title: Influence of additives on melt viscosity, surface tension, and film formation of dry powder coatings.
abstract: BACKGROUND
  Limited information on thermally cured dry-powder coatings used for solid dosage forms has been available in the literature.
  
  
  OBJECTIVE
  The aim of this study was to characterize the film formation process of Eudragit L 100-55 dry-powder coatings and to investigate the influence of film additives on melt viscosity and surface tension.
  
  
  METHODS
  The coating process employed no liquids and the plasticizer was combined with the polymer using hot melt extrusion. Thermoanalytical methods including differential scanning calorimetry and thermogravimetric analysis (TGA) were used to investigate the thermal properties of the dry-coating formulations. The rheological behavior of the coating formulations were characterized with the extrusion torque, and the surface energy parameters were determined from contact angle measurements. The influence of the level of triethyl citrate (TEC) as plasticizer and polyethylene glycol (PEG) 3350 in the polymer film on film formation was investigated using a digital force tester.
  
  
  RESULTS
  TGA confirmed thermal stability of all coating excipients at the investigated curing conditions. Increasing TEC levels and the addition of PEG 3350 as a low melting excipient in the coating reduced the viscosity of the polymer. Plasticization of the polymer with TEC increased the surface free energy, whereas the admixture of 10% PEG 3350 did not affect the surface free energy of Eudragit L 100-55. The spreading coefficient of the polymers over two sample tablet formulations was reduced with increasing surface free energy. During the curing process, puncture strength, and elongation of powder-cast films increased. The effect of curing time on the mechanical properties was dependent on the plasticizer content.
  
  
  CONCLUSIONS
  The incorporation of TEC and PEG 3350 into the Eudragit L 100-55 powder coating formulation improved film formation. Mechanical testing of powder-cast films showed an increase of both elongation and puncture strength over the curing process as criterion for polymer particle fusion, where film formation progressed faster at high plasticizer levels.
authors.affiliation: Drug Dynamics Institute, College of Pharmacy, The University of Texas at Austin, Austin, TX 78712, USA. sauer@mail.utexas.edu
authors.forename: Dorothea
authors.initials: D
authors.lastname: Sauer
authors.forename: James W
authors.initials: JW
authors.lastname: McGinity