During the development of coating formulae and processes, common problems tend to recur. This section brings together a collection of 25 typical queries and provides suggested solutions.
- 1 Question 1: What major process and formulation parameters do I need to take into account in the change from organic solvent coating to aqueous coating?
- 2 Question 2: Do I need to stir a coating suspension and for how long can I keep it?
- 3 Question 3: What quality of water should I be using?
- 4 Question 4: When do I need to add a plasticizer to a coating formula?
- 5 Question 5: Are there any detrimental effects caused by using high pigment concentrations?
- 6 Question 6: What is the effect of restrictions in the use of certain organic solvents?
- 7 Question 7: What problems are there for coating moisture-sensitive tablets or tablets containing water-soluble materials?
- 8 Question 8: How do I cure logo bridging on an existing tablet design?
Question 1: 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 water-insoluble 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, the core cannot be protected by the initial application of a relatively large quantity of spray material.
As a consequence of changing from organic solvent-based systems to aqueous-based processing, the following phenomena may also be observed.
- A decrease in adhesion of the film for the core. This may be remedied by a formula
modification, as described in “Common Defects in Film Coating Process: Causes and Possible Solutions”.
- The coated tablets have a distinctly matt appearance compared with organic solvent-based processing.
- Shade changes, compared with the organic solvent-based process may be observed even
when utilizing the same pigments.
Question 2: Do I need to stir a coating suspension and for how long can I keep it?
Answer: A well-milled suspension or a good-quality commercial coating system will need relatively little stirring. However, with formulations containing large quantities of iron oxides and/or talc, stirring should be more or less continuous, especially with talc as a constituent. Cellulosic systems in organic solvents, because of their relatively lower viscosity, will generally settle out more quickly than corresponding aqueous systems.
Many aqueous-based coating formulae are susceptible to microbial growth. Large quantities of polymer solution made up for incorporation into batches of final coating suspension will need to be preserved. Commercial coating systems can be constituted in small quantities minimizing waste at the end of the processing period and the consequential need to store and preserve suspension. Unpreserved coating suspensions should be discarded at the end of a working shift and certainly within 12 h of make-up to prevent undue microbial growth.
Note that foam generation either from reconstitution of a commercial system or from milling of an ‘in house’ mixture should be minimized. Unfortunately, foam on coating suspensions is very stable and difficult to remove. Excessive aeration makes for difficult handling of the suspension.
Question 3: What quality of water should I be using?
Answer: Compendial purified water should be used for making coating suspensions of aqueous systems.
Question 4: When do I need to add a plasticizer to a coating formula?
Answer: Generally, plasticizers are added to coating formulae to make them more universally applicable and to avoid potential coating problems, e.g. cracking, poor adhesion. Some acrylic systems do not need a plasticizer, e.g. NE30D, due to the specialized nature of the polymer used in this latex preparation.
For many cellulosic systems, water is a plasticizer but reliance on it is not recommended as it is not permanent within the film and can give rise to problems on storage.
Question 5: Are there any detrimental effects caused by using high pigment concentrations?
Answer: Excessively high pigment levels can give rise to brittle films which are rather rough in appearance. However, if moisture vapor permeation is a problem then increasing pigment content slightly will usually be advantageous, but excessive quantities may actually increase permeation through destruction of the integrity of the film.
It should be noted that the deleterious effect of pigments, can to some extent, be overcome by the use of good-quality small particle size pigments.
Question 6: What is the effect of restrictions in the use of certain organic solvents?
Answer: Legislation in many parts of the world, with environmental and worker protection considerations in mind, has the effect of removing certain solvents from use as process solvents. Frequently, these measures involve chlorinated hydrocarbons which are used as cosolvents with alcohols in solubilizing cellulose ethers.
Obviously consequential changes for the future would be:
- a move to aqueous spraying;
- a move to totally enclosed coating processes with solvent recovery system;
- a move to polymers with different solubility requirements, e.g. the acrylates.
Question 7: What problems are there for coating moisture-sensitive tablets or tablets containing water-soluble materials?
Answer: First, attention should be given to the drying conditions in terms of air temperature and quantity. For aqueous processing both of these should be high. As an example, in a Model 120 Accelacota an inlet temperature of 75–80°C coupled with an air volume of 56–60m3/min and a low spray rate should be used.
With an aqueous system, maximize the solids content to above 12 %w/w if possible so that a low water content is used. The intrinsic permeability of the film should be determined experimentally for a moisture-sensitive core. Adjust pigment and plasticizer levels to minimize moisture vapor transmission.
For particularly troublesome cores, consider a change to non-aqueous coating if this is feasible.
Question 8: How do I cure logo bridging on an existing tablet design?
Answer: The aim here should be to increase contact between the tablet core and the film; perhaps the most effective way of doing this is to reduce the internal stress in the film itself.
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