Solvent or Aqueous Film Coating

By Suresh Pareek & Chetan Rajsharad

Chetan Rajsharad

Suresh Pareek

Tablet coating, which is known to pharmaceutical industry for more than 200 years has undergone several changes during last 25 years owing to the rapid development in pharmaceutical technology. As the industry is continuously striving to provide modern medicines, the volumes required to be produced has increased manifold. Due to the increased demand, the pressure on the industry to manufacture pharmaceuticals like sugar coated tablets in large quantities in short period of time, has increased. Hence, it became necessary to invest efforts towards developing the tablet coating technology.

Research scientists in industry as well as academia have risen to this challenge and developed the process of film coating. The coating equipment available were meant for sugar coating; in order to adapt the process of film coating, one had to change the solvent medium from water to organic solvents as water being less volatile was causing problems in sufficient drying. Certainly the use of organic solvents was much easier to use and provide good opportunity to convert sugar coating, a time consuming and skilled operation, to a much faster process of film coating.

Different solvents which were used (in some cases still being used) for different application are (either alone or in combination) :

• Ethyl Alcohol
• Methyl Alcohol
• Isopropyl Alcohol
• Methylene Di chloride
• Acetone
• Chloroform

Such an effort to reinvent the tablet coating technology has not stopped just by developing the process of film coating. As the consumption of organic solvents increased alarmingly, the cost of these organic solvents increased. Moreover, the people concerned with the protection of the environment, raised an objection to high usage of toxic organic solvents. Hence, companies are attempting to eliminate the usage of organic solvents completely due to:

The hazards associated with using both flammable and potentially toxic solvents.

Dealing with the environmental issues that are associated with organic solvents.

The organic solvents are expensive and are likely to become more expensive in future.

The cost of insurance of the manufacturing facility is now very high if one uses the organic solvents.

The best alternative to overcome these adverse situation is to revert back to water as the solvent medium for tablet coating.

Scientists have observed several problem when converting this solvent system from organic to aqueous :

• Increase in coating suspension preparation time
• Increase in coating time
• Negative impact on the drug stability; if water is not effectively removed from coating
• Increased likelihood that the harsher coating condition (working at elevated temperatures) may affect the drug release characteristics (change in DT and dissolution profile)

The challenge is therefore, to develop suitable coating products and process to effectively remove the water from the system and yet work under suitable working conditions so as to maintain the product stability and the drug release characteristics.

From the above figures, it is clear that water requires much higher amount of energy for vaporistion. This energy for evaporation is a combination of Temperature and Air CFM (volume). In aqueous or water-based system, process parameters and set up of coating equipment play an important role. By the use of more efficient coating products and by controlling the process parameters and coating equipment set up, the whole system can be made efficient enough to make this conversion from organic solvent to aqueous possible.

Another important factor is that the compressed air should be free from moisture; air line should be fitted with suitable moisture trap and air filter or dehumidified air can be used. Similarly, number of Baffles, Inlet air CFM, Exhaust CFM and Pan RPM play an important role. It is necessary and advisable to use peristaltic pump in case of conventional coating set up.

The example has been considered for 42” Conventional pan with 80 kg tablet load of 200 mg each tablet totaling to a batch size of 4,00,000 Nos. The comparison was done for solvent and aqueous system. Considering a theoretical weight gain of three percent total film coating material required will be 2.4 kg (Sr.No.8 in the table) with a reconstitution level of five percent for organic system the total solvent required will be (100-5) / 5 X 2.4 kg. Out of this 65 percent will be MDC i.e. 29.6 kg and IPA will be 35 percent i.e. 16 kg. Taking an average cost of Rs 70 per kg for IPA and Rs 48 for MDC the total cost of solvents works out to Rs 2540 for 80 kg of tablets per batch. Assuming two batches are coated per day i.e. 50 batches per month for 25 days working, total saving is the solvent cost per month will be Rs1,27,000 totaling to Rs15.25 lakhs per annum. The saving calculated above are based on the assumptions ment-ioned and may differ as per actual conditions. There may be some changes required in the conventional coating pan set up in the following:

• Drying Air Capacity – Air Volume and heating capacity to be increased
• Increase in number of Baffles
• Exhaust Air volume
• Peristaltic Pump

The cost for the above modifications will be approx. Rs 1.50 lakhs which is recoverable in one month.

The coating process parameters have been considered for the same batch size as in the example in Table 1. Because of the difference in reconstitution level of organic (five percent ) and Aqueous Instacoat Aqua III (15 percent) the total film coating solution is 48 kg for solvent system and 16 kg for aqueous system. With a spray rate of 300 gm per minute for solvent system as compared to 120 gms for aqueous system time taken for aqueous system is 133 minutes as against 160 minutes for solvent based system thus there is a time saving of about 27 minutes per batch-16 percent.

These enable the users to successfully migrate from solvent to aqueous system, technical experts from Ideal Cures are in a position to offer services for taking commercial batches as well as for providing the technical training to the production staff.

E-mail: srp19@vsnl.com
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