Surface coating with poly(trifluoroethyl methacrylate) through rapid expansion of supercritical CO2 solutions

Abstract

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, 60:40wt/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 (Ppre, Tpre) 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 30nm to several microns, depending on processing parameters. Rapid expansion of fluoropolymer solutions (0.3–1.0wt%) with Ppre of 331bar initiated from unsaturated conditions produced nanoparticles with a narrow size distribution of ∼30–70nm; whereas larger particles with broader size distributions and a lower degree of agglomeration were obtained when supersaturated solutions were expanded with Ppre of 172bar, especially at Tpre (80°C) – higher than the glass transition temperature (73°C) of poly(TFEMA). The surface coverage by the fluoropolymer increased with increasing Ppre and poly(TFEMA) concentration, but decreased with increasing Tpre. In addition, the hydrophobicity of the coated substrate, determined by water contact angle and water vapor transmission rate measurements, increased with increasing surface coverage.