Achieving Process Understanding and Control in Film Coating - Pharmaceutical Technology

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Achieving Process Understanding and Control in Film Coating
The authors describe a QbD study that was performed to optimize a coating system.


Pharmaceutical Technology
Volume 36, Issue 9, pp. s18-s21

Gloss quality

Gloss is a measure of the proportion of light that has a specular reflection from the surface. A smooth surface has a high gloss, while a rougher surface has less, because the light reflected is diffused. High percentages of solid in the dry blend can make a rougher surface that can affect the gloss. Process conditions, such as coating temperature and spray rate, may also have a significant effect on the film surface rheology, which can affect the gloss. A novo-curve gloss meter was used to measure the film-coating gloss on the tablet coating with 60 light reflection.

Regression analysis showed a weak model with R2 (adjusted) of 40.45% even though the lack-of-fit p-value was 0.000. A low R2 (adjusted) means that other significant factors were not accounted for or that the process is not in control. A weak model resulted in low predictability. It is known, however, that the gloss can be induced by the batch weight and the pan rolling time after the spraying is completed.

The statistical summary of the gloss data showed a broad range of gloss results. Analysis of variables (ANOVA) was used to deduce the main effect. Based on the ANOVA result, high levels of TiO2 and red dye had a negative effect on the gloss. High percentage of weight gain had a positive effect on gloss. High temperature and high spray rate had a positive effect on gloss but had no effect above a 45 C exhaust temperature and a 20 g/min spray rate. High solids percent is expected to cause more roughness, which can affect the light reflection. MCT level did not show a significant effect on gloss. The requirement for color matching constrains the TiO2 and red dye levels, but a minimum coating exhaust temperature of 45 C and spray rate of 20 g/min are recommended to achieve the maximum gloss.

Slip quality

Slip (i.e., surface friction) of the film coating was measured using a texture analyzer. Five tablets were measured for each experiment (i.e., a total of 2250 samples) to obtain a statistical justification, and the data were analyzed by regression. The R2 (adjusted) of 68.80% was marginally acceptable. Regression analysis indicated that spray rate, exhaust temperature, and level of MCT also have a significant effect on slip. The regression model predicted that a minimum slip of 27.56 g-force could be achieved with the recommended material composition and process conditions. Because the material composition must keep constant to match the target color, the optimal response was adjusted using weight gain and titanium dioxide and red-dye levels.

Adhesion quality

Adhesion quality was determined by measuring the tension force on a tablet. A tablet was placed on a piece of double-sided tape adhered to the top of the texture analyzer's flat platform. Another piece of tape was pressed flat to the bottom of a 25-mm, stainless-steel, cylindrical probe, compressed to 800-g force onto the tablets for 10 s, then pulled apart at 1 mm/s. Regression analysis had a R2 (adjusted) of 59.04%, which suggested unaccounted-for factors or high experimental error. The data distribution for the adhesion data was a non-Gaussian distribution, which was fitted as a 3-parameters Weibull distribution. The probability of failure based on the design space was estimated to be low (0.94%), which indicated that adhesion was not sensitive to material factors (i.e., TiO2, red dye, and MCT levels) or process factors (i.e., exhaust temperature and spray rate).


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