Frictional and flow properties – ring shear cell testing results

Typical yield loci for the powders are displayed in Fig. 5.5. The internal frictional properties of the powders, which can be described by effective friction angle, e , and the flow function are obtained from the measurement. It is shown that, the internal friction properties of the powders are comparable, except for the lower values of DCPA and lactose. According to the flow function values, the sample powders can be divided into three groups (Table 5.2) according to Jenike’s classification (1967):

Table 5.2 The flowability of the sample powders.

Flow function, ffc Type of flow Powder

2 < ffc < 4 Cohesive DCPD 4 < ffc <10 Easy flowing MCC Avicel PH 101 MCC Avicel PH 102 Mannitol DCPA ffc < 10 Free flowing Lactose Mannitol Parteck M200

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Figure 5.5 Yield loci for the sample powders under 8 kPa normal load, SIGMA1 is major principal stress, FC is the unconfined stress, FFC is the flow function, RHOB is the density of

the bulk, PHIE, PHILIN, PHISF is effective friction angle, linear friction angle and friction angle for steady flow, respectively.

The shear stresses of the powders obtained in the ring shear cell tester are plotted as a function of the applied normal stresses (see in Fig 5.6). The wall friction angle, w and

corresponding coefficient, w , are calculated by adopting the linear regression between shear

and normal stress, as shown in Fig 5.7. It can be seen that di-calcium phosphate has the greatest friction coefficient, while MCC has the smallest.

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Figure 5.6 The variation of the wall shear stress with normal stress for the sample powders.

Figure 5.7 Wall friction coefficient of the sample powders again smooth stainless steel wall material with roughness of 0.31 ± 0.02 µm.

Effects of lubrication

MgSt (0.25 – 1.5% w/w) was added to MCC (Avicel PH102) and DCPD in order to investigate the effects of bulk lubrication on the frictional and flow properties. The variation of the flow function with increasing MgSt concentration is shown in Fig. 5.8. It can be seen that the flow function increases significantly with the amount of MgSt added for MCC, especially at small MgSt concentrations. On the other hand, the flow function of DCPD is insensitive to the lubricant.

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Figure 5.8 The variation of flow function of DCPD and MCC 102 as a function of the concentration of MgSt.

Figure 5.9 shows the wall shear stress as a function of the normal stress for the feed powders mixed with different amounts of MgSt; the gradients are equal to the coefficients of friction. In the case of the unlubricated powders, the gradients for DCPD and MCC are ~ 0.5 and ~ 0.09, respectively. The gradients for DCPD reduces to ~ 0.1 when 0.75% (w/w) MgSt was added to the bulk, while for MCC the values are unaffected by the addition of the lubricant. The influence of the amount of lubricant on the wall friction in terms of the angle of wall friction, _w, is shown in Fig. 5.10, together with the corresponding effective angles of friction,

e

 . The value of e for unlubricated DCPD is only slightly greater than that for MCC and it does not decrease with increasing MgSt concentration unlike MCC.

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b)

Figure 5.9 The variation of the wall shear stress with normal stress for a) DCPD and b) MCC 102, with various amounts of MgSt in the bulk.

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Figure 5.10 Frictional angles of MCC 102 & DCPD as a function of concentration of MgSt.

Effects of moisture content

In order to investigate the effects of moisture content, MCC (Avicel PH 102) and DCPD were mixed with water (2.5 – 15%, w/w). The actual moisture contents are measured using a moisture analyser (MA30 Goettingen, Sartorius, Germany) shown in Table 5.3. The effect of moisture content on the flow function of the powders is shown in Fig. 5.11. For both powders, the flow function is almost constant at low moisture contents (i.e. added water < 5% w/w), but decreases at higher moisture contents (added water > 5%). It may be noted that the variations of the flow function for both powders are not significant (i.e. ∆ffc < 3).

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Table 5.3 The moisture content of DCPD and MCC with various water contents. Powder Water content (w/w) Moisture content (w/w)

DCPD 0 5.6 ± 0.2 % 2.5% 8.4 ± 0.1% 5% 9.6 ± 0.2 % 7.5% 12.4± 0.2% 10% 15.2 ± 0.3 % 15% 20.4 ± 0.2 % MCC PH 102 0 6.3 ± 0.4 % 2.5% 9.1 ± 0.2 % 5% 11.4 ± 0.2 % 7.5% 14.1 ± 0.3 % 10% 16.4 ± 0.4 % 15% 22.0 ± 0.3%

The wall friction coefficients of the powders increases with increasing moisture content (Fig 5.12). At low moisture contents (< 2.5% added water), the wall friction coefficient of DCPD increases with the amount of added water. The influence of moisture content on the values of

e

 is shown in Fig. 5.13. The trends can be summarised by dividing the moisture contents into three regions with boundaries at 5 and 10% added water:

Table 5.4 The effect of moisture content on the internal effective friction angle for MCC and DCPD. Added water (%) Moisture content of DCPD (%) Change in e (DCPD) Moisture content of MCC (%) Change in e (MCC) < 5 5.6 – 9.6 % Decrease 6.3 – 11.4 % Decrease 5 – 10 9.6 – 15.2 % Increase 11.4 –16.4 % Increase > 10 > 15.2 % Decrease > 16.4 % Increase

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Figure 5.12 Wall friction coefficients of DCPD and MCC PH 102 at various moisture contents.

Figure 5.13 Effective internal frictional angle of DCPD and MCC with various moisture contents.

Properties of binary mixtures

In order to investigate the effects of flowability on the maximum pressure and nip angles, two binary mixtures of MCC (Avicel PH 102) and DCPD, mannitol and MCC (Avicel PH 101)

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were characterised and roll compacted. The frictional and flow properties of the mixtures are shown in Figs 5.14 – 5.16. The variation of the values of w and wall friction coefficient indicate almost ideal mixing behaviour (Figs 5.14 & 5.15). This also the case for the flow functions (Fig. 5.16) corresponding to the DCPD and MCC mixtures. For the mannitol and Avicel PH102 mixtures, the data are characterised by a relatively sharp transition region.

Figure 5.14 Effective friction angle of binary mixtures as a function of MCC content.

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Figure 5.16 Flow function of binary mixtures as a function of MCC content.