Measuring the hardness of a tablet is not a reliable indicator for tablet strength as some formulations when compressed into very hard tablets tend to 'cap' or lose their crown portions on attrition. Such tablets tend to powder, chip and fragment. (1)
They not only lack elegance and consumer acceptance but also spoil the areas of manufacturing such as coating and packaging.
In friability test the tablets are prone to abrasion hence enabling us to check for the tablet strength under application of force in different manner.
The friability test is carried out in an instrument called a friabilator. A friability testing apparatus should stimulate the conditions that the product will be exposed to during the process of production. This test is a method to determine physical strength of uncoated tablets upon exposure to mechanical shock and attrition.
The commonly used friabilator in laboratories is the Roche friabilator (2).
This instrument consists of a plastic chamber for placing the tablets which is attached to a horizontal axis. The drum has an inside diameter of 287mm and is about 38mm in depth, made of a transparent synthetic polymer with polished internal surface. A set of pre weighed tablets [if one tablet weigh 650mg or less then approx 6.5g of total weight should be taken and for more than 650mg/tablet weight, 10 tablets should be taken] (3) are placed in the plastic chamber revolving at 24-25rpm for 4 min. The tablets are subjected to combined effects of abrasion and shock. The tablets are dropped at a distance of six inches on each revolution.
If the tablet size or shape becomes irregular adjust the drum so that base forms an angle of about 10 degrees with bench top and the tablets fall freely when drum is rotated.
The instrument is operated for 100 revolutions after which the tablets are dusted and reweighed.
Conventional compressed tablets that lose less than 0.5% to 1% of weight are considered acceptable.
In case of hygroscopic tablets a humidity-controlled environment (relative humidity less than 40%) is required for testing.
Tablets prone to capping during the test are considered unfit for commercial use.
1) Punches that are in poor condition or worn at their surface edges, resulting in 'whiskering' at the tablet edge and show higher than normal friability values.
2) Friability test is influenced by internal factors like the moisture content of tablet granules and finished tablets. Moisture at low and acceptable level acts as a binder
There is no standard method established for evaluating friability of pellets. The friability of pellets was determined using a rotating drum like apparatus (Roche friabilator). But due to the low weight of pellets the mechanical stress applied is less. This can be corrected by adding glass or steel balls to increase stress.
The best method for testing the friability of pellets was the AIR STREAM method. (5)
In this method the fines were removed through sieving and approximately 8g(m1)of pellets were filled in glass apparatus. The apparatus was closed using a sieve lid and the pellets were subjected to air stream. After 16 min the pellets were removed and reweighed (m2). Each batch was tested 3 times (n).The friability was calculated as percentage weight loss according to the equation:
An abrasion drum was a modified USP friability tester.
This drum can generate two different types of motion depending on how the abrasion drum is mounted to the friabilator arm.
One motion generates cascading movement from one lamella to other, while the other motion raises and drops the spheres from a distance approx 200mm.
This method was made more effective by adding 1mm glass beads to the pellets in order to increase stress level on pellets (Generally 10g of pellets and 25g of glass sphere are taken and rotated for 25rpm for 10 min).
(1) Lachman L, Lieberman H, Kanig J. The theory and practice of industrial pharmacy. Varghese publication house, 3rd edition, page-299.
(2)Japanese pharmacopeia, 14th edition page-1327.
(3) US Pharmacopoeia 29 NF 24. http://www.pharmacopeia.cn/v29240/usp29nf24s0_c1216.html (accessed on 20 Sept' 2010)
(4)www.meggle-pharma.de/de/literature/articles/332/332.pdf (accessed on 25 Sept' 2010)
(5) Pharmaceutical pelletization technology, 1st Ghebre-Selassic(ed.), Marcel dekker, NewYork ,1989; page- 261-265.
(6)Pisek R, Korselj V,Vrecer F. Comparison of Direct Rotor Pelletization(Fluid Bed) and Hign Shear Pelletization Method for Pellet Production. Pharm and Biopharm 2002 (53): 327-333.
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