Rapid expansion of supercritical solutions (RESS) of poly(trifluoroethyl methacrylate), poly(TFEMA), was performed to produce ultrafine particles for spray coating application to improve the hydrophobicity of moisture-sensitive biodegradable materials. Carbon dioxide (CO2) was used as the RESS solvent. Thermoplastic starch/poly(butylene adipate-co-terephthalate) (TPS/PBAT, 60:40 wt/wt) blend was used as the coating substrate. The objectives of this work were to determine the capacity of the RESS process for coating TPS-based material with poly(TFEMA), and to investigate the effect of RESS parameters i.e. pre-expansion pressure and temperature (P-pre, T-pre) and poly(TFEMA) concentration - on the surface morphology and hydrophobicity of the coated materials. It was found that RESS produced poly(TFEMA) particles precipitated onto the surface of the TPS/PBAT substrate, with particle sizes ranging from 30 nm to several microns, depending on processing parameters. Rapid expansion of fluoropolymer solutions (0.3-1.0 wt%) with P-pre of 331 bar initiated from unsaturated conditions produced nanoparticles with a narrow size distribution of similar to 30-70 nm; whereas larger particles with broader size distributions and a lower degree of agglomeration were obtained when supersaturated solutions were expanded with Ppre of 172 bar, especially at T-pre (80 degrees C) - higher than the glass transition temperature (73 degrees C) of poly(TFEMA). The surface coverage by the fluoropolymer increased with increasing Pp and poly(TFEMA) concentration, but decreased with increasing T-pre. In addition, the hydrophobicity of the coated substrate, determined by water contact angle and water vapor transmission rate measurements, increased with increasing surface coverage. (C) 2014 Elsevier B.V. All rights reserved.