Polymers 

Polymers like methyl cellulose, hydroxypropyl methylcellulose, ethyl cellulose and others became the main coating materials. 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.

If you use hydroxypropyl methylcellulose (HPMC) and dissolved in water, you must face problems, such as :
  • 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. 
Selecting the correct polymer 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.


To overcome the problem of using HPMC as aqueous dispersion coating, some formulators have used the combination of HPMC and HPC. HPC provides better film adhesion to the substrate then HPMC, but, other mechanical properties of HPC are not comparable to HPMC and theprize also more expensive than HPMC, 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.

Plasticizer
Success of enteric coating efficiency mostly relies on the addition of plasticizers. Plasticizers are a group of auxiliary components that improve elasticity of the polymeric film which is generally rigid and breakable.

Plasticizer function :
  • Plasticizers reduce the minimum film forming temperature (MFFT) of the polymers
  • softening the polymeric film at lower temperature
  • improves the spreadability of the polymer on the surface of the coating substrates
  • generates a smoother surface texture of the coating layer. 
A wide range of plasticizers are available to the formulator such as phthalate esters, phosphate esters, other esters like citrates, stearates, sebacate, oleate, adipate etc. oils, glycerol, glycols etc.

The type of plasticizer should be selected carefully as it influences the film brittleness, compatibility with the coating substrates and product stability. Hydrophilic plasticizer, triethyl citrate, is reported to improve the property of Eudragit L 30 D-55 film in the soft gelatin capsule formulations regardless of the type of filled liquid whereas hydrophobic plasticizer, tributyl citrate, gives satisfactory enteric protection only with hydrophobic filled liquid. The latter plasticizer could migrate to the hydrophobic filled liquid upon storage, resulting in the reduction of the enteric protection.

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 dissolution 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. 
Besides the plasticizer type, the amount of plasticizer is important for film flexibility.
Insufficient amount of plasticizer causes the film blistering which could lead to a premature drug release in acidic media.

However, high amount of plasticizer reduces the strength of the film, may accelerate the water uptake into the cores upon storage, reduces the modulus of elasticity (a desired effect) and thus reduces the possibility of logo bridging.

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:

1. Pigments/Colorant

The commonly used colorants in coating are water soluble dyes. However, the overall color 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 color difference among various tablets within the same batch may become very visible.

2. Opacifier

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


3. Anti-tacking agent

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.

4. Film adhesion enhancer


5. Sweetners


6. Flavours

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.

7. Anti foaming agent

The concentration and the properties of each excipients can affect the quality of the resulting film
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.


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