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Knowledge of the physicochemical properties of various excipients used in film-coated dosage forms and how they interact with each other, as well as knowledge of the parameters of the coating process (temperature, air velocity, etc.) allows the technologist to avoid the occurrence of most defects related to internal stress.
Another example is the formation of an uneven or rough film, which is the result of premature drying of sprayed liquid droplets or the result of spraying too viscous coating solution. Uneven staining or color unevenness of the coating may result from poor distribution of the coloring agent in the coating. This defect is usually found either when using soluble dyes in aqueous coatings (when the coloring agent can migrate), or when the solvent remaining in the film changes, or when the plasticizer migrates (if the coloring agent is soluble in it).
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The use of pigments in the coating process significantly reduces the formation of this defect. However, uneven staining may result from insufficient pigment distribution in the coating solution. In conclusion, it can be noted that some defects can arise due to internal stresses, which increase in the film when it dries. One example of such defects can be the formation of cracks that occur when the tensile strength of the film is exceeded.