Plasticizers are generally non-volatile, high boiling, non-separating substances. Plasticizers used in a polymeric system should be miscible with the polymer and exhibit little tendency for migration, exudation, evaporation, or volatilization. Many compounds can be used to plasticize polymers. Phthalate esters such as diethyl phthalate, sebacate esters such as dibutyl sebacate, and  citrate esters such as triethyl citrate and tributyl citrate are commonly used as plasticizers. Various glycol  derivatives including propylene glycol and polyethylene glycol have also been  used to plasticize polymeric films. 

Effect of Plasticizers on Mechanical Properties of Films
  • Decrease in tensile strength. Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. Tensile strength is the opposite of compressive strength and the values can be quite different.
  • Decrease in elastic modulus. An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it.
  • Increase in film elongation
  • E.g. the effect of plasticizers on the mechanical properties of cast film of H P MC i.e.; Methocel ES was studied and it was found that the low molecular weight PEGs had a better plasticizing effect as compared to higher molecular weight due to viscoelastic effect of former.
Effect on Residual Internal Stress
  • Plasticizers reduce internal stress within the films
  • They also decrease the surface tension at the polymer surface
  • When the effect on residual internal stress of plasticizer was evaluated on CAP films, the triacetin among the triacetin, DEP and Citroflex 2A had the best plasticizing effect due to lowering of residual internal stress within the film
Effect on Permeability of Films
  • Plasticizers play a very significant role in optimizing the permeability characteristics of film coat to retard the entry of water vapor and other gases.
  • As plasticizers modify the structure of polymers, they can alter the diffusion or the dissolution of permeants across the polymers
  • E.g.; the water absorption coefficient for HPMC films plasticized with PEG 400 and 1000 for both the plasticizers are higher

Plasticizers Disadvantage in Film Coating Tablet
Plasticizers also have the disadvantage of increasing film permeability, depending on the type of plasticizer. For instance, hydrophilic plasticizers are low barriers to moisture and tend to have a more pronounced effect on increasing water vapor permeability compared to oxygen permeability. This would result to the lost of controlled  release of polymeric coating.

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