Issue link: https://www.e-digitaleditions.com/i/807837
Temperature The temperature of the tablet bed is another impor- tant parameter. For the best result, it must exceed the glass transition temperature of the coating's polymer- plasticizer system. The glass transition temperature is the minimum temperature at which the coating will form a film. If the bed temperature falls below it, you can nor- mally tell because the tablets will not be as shiny as those coated at or above this critical temperature. The more plasticizer a coating formulation contains, the lower its glass transition temperature. Spray gun setup The heart of any tablet coater is the spray gun system. For efficient coating, the spray guns must be checked and maintained regularly. The slightest damage to the tip of the nib or needle could have a large detrimental effect on the spray pattern and the droplet size distribution. Don't waste time trying to gauge the spray pattern and droplet size from outside the coater. The best way to assess them is to pass a sheet of paper through the spray at the start of the process at the same speed the tablets travel. While it's not a precise method, it gives you an idea of droplet size, size distribution, and how the droplets are deposited. In fact, spray deposition—how droplets behave when they impinge the tablet core—is very important to effi- ciency. The droplets must have the correct surface tension so they burst on contact with the tablet surface and form a liquid film, which is rapidly dried by the warmth of the tablet cores and air movement. For this to work as it should, most coating solutions require a viscosity of 250 to 500 centipoise. If the solution is too wet when it impinges the tablet surface, it's likely to splash back and create tiny droplets that quickly dry and are carried away from the tablet bed. Conversely, if the surface tension of the droplets is too high, they're likely to bounce off the tablet and be carried away. If all the guns are identical and maintained to the correct standard, then the spray pattern and droplet size distribution should be similar for them all. An example of an efficient spray is shown below. Once the spray guns are set up, the atomizing air determines the droplet size and size distribution. For optimal results, keep the droplet size between 20 and 30 microns. If the atomizing air is too strong, beards are likely to form on the spray guns, which will alter the spray pattern. Bearding is less of a problem today thanks to the availability of hornless spray caps. Examples of efficiency improvement It's sometimes hard to convince people to improve effi- ciency. In one case, I was accused of not coating correctly because, after 7 hours of coating a product, there was excessive weight gain on the tablets and the coater was clean. Meanwhile, the other coaters at the site were so fouled with overspray that it was impossible to see inside the drum after 1 hour of coating. And there were still about 10 hours left in the process! I spent 2 weeks at that site and I did finally persuade the technicians to change their outlook and initiate new routines. As a result, they were able to optimize their coating processes and thus forego the purchase of a new coater. They also increased capacity by about 60 percent and they no longer needed to clean after every batch. That combination of changes reduced processing times to around 8 hours. At another site it had become routine to close down the coating operation every 6 months to dismantle and clean exhaust ducts blocked by coating solids (photos). That's an expensive undertaking and shouldn't be neces- sary. And just how does that much material get into the ducts? Little by little: Every time the employees had encountered a coating problem, they removed water from the coating solution and replaced it with 96 percent 26 April 2017 Tablets & Capsules Once the spray guns are set up, the atomizing air determines the droplet size and size distribution. This is an example of an efficient spray. The buildup of solids in exhaust ducts indicates poor efficiency. It can also lead to turbulence in the drum, further diminishing efficiency. At this site, it was routine to stop the coating operation every 6 months to dismantle and clean the ducts.