Tablet Evaluation Tests/Mechanical strength of tablets
From Pharmpedia
Contents |
Mechanical strength of tablets
The mechanical strength of a tablet provides a measure of the bonding potential of the material concerned and this information is useful in the selection of excipients. An excessively strong bond may prevent rapid disintegration and subsequent dissolution of a drug. Weak bonding characteristics may limit the selection and/or proportion of excipients, such as lubricants, that would be added to the formulation.
The mechanical properties of pharmaceutical tablets are quantifiable by the friability5, hardness or crushing strength6-8, crushing strength-friability values7-8, tensile strength9-11 and brittle fracture Index9, 12.
Friability
| TOC :Tablet Evaluation Tests |
|
Introduction |
Friction and shock are the forces that most often cause tablets to chip, cap or break. The friability test is closely related to tablet hardness and is designed to evaluate the ability of the tablet to withstand abrasion in packaging, handling and shipping. It is usually measured by the use of the Roche friabilator. A number of tablets are weighed and placed in the apparatus where they are exposed to rolling and repeated shocks as they fall 6 inches in each turn within the apparatus. After four minutes of this treatment or 100 revolutions, the tablets are weighed and the weight compared with the initial weight. The loss due to abrasion is a measure of the tablet friability. The value is expressed as a percentage. A maximum weight loss of not more than 1% of the weight of the tablets being tested during the friability test is considered generally acceptable and any broken or smashed tablets are not picked up3. Normally, when capping occurs, friability values are not calculated. A thick tablet may have less tendency to cap whereas thin tablets of large diameter often show extensive capping, thus indicating that tablets with greater thickness have reduced internal stress2.
Hardness or Crushing strength
The resistance of tablets to capping, abrasion or breakage under conditions of storage, transportation and handling before usage depends on its hardness. The small and portable hardness tester was manufactured and introduced by Monsanto in the Mid 1930s. It is now designated as either the Monsanto or Stokes hardness tester. The instrument measures the force required to break the tablet when the force generated by a coil spring is applied diametrally to the tablet. The Strong-Cobb Pfizer and Schleuniger apparatus which were later introduced measures the diametrically applied force required to break the tablet.
Hardness, which is now more appropriately called crushing strength determinations are made during tablet production and are used to determine the need for pressure adjustment on tablet machine. If the tablet is too hard, it may not disintegrate in the required period of time to meet the dissolution specifications; if it is too soft, it may not be able to withstand the handling during subsequent processing such as coating or packaging and shipping operations. The force required to break the tablet is measured in kilograms and a crushing strength of 4Kg is usually considered to be the minimum for satisfactory tablets2. Oral tablets normally have a hardness of 4 to 10kg; however, hypodermic and chewable tablets are usually much softer (3 kg) and some sustained release tablets are much harder (10-20 kg).Tablet hardness have been associated with other tablet properties such as density and porosity. Hardness generally increase with normal storage of tablets and depends on the shape, chemical properties, binding agent and pressure applied during compression7,8.
Another measure of the mechanical strength of pharmaceutical tablets that have been used is the crushing strength-friability ratio (CSFR)7,8. The CS provides a measure of tablet strength while F is a measure of tablet weakness. Studies have shown that the higher the CSFR values, the stronger the tablet7,8.
Tensile strength
A non-compendial method of measuring the mechanical strength of tablets that is now widely used is the tensile strength. This is the force required to break a tablet in a diametral compression test. The radial tensile strength, T, of the tablets can be calculated from the equation:
T = 2 F / π d H (1)
where F is the load needed to break the tablet, and d and H are the diameter and thickness respectively. Several precautions must be taken when using the equation. Various factors e.g. test conditions, deformation properties of the material, adhesion conditions between compact and its support and tablet shape may influence the measurements of the tensile strength 6.
