Most of enteric coating polymers dissolve well in organic solvents and easy to evaporate, giving a stable coating solution that facilitates faster coating processes.

But, the practical use of organic solvents in pharmaceutical formulations has decreased since organic solvent residues in final products are restricted by the authorities in some countries, why?
  1. Organic solvents are highly flammable, require special handling and equipment
  2. Toxic to operators and harmful to the environment
  3. Organic solvents are more costly, add cost to the final product
  4. Toxicity potential of traces of the residual solvents in the tablet coating 
These concerns encourage the use of aqueous dispersion systems with 30-40% wt. dry polymer dispersed in water systems, assisted by surfactants. The last years efforts have been made to develop ready to use dispersions which include all auxiliary components such as plasticizers, opacifiers, and antifoaming agents.

The most popular material for aqueous dispersion of enteric coating is EUDRAGIT L30D. This is a copolymer that is anionic in character and based on polymethacrylic acid and acrylic acid esters.

The Organic Solution and Aqueous Dispersion Differences of Film Formation Process.

The film formation process of organic solvents and aqueous dispersions is basically different.
The polymer in the organic solutions undergoes sol to gel transitions during solvent evaporation whereas polymer particles in aqueous dispersions deposit layer by layer on the surfaces of the coating substrates.

When water evaporates, polymer particles approach each other, due to capillary force, and gradually fuse to a uniform layer. The size of polymer particles influence the film formation. The smaller the particles are, the larger the contact area between the polymer particles becomes. This accelerates polymer coalescence. By consequence a lower amount of dry polymer is required for the enteric protection
A special advantage of aqueous synthetic polymer dispersions employed is that the water is merely a dispersing agent and not a solvent for the polymer. This means that water is not retained by the lacquer substance during the formation of the film but evaporates rapidly and almost completely. This aspect is extremely important especially for coating drugs that are highly moisture sensitive such as aspirin.

Disadvantages/Limitation of aqueous dispersion enteric coating :
  1. coating process take longer time than organic solvent systems as there is more energy required to evaporate water than for solvents.
  2. deterioration of heat- and/or moisture-sensitive drugs during coating processes could be increase.
  3. susceptible to coagulation because of a number of factors, such as additions of fine powder pigments or wetting agents, high shear gradients during mixing and pH change. 
  4. Poor film adhesion.
  5. Poor tablet finish due to high viscosity of coating solution
  6. Uneven surface of finish product
  7. Non-uniform colour of finished product

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