Interrelationships Between Coating Uniformity and Efficiency in Pan Coating Processes

Abstract

The relationships between coating uniformity and efficiency were explored for tablet coating processes in pan coaters. The factors affecting the size of the spray zone were modeled using one-dimensional deposition analysis of spray droplets. This model was incorporated into the analytical model developed for coating uniformity by Choi et al. (AAPS PharmSciTech 22(7), 2021) that farther elucidated the effects of tablet shape and bed porosity. The results were compared with literature data on coating efficiency. The variables examined included tablet shape and size, coating time, pan speed, atomizing and pattern air flow rates, bed porosity, spray rate, batch size, coating solution concentration, spray gun-to-bed distance, and pan diameter. It is shown that, except for pan diameter and atomizing air flow rate, variables that improve coating efficiency adversely affected coating uniformity and vice versa. Implications of these relationships are discussed to improve formulation, process, and equipment designs.

Appendices

Appendix 1. Tablet diameter, surface area, and shape factor relationships

In deriving the mathematical relationships, tablet size is expressed in terms of its characteristic spherical diameter (i.e., the volume-equivalent diameter, d_v), which is given by:

$${d}_{v}={\left(\frac{6{v}_{\mathrm{tab}}}{\pi }\right)}^\frac{1}{3}={\left(\frac{6}{\pi }\frac{{w}_{\mathrm{tab}}}{{\rho }_{\mathrm{tab}}}\right)}^\frac{1}{3}$$

(A1)

where v_tab, w_tab, and $\rho$ _tab denote the volume, weight, and density of the tablet. Note that the surface area of a sphere with this diameter (a_v) is given by

$${a}_{v}=\pi {d}_{v}^{2}$$

(A2)

Substituting Eq. A1 and ${v}_{\mathrm{tab}}={w}_{\mathrm{tab}}/{\rho }_{\mathrm{tab}}$ into A2 gives

$${a}_{v}=\pi {\left(\frac{6{v}_{\mathrm{tab}}}{\pi }\right)}^\frac{2}{3}\approx 4.8{\left(\frac{w}{\rho }\right)}_{\mathrm{tab}}^{0.67}$$

(A3)

where w_tab and $\rho$ _tab are the weight and density of a tablet, respectively. For non-spherical tablets, the surface area of the tablet (a_tab) is larger than a_v. Substituting this equation into Eq. 27 results in

$$\psi \approx \frac{4.8{\left(\frac{w}{\rho }\right)}_{\mathrm{tab}}^{0.67}}{{a}_{\mathrm{tab}}}$$

(28)

Appendix 2. Shape factor, coating efficiency, and film thickness relationships

For small film volumes (v_f) relative to the total tablet volume, the film thickness (l_f) can be approximated by dividing the film volume by the tablet surface area:

$${l}_{f}={v}_{f}/{a}_{\mathrm{tab}}$$

(B1)

v_f is the weight of film on a tablet (w_f) divided by the density of the film (${\rho }_{f}$): i.e.,

$${v}_{f}={w}_{f}/{\rho }_{f}$$

(B2)

w_f in the pan coating process is dependent on the amount of the coating applied to the tablet (w_c) and the coating efficiency (η_c):

$${w}_{f}={w}_{c}{\eta }_{c}=\frac{{W}_{c}}{N}{\eta }_{c}=\frac{{W}_{c}}{{W}_{u}/{w}_{u}}{\eta }_{c}$$

(B3)

Note that w_c is given by the total coating quantity divided by the number of tablets (= W_c/N) and the number of tablets is given by the feed weight of the uncoated tablets (W_u) divided by the uncoated tablet weight (w_u).

Combining Eqs. 28, B1, B2, and B3 results in the relationship between w_c, η_c, ${\rho }_{f}$, a_tab, and l_f:

$${l}_{f}=\frac{{w}_{c}{\eta }_{c}}{{\rho }_{f}{a}_{\mathrm{tab}}}=\frac{0.21\psi }{{\left(\frac{w}{\rho }\right)}_{\mathrm{tab}}^{0.67}}\frac{{w}_{c}{\eta }_{c}}{{\rho }_{f}}$$

(B4)