for force measurements, and electronic signal processing, and therefore are preferred.

However, the result of Strong cob hardness tester is 1.6 times accurate than the Heberlain hardness tester and 1.4 times more accurate than the Pfizer hardness tester.

Most commonly used apparatus for hardness is electronically operated hardness tester i.e. HEBERLAIN HARDNESS TESTER.

HEBERLAIN HARDNESS TESTER:

This apparatus usually have two platens between which tablets is placed and compressed and the value of the hardness is measured. The platens should be parallel. Their faces should be polished smooth and precision-ground perpendicularly to the direction of movement. Perpendicularity must be preserved during platen movement, and the mechanism should be free of any bending or torsion displacements as the load is applied. The contact faces must be larger than the area of contact with the tablet.

Either the rate of platen movement or the rate at which the compressive force is applied (i.e., the loading rate) should be constant. Maintaining a constant loading rate avoids the rapid buildup of compressive loads, which may lead to uncontrolled crushing or shear failure and greater variability in the measured breaking force. However, constant loading rate measurements may be too slow for real time monitoring of tablet production.

UNIT of measuring hardness is kg/cm2.

CRITERA: Tablet hardness should lies between 5 to 10 kg/cm2.

RESULT LIMIT: ± 5%.

Thickness and diameter of the tablets can also be checked by this Heberlain hardness tester. The mode of the apparatus is set according to the test (hardness, thickness or diameter

Factors Affecting the Hardness:

Compression of the tablet and compressive force. 

Amount of binder. (More binder à more hardness) 

Method of granulation in preparing the tablet (wet method gives more hardness than direct method, Slugging method gives the best hardness).

Limits:

5 kilograms minimum and 8 kilograms maximum.

Make hardness test on 5 tablets and then take the average hardness