From Solvent to Aqueous Film Coating

Since ages tablets / pills has been the most preferred dosage form to deliver any medically active substance to human body.

These tablets/ pills have to be coated for various reasons using a wide variety of materials and processes. The majority of tablets are coated for cosmetic reasons and for identifi-cation of specific brand in market place, however, a number of products are now coated to provide some functional benefits like enteric coating, moisture protective coating, coatings for control release, flavour coating, taste mask coating etc.

Until about 1950, sugar was the first choice as coating agent for pharmaceutical preparations and much time and efforts were spent in perfecting the sugar coating techniques. Nobody ever was concerned about the problems like material cost, toxic effects due to coating or pollution etc. because the solvent used was always water. How- ever, sugar coating technique was time consuming, affecting the productivity and the quality of finished product was dependent on the skills of operator. Many a times the companies had to reschedule their production plans due to the non-availability of skillful coating operator. These problems led to the development of film coating technique which was mainly based on solutions of different polymers in various organic solvents. All these solvents are toxic in nature. As the level of understanding regarding the toxic effects of these solvents is increasing, industrial hygiene rules and FDA regulations are being tightened world over, limiting the use of these solvents and exposure of workers to these solvents. Another area of concern is the cost of these solvents, which can only be expected to increase in coming times. In today's competitive business environment any cost saved will improve the market viability and success of any product. We are, therefore, left with no other choice but to eliminate the use of organic solvents and start using water as the solvent system for tablet coating. Like any other system, aqueous film coating has some disadvantages. The main reason for using organicsolvents was to avoid possible decomposition of active ingredients and many other process related problems such as over wetting, picking and sticking etc. which may occur with aqueous coating systems. However, research and experience of industry has indicated that the decomposition of active ingredients and possible coating difficulties are not so serious issues in actual applications and all such problems can be sorted out by scientific evaluation of the reasons for these problems. Most of these problems could be categorized as :

1) Material related problems
2) Coating instrument related problems
3) Coating process related problems

In this article we will discuss various issues related to Materials used for Aqueous Film Coating.
A large number of problems observed during conversion from organic solvent based coating to aqueous film coating are related to material selected for coating formulation. Some of these problems could be :
a) Poor film adhesion
b) Poor tablet finish due to high viscosity of coating solution
c) Uneven surface of finish product
d) Non-uniform colour of finished product
e) Longer coating cycle time





To understand these coating problems, we will have to understand the properties and role of various materials used in film coating formulations.

Polymers

As the tablet coating technique was changed from sugar coating to film coating, polymers like methyl cellulose, hydroxypropyl methylcellulose, ethyl cellulose etc. became the main coating materials in place of sugar. The higher viscosity grades of HPMC though provided film with good tensile strength but produces films having poor adhesion with the core surface and very often one can easily peal-off the film from the tablet surface. The same HPMC when dissolved in water give rise to many other problems like -

• high solution viscosity
• water is a poor solvent for HPMC as compared to organic solvents, therefore, solution preparation is difficult
• water has much higher surface tension than organic solvents, material wetting is difficult resulting in poor film adhesion
• films produced using water as solvent has poor mechanical proper- ties like low tensile strength, higher modulus of elasticity and low film adhesion.

Therefore, the selection of correct polymer system is very critical for the success of aqueous coating formulation. By selecting the lower viscosity polymers, the solid content in the coating formulation can be increased which will result in lesser amount of water required which in turn can increase the coating speed. However, the lower viscosity HPMC produces the films with lower tensile strength. As described earlier the film produced by HPMC using water as solvent system may have poor film adhesion resulting in easy peel-off from the tablet surface. To overcome this problem some formulators have used the combination of HPMC and HPC. HPC provides better film adhesion to the substrate then HPMC, however, other mechanical properties of HPC are not comparable to HPMC, moreover, the cost of HPC is much higher then HPMC and thus makes the formulation economically non-viable. Various other polymers are also used in developing aqueous film coating formulations like Sod. CMC, PVA, PVP, Sod. Alginate, PEG etc. either alone or in combination.

Plastisizers
The next most important component of the coating formulation is plasticizer. A wide range of plasticizers are available to the formulator such as phthalate esters, phos-phate esters, other esters like citrates, stearates, sebacate, oleate, adipate etc. oils, glycerol, glycols etc. The important factors to be considered here are :

- Water solubility of the plasticizer : Hydrophobic plasticizers will create problems in solution preparation and can affect the D.T. and dis- solution profile of the finished product.

- Water vapour transmission rate through the film : Higher concentration of plasticizer in the film generally tends to increase the water vapour permeability.

Concentration in the coating formulation :
Higher concentration of plasticizer reduces the modulus of elasticity (a desired effect) and thus reduces the possibility of logo bridging but also reduces the tensile strength of the film (undesired effect).

- Film adhesion generally tends to increase with increased concen tration of plasticizer.

- Higher concentration of plasticizer can lead to its bleeding (making the tablet surface feel oily).

- in most of the cases presence of plasticizer improves the gloss level in the finished product (depending on the quality and concentration of the plasticizer).

- Volatility of the plasticizer : Aqueous coating generally need higher drying capacity during the coating cycle due to less volatility of water, if the plasticizer is more volatile e.g. propylene glycol, much of the plasticizer may get lost during the coating process.

Therefore, one needs to strike a balance between the desired and undesired effects of the plasticizer and optimize its concentration in the coating formulation. Many a times use of combination of plasticizer becomes necessary to achieve the most optimum results.

Additives
The properties and composition of other components of the film coating formulation also need to be considered and optimized to get the most desired effects without affecting the quality of the film. Various other components which could be used in coating formulation are -

- Pigments
- Opacifier
- Anti-tacking agent
- Film adhesion enhancer
- Sweetners
- Flavours
- Anti foaming agent

The concentration and the properties of each of these excipients can affect the quality of the resulting film, e.g.

- The commonly used colourants in sugar coating are water soluble dyes. However, the overall colour effect of these dyes depend on the dye concentration at a particular point, thickness of film at that point and the residual moisture content in the film at that point. As these parameters can differ from tablet to tablet, the colour difference among various tablets within the same batch may become very visible.

- The opacity of the film depends on the difference between the refractive index of the polymer and other components of the coating formu- lation. The lake colours used in film coating has refractive index similar to that of various polymers, thus the opacity of lake colours is very poor.

- The most commonly used anti- tacking agent is Talc, which if used in higher concentration tends to settle down from the coating suspension, thus affecting the composition of suspension during the coating process. Further, it is poor opacifier and tends to produce translucent films.

- As the aqueous film coating need higher drying capacity, the volatile matter in the flavours used may get lost, changing the nature of the flavour. These volatile matters may also interact with other components of the coating formulation and can affect their properties. One, there- fore, need to use specific flavours and incorporate them in the coating formulation in such a manner so that it does not affect the film quality.

It, therefore, once again becomes a lot of balancing act while developing the optimized coating formulation.

Effect of Residual Moisture

One should keep in mind that water is less volatile than organic solvents, and will require much better drying capacity resulting in higher energy cost to the coating process. However, exceptions do exist in optimized film coating formulations which have a very low affinity for water, and therefore, can be run at lower temperatures, higher spray rates. Ideal Cures Pvt. Ltd. has developed few such products (under INSTACOAT range of products) which dries faster and the whole coating process can be completed in the same or sometimes little less time as compared to organic solvent based coatings.

The use of organic solvents raises the possibility of residual solvent in the finish product which is increasingly becoming a concern to the regulatory agencies due to their adverse effects on the consumer health.