Is it the Press or the Power?
Friability limits must be less than 1%. It’s commonly thought that friability is a formula related defect, and for the most part this statement is correct. However, tablet friability can be compensated for on a tablet press and in tooling design.
During compression, particles need to bind and lock together. If the powders are not cohesive, or the bond that holds particle together is broken, the result is particle loss; a friability problem.
Step 1: Study the effects of Weight Control. Controlling friability requires accurate control of tablet weight. Set the machine so weights are on target with very little variation. If weights are high this means the tablet will be harder, and low weighs means the tablet will be softer.
Insufficient or non effective binders require higher compression force to hold the particle together. A well granulated formula with a good binder can handle a wider range of compression forces and is less subject to friability failure. If the particles within the formula are friable, high compression forces can and will break particles apart. Lower compression forces may not be enough to bond particles together, again causing friability failure. The bottom line is friability can be caused by both low compression force and high compression forces and this depends on the nature of the raw materials, the process, and the type of binder in use.
Step 2: Study the effects of Dwell Time. Determine if friability is from too much, or too little force. Again, tablet weights must be accurate as described above. Slow the machine down. If friability issues improve then longer dwell is beneficial. However, if there is no improvement or if the tablet becomes more friable then too much dwell time is breaking apart the particles, or what holds the particles together (binder). Some products are very sensitive to the slightest change in pressure/force, this is why weights must be accurate. At some point all particles will reach maximum compressibility. When they reach that point the ability to lock to other particles is broken. If the binder that locks granules together breaks apart from force the result is the same.
Step 3: Study the effects of Tablet Expansion. When a tablet is ejected it expands. If the expansion is too rapid, or too great the particles break apart. A common fix is to add a taper in the die to allow for a more gentle release. Measure the expansion. The taper in the die should be about 70-80% of the expansion. In other words if the tablet expands .010” then the taper should be .007”-.008” at 6mm depth.
Step 4: Study the effects of Air Release. Entrapped air can cause the tablet to break apart. Slowing the press will extend dwell time, and give the air more time to escape. Press speed and the amount of clearance between the upper punch tip and the die also contributes to the air release. For many powders increasing the venting by increasing the taper or increasing the tip/die clearance, slowing the machine down, and using pre-compression correctly all contribute to improved friability.
Step 5: Study the effects of Tooling Condition. Die wear rings can cause friability issues. Make certain dies are inspected and replaced when wear rings are detected. Cup depth – The deeper the cup the more sensitive the tablet is to air entrapment and compressibility. Lettering, bisects, and logos all should be on the upper punch, and it is best to leave the lower punch cup plain if possible. Cup edge land and a compound radius within the cup will also contribute to better friability results. Polished punches and a formula with a well blended lubricant all contribute to the success of a tablet.
Friability is a defect: it has many root causes and for anyone to say it is simply a formula issue is somewhat misleading. Contact me to discuss your friability issues.