Processing behaviour

Roll compaction experiments with powders of distinctive properties, and various process parameters were carried out to explore the dependency of the process behaviour and quality of the produced compacts. The maximum pressure generated, and the nip angle in roll compaction was considered as critical factors describing the process. The density and fracture energy of the ribbons that is related to the strength and fracture potential in granulation were employed to evaluate the products. In this section, the experimental results revealing the effects of process parameters (i.e. roll gap, roll speed), powder properties (i.e. lubrication, moisture content and binary mixtures) are presented.

Effects of roll gap

Roll compaction of MCC (Avicel PH 102) and DCPD were carried out with a fixed roll speed of 1 rpm. The roll gap was varied in the range 0.6 to 2.0 mm in order to evaluate the effects

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on the behaviour of these two powders. It was noticed that DCPD powder could not be successfully compacted at the roll gap of 2.0 mm due to the leakage and insufficient compaction pressure generated (no compaction pressure recorded during the process). The variation of the maximum pressure and nip angle with the roll gap are shown in Figs 5.17 and 5.18. The maximum pressure reduces with increasing roll gap (see in Fig. 5.17). For the nip angle, the value increases with the roll gap for MCC, but decreases for DCPD (Fig. 5.18).

Figure 5.17 Effects of roll gap on maximum roll pressures.

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Effects of roll speed

In order to investigate the effects of roll speed, MCC (Avicel PH 102) and DCPD were compacted at a constant roll gap S=1.0 mm with roll speeds in the range 0.5 to 5 rpm (Petit- Renaud et al., 1998). The maximum pressures for the powders at different roll speeds are shown in Fig. 5.19. At the lowest roll speed considered (i.e. 0.5 rpm), a high maximum pressure is obtained for DCPD, which has a larger effective friction angle and wall friction angle than MCC. As the roll speed increases, a rapid decrease in the maximum pressure is observed for DCPD, which is consistent with previous work (Inghelbrecht and Remon, 1998, Petit-Renaud et al., 1998). Ribbons could not be produced at high roll speeds because the compaction pressure was too small. For MCC, the maximum pressure decreases slightly as the roll speed increases.

Figure 5.19 Effects of roll speed on maximum roll pressures.

The nip angles at the various roll speeds were determined using the proposed approach (Section 4.3.1) and are shown in Fig. 5.20. The values calculated using the methods employed by Johanson (1965) and Bindhumadhavan et al. (2005) are also superimposed. It can be seen that, for MCC (i.e. the easy flowing powder), the values of the nip angle were almost constant, while for DCPD they decreases sharply. Similar results were obtained using

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the present method (Section 4.3.1) and that proposed by Bindhumadhavan et al. (2005). However, the current approach shows advantages in the measurement of the nip angle for cohesive powder like DCPD at higher roll speed (i.e. 3 rpm, shown in Fig. 5.20) In contrast, constant nip angles were obtained using Johanson theory (1965), as the effect of roll speed was not considered. In addition, Johanson theory (1965) slightly underestimates the nip angle for MCC but overestimate the value for DCPD.

Figure 5.20 Effects of roll speed on nip angle in roll compaction.

Effects of lubrication

To explore the influence of the lubrication, two powders (DCPD, MCC Avicel PH 102) showing different sensitivity to MgSt (Vromans et al., 1988, Zuurman et al., 1999) were used. The powders are both commonly used pharmaceutical excipients but with distinctive particle sizes, surface topographies and sensitivities to lubrication with MgSt; it has been reported that the lubrication of DCPD with MgSt is very insensitive to the mixing conditions (Vromans et al., 1988), unlike MCC (Zuurman et al., 1999). MgSt of 0.25 and 1% (w/w) were employed for bulk and wall lubrication conditioning (i.e. lubricating the roll surfaces). The bulk lubricated powders were prepared by mixing MgSt and primary powders at 72 rpm

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rotating speed in a double-cone blender (5 min mixing time for MCC, 10 min mixing time for DCPD). Wall lubrication was achieved by spraying an ethanol suspension of MgSt on the roll surfaces.

The corresponding maximum roll pressures and nip angles are shown in Figs 5.21 and 5.22. For MCC, the maximum pressure and nip angle were unaffected when the roll surfaces were lubricated but they were reduced by bulk lubrication. For DCPD, both roll and bulk lubrication resulted in reductions in the nip angle and maximum pressure. However, the values of the maximum pressure and nip angle reached a constant value at 0.25 % w/w MgSt. Provided that there is sufficient MgSt to induce the maximum possible lubrication, the reduction in the nip angle and maximum pressure values are similar.

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Figure 5.22 Effects of lubrication on nip angle in roll compaction.

Effects of moisture content

Attempts were also made to roll compact DCPD and MCC (Avicel PH 102) powders with additional water. It was found that jamming occurred during the processing of DCPD. Therefore, the powder could not be successfully pressed with added water. The experimental measured maximum pressure and nip angle for MCC with various amounts of added water are shown in Figs 5.23 and 5.24. Initially, with increasing amount of added water, the maximum pressure keeps constant (amount of added water < 2.5% w/w), then increases sharply (see in Fig. 5.23), consistent with the observation by Wu et al. (2010b). However, the results show that the maximum pressure increases with the amount of added water even at high concentration which indicates the water acting as a binder(> 10% w/w), opposite to the work by Wu et al. (2010b) which reported the pressure generated for most moist MCC powders decreases due to the reduction in cohesion and increase in flowability that causes powder passing through the roll gap without compaction.Despite the constant values at low amounts of added water (< 2.5% w/w), the nip angle generally decreased with increasing moisture content, similar to the results from Wu et al. (2010b) (see in Fig. 5.24).

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Figure 5.23 Effects of water contents on maximum roll pressures for MCC 102.

Figure 5.24 Effects of water content on nip angle in roll compaction for MCC 102.

Binary mixtures

The maximum pressure and nip angle of binary mixtures of MCC (Avicel PH 102) and DCPD are shown in Fig. 5.25. It is clear that the maximum pressure increases monotonically with increasing concentration of MCC. The addition of MCC leads to an increasing flowability of the mixture and improved feeding in roll compaction, and thus an increase of

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the nip angle. However, the nip angle is primarily determined by MCC when its concentration is greater than 75%.

Figure 5.25 The maximum pressure and nip angle as a function of MCC content for MCC 102- DCPD mixtures (S=1.0 mm, u=1.0 rpm).

In this work, powders with constant volume were fed into the system and roll compacted as introduced in Section 4.3.1. During the compaction, the rolls rotate several loops (revolutions). The number of loops in a single run depends on the feeding of the material (e.g. 6 loops for MCC 102 and 12 loops for DCPD).In order to evaluate the reproducibility of the compaction results, the pressure profile from the beginning (1st loop) till the end (last loop) for a single compaction run were recorded and compared. For the binary mixtures of mannitol and MCC (Avicel PH 101), reproducible compression pressures and nip angles could not be achieved during the process. As shown in the pressure profile of the 8 loops of roll compaction (Fig. 5.26a), the compaction pressure increases gradually every loop. In addition, the corresponding nip angle increases with the processing time/loop number. In contrast, it can be seen that for pure MCC, the value of the maximum pressure remained constant after the first loop (Fig. 5.26b).

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b)

Figure 5.26 Typical pressure profiles obtained for a) Mannitol-MCC 101 mixture and b) DCPD- MCC 102 in one run (S=1.0 mm, u=1.0 rpm), angle = 0 represents the minimum roll gap.

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