Challenges in coating

David Graham. Technical Sales Manager at Coating Systems.One of the biggest changes in recent years that has impacted tablet coating technologies is the introduction of 21 CFR Part 11, which deals with electronic records and electronic signatures. This regulation has helped simplify and automate data recording by moving towards paperless systems that can capture events in real time. Although this has been beneficial for some companies, for others it has not given any real benefit and they have adopted procedures to work around the requirements.

As has been seen with other pharmaceutical manufacturing equipment, there is a growing drive to utilise simpler, more user-friendly machines that are more cost‑effective and flexible. Flexibility in batch size for a single machine is particularly critical in the tablet coating industry and, as such, machines are being developed that feature many batch size options. Usually a coating pan has the option to work from a full load down to 50% of capacity. Nowadays, however, the industry is pushing for a 70% reduction on batch sizes. Given the industry standard 60 in. drum, a nominal 350 kg load would be reduced to 105 kg. This is achievable with the correct set up and knowledge. Machines that offer a wide range of operation are essential considering the fact that many tablet coating manufacturers use a variety of product shapes, and require different batch sizes and different machine loading options. Addressing flexibility issues requires an equipment provider to work closely with customers to meet their needs and not the industry offering what it wants to manufacture. Traditionally, you have drums of 60 in., 48 in., and 24 in., giving batches of 350 kg, 150 kg and 12 kg, respectively. There isn’t much variation on this to simplify machine production and cost.

Main challenges

Cost is always a principle driver for manufacturers — the faster you can coat a batch the more economical the machine, but it is also important to balance coating time versus quality by achieving the maximum spray rate with minimal application time. Time savings can be made by loading and unloading batches more quickly, although care must be taken to avoid damaging the tablets in the process, and also in cleaning by using automatic methods or by cleaning when production is not taking place. In many cases, advances in coating systems have reduced the cost of ownership by reducing unproductive time loading, unloading and cleaning. If the manufacturer can get an extra batch completed within one operator shift then the machine’s profitability is greatly increased.

The cost of coating materials is also an important consideration; however, it is possible to reduce the volume of liquid for a given coating application to help balance the cost (the less water you spray the less you need to dry). The concentration of coating solutions can give disadvantages if the cost of the materials is greater than the operational cost.

Rapid coating times require faster drying and better mixing, which has led to advances in technology; for instance, the industry is now moving away from traditional spray nozzles to more high‑tech systems, such as pumps that feature an individual spray line to nozzle with anti‑bearding properties. Most companies have adopted similar models of operation and a standard practice for coating has evolved. However, a lot of the technologies used in coating have now reached a plateau; for instance, mixing is now at a nominal level for average strength tablets because if it was any faster then the damage to the tablets would be detrimental to the coating process. Air flow is also limited because of pressure on the tablet bed — pulling more air through the tablet bed increases the pressure on the coated tablets, but does not necessarily reduce the drying time. As current practices for coating is sufficient, manufacturers are unlikely to change their processes unless they see any major advantages. One weakness in current operating models, however, relates to coating validation. The industry validates spray systems based on nozzle pressure, but this can be difficult because a blocked pipe will always report the correct pressure without flow.

Another area where there is room for improvement is in machine operating efficiency because the standard coating process for all machines is very wasteful. A standard model will take air from the outside and heat it up to 60 °C, and will have exhaust air at about 45 °C. Fortunately, technology is now available to help the industry to adopt environmental considerations; for instance, with a plate heat exchanger, the exhaust air can be used to preheat the incoming air to about 30 °C (depending on the installation), saving on the energy needed for heating. This will lead to cost savings and will also support government initiatives aimed at encouraging companies to be more environmentally friendly.