Tablet hardness testing

Tablet hardness testing is a laboratory technique used by the pharmaceutical industry to determine the breaking point and structural integrity of a tablet and find out how it changes "under conditions of storage, transportation, packaging and handling before usage"[1] The breaking point of a tablet is based on its shape.[2] It is similar to friability testing,[1] but they are not the same thing.

Tablet hardness testers first appeared in the 1930s.[3] In the 1950s, the Strong-Cobb tester was introduced. It was patented by Robert Albrecht on July 21, 1953.[4] and used an air pump. The tablet breaking force was based on arbitrary units referred to as Strong-Cobbs.[3] The new one gave readings that were inconsistent to those given by the older testers.[3] Later, electro-mechanical testing machines were introduced. They often include mechanisms like motor drives, and the ability to send measurements to a computer or printer.[3]

There are 2 main processes to test tablet hardness: compression testing and 3 point bend testing. For compression testing, the analyst generally aligns the tablet in a repeatable way,[2] and the tablet is squeezed between a fixed and a moving jaw. The first machines continually applied force with a spring and screw thread until the tablet started to break.[3] When the tablet fractured, the hardness was read with a sliding scale.[3]

List of common hardness testers

There are several devices used to perform this task:

  • The Monsanto tester was developed 50 years ago. The design consists of "a barrel containing a compressible spring held between 2 plungers". The tablet is placed on the lower plunger, and the upper plunger is lowered onto it.[1][5]
  • The Strong-Cobb tester forces an anvil against a stationary platform. Results are viewed from a hydraulic gauge.[5] The results are very similar to that of the Monsanto tester.[6]
  • The Pfizer tester compresses tablet between a holding anvil and a piston connected to a force-reading gauge when its plier-like handles are gripped.[5]
  • The Erweka tester tests a tablet placed on the lower anvil and a weight moving along a rail transmits pressure slowly to the tablet.[5]
  • The Dr.Schleuniger Pharmatron tester operates in a horizontal position. An electric motor drives an anvil to compress a tablet at a constant rate. The tablet is pushed against a stationary anvil until it fractures. A reading is taken from a scale indicator.[5]
  • Kraemer Elektronik's tablet testing system was the first automatic tablet hardness testing system for auto-regulation at tablet presses. Invented by the german mechanical engineer Mr. Norbert Kraemer in Darmstadt/Germany. The tablets are separated by a patented feeder chute and moved on a horizontal starwheel through different testing stations. The Kraemer Elektronik automatic tablet testing system measures weight, thickness, diameter/length, width and hardness of tablets and capsules.[7]

Units of measurement

The units of measurement of tablet hardness mostly follows standards used in materials testing – the International System of Units.

  • Kilogram (kg) – The kilogram is recognized by the SI system as the primary unit of mass.
  • Newton (N) – The Newton is the SI unit of force; the standard for tablet hardness testing. 9.807 Newtons = 1 kilogram (at one G, earth surface gravity).
  • Pound (lb) – Technically a unit of force but can also be used for mass under earth gravity. Sometimes used for tablet strength testing in North America, but it is not an SI unit. 1 kilogram = 2.204 pounds.
  • Kilopond (kp) – Not to be confused with a pound. A unit of force also called a kilogram of force. Still used today in some applications, but not recognized by the SI system. 1 kilopond = 1 kgf.
  • Strong-Cobb (SC) – An ad hoc unit of force which is a legacy of one of the first tablet hardness testing machines.[4] Although the SC is arbitrary, it was recognized as the international standard from the 1950s to the 1980s. 1 Strong-Cobb represented roughly 0.7 kilogram of force or about 7 newtons.[8] Although the Strong-Cobb unit is arbitrarily based on the dial reading of a hardness tester, it became an international standard for tablet hardness in the 1950s until it was superseded by testers using SI units in the 1980s.[6] The Strong-Cobb is a unit with a very unusual name for a unit of measurement since it is named after the company, Strong-Cobb Inc. The inventor of the hardness tester was Robert Albrecht,[4] the plant engineer for the Strong-Cobb Company. He sold the patent to the company for $1.00.

Sources
  1. Joseph Price Remington (2006). Remington: The Science And Practice Of Pharmacy. Lippincott Williams & Wilkins. ISBN 0781746736.
  2. "Tablet hardness testing". Sotax. Archived from the original on 5 January 2013. Retrieved 16 February 2013.
  3. "Some Information on Tablet Hardness Testing". Engineering Systems. Retrieved 16 February 2013.
  4. US 2645936, Robert, Albrecht, "Tablet hardness testing machine", published 1953-07-21, assigned to Strong Cobb & Company Inc.
  5. "Quality control of solid dosage form". Scribd. Retrieved 16 February 2013.
  6. McCallum, A.; Buchter, J.; Albrecht, R. (1955). "Comparison and correlation of the Strong Cobb and the Monsanto tablet hardness testers". Journal of the American Pharmaceutical Association. 44 (2): 83–85. doi:10.1002/jps.3030440208. PMID 14353719.
  7. Charles, Ischi AG (2017). "Automated tablet & capsule testing technology". Scribd.
  8. Russ Rowlett (September 1, 2004). "How Many? A Dictionary of Units of Measurement". University of North Carolina. Retrieved 16 February 2013.

Further reading
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