Some authors have suggested the determination of axial tensile strength because of the sensitivity of the radial tensile strength measurements to crack propagation variations13-14. The axial tensile strength (Tx) can be calculated from the following relationship:
Tx = 4 F / π d2 (2)
Tensile strength has been used in combination with indentation hardness to evaluate tabletting performance of materials12. The indentation hardness is a time-dependent property used to measure the plastic yield of a material. It can be determined by either static methods (e.g. the Brinell, Vickers and Rockwell hardness tests) or the dynamic methods15. The static indentation methods involve the formation of a permanent indentation on the surface of the material tested and the hardness is determined by means of the load applied and the size of the indentation formed16. In the dynamic indentation tests, either a pendulum is allowed to strike from a known distance or an indenter is allowed to fall under gravity unto the surface of the test material. The hardness is then determined from the rebound height of the pendulum or the volume of the resulting indentation. Using an apparatus consisting of a steel sphere pendulum acting as an indenter, Hiestand et al.12 estimated the hardness (i. e. the mean deformation pressure) of compacted materials by dividing the energy consumed during the impact by the volume of indentation.
Brittle fracture index (BFI)
Hiestand et al.17-18 have studied the effects of decompression on the tabletting performance of pharmaceutical materials and stated that whether or not fracture occurs during the shear deformation which accompanies decompression depends on the ability of the materials to relieve stresses by plastic deformation without undergoing brittle fracture and this ability is a time-dependent phenomenon. Those materials that relieve stress rapidly are less likely to cap or laminate. The brittleness test is based on the Griffith fracture theory which teaches that, for crack growth to occur, the energy stored at the tip of a crack must just exceed the energy required to form two new surfaces resulting from the propagation of the crack. Also, the amount of energy stored at the tip of a crack is a function of the dimensions of the crack.
In the light of this theory, Hiestand et al.17 showed that when compacts are made with a small axially-oriented round hole at their centre, the compact is nearly always weakened. Under the conditions of the tensile strength test, elasticity theory predicts that the stress concentration factor for the hole should be about 3.0. Hiestand18 showed that for isotropic materials, the ratio of compressive stress at the centre of a compact to the tensile stress, which causes fracture, has a value of 3.7. However, recent studies have shown that for a ratio of hole diameter to disc of about 0.1, the stress concentration factor, i.e. the ratio between tensile stress at the inner boundary of the hole and the tensile stress of a tablet having no hole, should be around 1019,20. Thus, the BFI is obtained by comparing the tensile strength of tablets with a hole at their centre, which acts as a built-in stress concentration defect, with the tensile strength of tablets without a hole, both at the same relative density18,21. The brittle fracture index (BFI) of the tablets was calculated using the following equation
BFI = [(T / To) – 1 (3)
Where T is the tensile strength of the tablet without a hole and To, to the tensile strength of a tablet with a hole. The theoretical value of BFI range is 0 - 1 when the stress concentration factor is 3. Since the BFI is an inverse measure of localized stress relief, it should indicate the tendency of a tablet to laminate or cap. In principle, BFI values in excess of unity may occur. In practice, however, one probably cannot make an intact tablet of a material with a BFI of 1. Therefore, the observed range of values may not exceed the 0 - 1 range. Where by the closer the value of BFI to 0, the less stress relief takes place. A high value of BFI is an indication of the tendency of the tablet to laminate during the compaction process. A low BFI value is desirable for minimal lamination and capping during production9,21.
Robert and Rowe 22 extended the determination of the BFI to compact of ‘tablet-sized’ dimensions. This allows the BFI to be measured at strain rates and conditions approaching those normally used in tabletting. They found the BFI values for microcrystalline cellulose, tablettose and heavy magnesium carbonate to be in good agreement with the results of previous workers12, 17 . Itiola & Pilpel21 using both granular and powdered metronidazole formulations studied the mechanical properties of the tablets and differentiated between the bond strength of the tablets as measured by their tensile strength and the tendency of the tablets to laminate or cap as measured by the brittle fracture index values. They found that tablets made from granules had lower tensile strength than those made from powders but were also less brittle.
The BFI have also been used to characterize the mechanical properties of pharmaceutical formulations9,23-25 and some local starches, namely cassava, potato and yam starches23,24. Tablets of these starches were shown to possess low tensile strength values, but also had low BFI values. Studies have also shown that the BFI is affected by the nature and concentration of binding agent, compression pressure and compression speed. Generally, the higher the BFI values, the more friable a tablet is likely to be25.
Retrieved from "http://www.pharmpedia.com/Tablet_Evaluation_Tests/Mechanical_strength_of_tablets"
This page has been accessed 2,357 times. This page was last modified 21:19, 14 February 2006. All content, except where otherwise noted, is licensed under a Creative Commons Attribution License.
