Issue link: https://www.e-digitaleditions.com/i/807837
14 April 2017 Tablets & Capsules must adjust the target weight gain. Table 1 illustrates the effect of tablet size on film thickness. Although film thickness is the target parameter of any coating process, it is very seldom used to determine the endpoint of coating runs. In fact, in most cases, it is not measured at all. Rather, for reasons of practicality, weight gain (WG) is usually used to determine the endpoint of the coating process. As shown in Equation 2, weight gain can be expressed as milligrams of film per square cen- timeter of substrate surface. WG ( ) = It can also be expressed as a percentage of tablet mass, as shown in Equation 3. WG(%)= Coating suspension As Equation 2 shows, high concentrations of solids enable you to conduct shorter coating runs because the process attains the target weight gain in less time. At a glance, this appears to be a favorable exchange, but there are two major limitations to maximizing the spray rate and concentration. The first limitation: A linear increase in the solids con- centration of the suspension leads to an exponential growth in its viscosity. That may pose a problem because the pump that delivers the coating to the spray appara- tus—usually a peristaltic pump—may not have the power to handle highly viscous liquids. Therefore, it is essential to determine the actual spray rates of a given liquid delivery system at different pump speeds and for different viscosi- ties. See Figure 1. The second limitation: High solids concentrations (high viscosities) negatively affect the atomization of the liquid, and the droplets may not be fine enough. Furthermore, the droplets may not spread over the tablet surface upon impingement, which will lead to uneven film formation. Drying conditions Drying conditions also strongly influence film smoothness and are reflected in the temperature and rela- tive humidity (RH) in the product bed. Drying condi- tions are a function of the spray rate and the drying capacity, i.e., how well the drying air can take up water. This is defined by the inlet-air temperature and air vol- ume, as well as by the heat loss of the equipment. The effect of the inlet-air humidity is often misunderstood. While higher inlet-air humidity leads to a higher RH in the product bed, it does not affect the total amount of water that can be evaporated. Blending The duration of a coating run, or more specifically, the spraying time, is defined by the target weight gain, the concentration of the solution, and the spray rate, as shown in Equation 2. The intensity of blending must be balanced with the duration of the spraying step. Obviously, a brief spraying step requires intense blending to achieve a homogeneous distribution of the film, whereas a process with long spray times can blend at a lower intensity. In drum coaters—also known as pan coaters—"blend- ing intensity" correlates directly with drum speed. In fluid-bed systems, blending is a function of the drying- air's velocity and volume, which means a single parameter controls both the blending and drying conditions and thus film smoothness. The ability to adjust the blending intensity and drying conditions independently is an important advantage of drum coaters over fluid-bed coaters. Mapping the process parameters As Figure 2 shows, it is possible to map out all of the parameters discussed above and their interdependence. The map illustrates how two target parameters—the uniformity and smoothness of the final coating—can be controlled. Coating uniformity depends on the spray duration, blending intensity, and film thickness. As noted above, thickness is a function of the target weight gain and the sur- face area of the substrate. Weight gain, in turn, is a function of the spray duration, coating concentration, spray rate, and tablet mass. Film smoothness is controlled by the droplet size, coating viscosity, and drying conditions. Table 1 Coated to the same target weight gain, smaller tablets exhibit significantly lower film thickness than larger ones Figure 1 The effect of suspension viscosity on spray rates Pump speed Actual spray rate 100 mPas 200 mPas 300 mPas Diameter (mm) 11 8 Height (mm) 2.5 2 Weight gain (%) 6.0 6.0 Film thickness (microns) 51 39 mg cm 2 (Concentration x Spray rate x Duration) (1,000 x Number of tablets x Tablet surface area) (2) (Concentration x Spray rate x Duration) (Total tablet mass) (3)