TABLETING
According to the above definitions, Avicel pH 102 would have a huge amount of fines (Table 1). Its average particle size is 100 µm. Nevertheless, it has an excellent flowability and is used for direct compression. Even Avicel pH 101, with an average particle size of 50 µm is sufficiently flowable to be processed on rotary tablet presses. Both examples show, that the amount of particles below 100 µm, 355 µm, or 500 µm is no suitable evaluation.
MAKING ASSUMPTIONS Nevertheless, based on this vague number the conclusion is made that the amount of fines is too large. Tis assumption is the beginning of the downwards spiral. To counteract the increase in fines, the specific roll force is increased. In principle, this is a correct consideration. As shown in Fig. 2, a larger roll force results in coarser granules. But the side effect – which is actually the main effect – is that the granules become harder and their re-compressibility decreases. Te larger granules will become too coarse. Terefore, an even smaller screen will be used (hoping that the higher roll force and the smaller screen balances both effects out). Often this is not the case, and the amount of fines (however defined) has decreased considerably. Again, the specific roll force is increased, with the known consequences. In the end, if no satisfying granule is achieved, the worst-case conclusion is: roller compaction is not possible or an additional re-circulation
Design priorities for dry granulation products must be established at the start of the development stage
step is required. Even if such a product finally comes to production, it rarely has optimal properties.
Table 1. PSD of Avicel according to specifications of DuPont
WHAT’S THE ANSWER? Te solution for escaping the spiral of “small screen – more force – smaller screen – even more force…” is to take the opposite direction. In roller compaction, screens larger than 1mm are standard. Less roll force is needed, and the granules get softer. Tis is especially important when evaluating coarser particles. Te re- compressibility becomes better, and the following tableting process becomes less problematic. Additionally, the amount of fines should not be taken as a decisive parameter. It gives no evidence about the resulting flowability of the granulate. Tis must be tested on a rotary press or filling system. It is worth pointing out that low roll
forces and large screens are no panacea. Te optimal settings are product- dependent and must be established during development.
Fig. 2. Influence of the specific roll force on the particle size distribution, example Avicel 101
Hartmut vom Bey is with Gerteis Maschinen + Processengineering & Barbara Fretter is with Solids Development Consult.
www.gerteis.com
www.scientistlive.com 47
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60
