Issue link: https://www.e-digitaleditions.com/i/398011
16 October 2014 Tablets & Capsules DC2 consists of thicker, more rounded particles. The parti- cle size and shape attributes of DC2 are tightly controlled, but the details of those attributes will not be discussed for proprietary reasons. Regardless, the DC2 particle morphol- ogy results in better dry powder flow, which was quantified using an internally developed funnel flow test (Figure 3) [9]. Figure 4 shows the results of flow-rate testing on four to five samples each of the K4M and K100M molecular-weight (MW) grades of DC2 and CR. A powder flow rate of 0 grams per second (g/s) was assigned to the K4M CR and the K100M CR samples because the glass funnel had to be tapped to clear it of powder, which had bridged within the funnel. No tap- ping was needed to clear the funnel of the Methocel DC2 samples. K4M DC2 flowed more readily than K100M DC2 (78 to 103 g/s versus 51 to 58 g/s). The dif- ferences in powder flow rates between the two MW grades of DC2 are likely due to the different cellulose raw materials used to achieve the desired MW of each product. Typically, cotton linters are used to produce higher-MW cellulose derivatives, while wood pulp is used to make lower-MW cellulose derivatives. Methocel DC2 and Methocel CR exhibit similar aver- age bulk densities (Figure 5a), but the bulk density data from the individual DC2 measurements are more repro- ducible than those of CR, an indication of more consistent morphological attributes. The K4M DC2 and K4M CR samples exhibited average bulk densities of 296 (±4) kilo- grams per cubic meter (kg/m 3 ) and 289 (±25) kg/m 3 . The K100M DC2 and K100M CR samples exhibited average bulk densities of 259 (±16) kg/m 3 and 272 (±35) kg/m 3 . As noted above, the differences in bulk density between the two MW grades are likely due to the different types of cellulose used as starting materials. Although the average bulk densities of the morphol- ogy grades are similar for a given MW grade, their per- meabilities to air under increasing consolidating load are notably different. One would expect that a powder with a more consistent density would be less permeable to air. Conversely, a lower density powder would have a greater void volume and thus be more permeable to air. Yet the permeability of Methocel DC2 is significantly greater than that of Methocel CR (Figure 5b). That is sur- prising because the bulk density of DC2 is similar (but more reproducible) than that of CR. DC2's higher perme- ability at a comparable density indicates that it exhibits more fluid-like behavior and can exit a hopper outlet more readily, travel through a feeder better, and fill a tablet press die more evenly. Powder permeability signifi- cantly influences the discharge rate from hoppers, and as noted above, DC2 flowed freely from the glass funnel, whereas CR had a zero discharge rate (Figure 4). The fluid-like flow of DC2 should allow greater control over the direct-compression process, resulting in better tablet- to-tablet consistency. Although the bulk density of DC2 is more reproducible and its permeability is higher, under an increasing consolidation load it densifies to a lesser extent than CR. In other words, DC2 exhibits more fluid- Figure 3 Comparison of Methocel DC2 and Methocel CR powder flow properties a. Start a. After 8 seconds a. After 14 seconds Figure 4 Both grades of Methocel DC2 exhibited higher powder flow rates than Methocel CR DC2 CR 100 80 60 40 20 0 DC2 CR K100M KM4 Powder bulk flow rate (g/s)