Some common practical questions and suggested answers During the development of coating formulae and processes, common problems tend to recur. This section brings together a collection of typical queries and provides suggested solutions. 1. Question: What major process and formulation parameters do I need to take into account in the change from organic solvent coating to aqueous coating? Answer: Looking first of all at the formulation-based parameters, there is a need to increase the solids loading of the coating suspension to something like 12 %w/w if using a typical HPMC-based formula. Maximizing solids will usefully minimize the water content. | Page 439 15 Some common practical questions and suggested answers During the development of coating formulae and processes common problems tend to recur. This section brings together a collection of typical queries and provides suggested solutions. 1. Question What major process and formulation parameters do I need to take into account in the change from organic solvent coating to aqueous coating Answer Looking first of all at the formulation-based parameters there is a need to increase the solids loading of the coating suspension to something like 12 w w if using a typical HPMC-based formula. Maximizing solids will usefully minimize the water content of the suspension but excessively viscous suspension will be difficult to spray. Commonly organic solvent-based formulae normally contain HPMC viscosity grades of 15 mPa s or even higher. These should be substituted by lower viscosity types such as 5 mPa s. Ethylcellulose is used frequently in organic solvent-based formulae and of course will in its simplest form have to be omitted from a totally aqueous formula due to its insolubility. However use of aqueous dispersions of ethylcellulose Surelease Aquacoat are recommended if a waterinsoluble functional coat is required. Regarding the tablet core formula this needs to be more robust to take into account the rather longer spraying times which may be necessary with water-based spraying. Moisture-sensitive actives are not necessarily a problem in a well-controlled process. The obvious difficulty from a processing point of view is that water a liquid with a relatively higher latent heat of evaporation has to be removed from the process. This necessitates higher process temperatures additional quantities of drying air and generally lower rates of spray application. The initial application of spray demands extra caution as unlike organic solvent-based spraying Page 440 the core cannot be protected by the initial application of a relatively large quantity of spray material.
