"Properties of siloxane coatings deposited in a reel-to-reel atmospheric pressure plasma system"

Barry Twomey

University College Dublin, Ireland
barry.twomey@ucd.ie

Atmospheric pressure plasmas have enormous potential for the surface engineering of polymers used in the packaging and textile industries. This study reports on the deposition of nanometre thick siloxane coatings on a PET web in a reel-to-reel atmospheric pressure plasma system. This system comprises two conductive liquid electrodes, between which a polymer film passes at speeds of up 20 m/min. Using a helium/oxygen plasma, coatings were deposited on the web from the siloxane precursors PDMS, TEOS, OMCTS and HMDSO. These liquid precursors retain their chemical functionality in the plasma and are carried intact to the substrate.

Through control of the precursor type, flow rate and input power the surface chemistry of the PET was tailored to form coatings which range from hydrophilic (SiOx type) to hydrophobic (SiO(CH3)x t-ype). Water contact angles in the range 10o – 110o were obtained for the coated PET. These contact angle were correlated with the chemical composition and bonding within the coatings, as determined by XPS. Increasing the hydrophilic coating properties was found to be associated with a decrease in the concentration of methyl functional groups in the coating. Coating thickness was measured using variable angle ellipsometry and typical values were in the range of 5-20nm. The coating morphology was characterised using optical profilometry and SEM.

The flow of liquid precursor into the helium/oxygen plasma was varied between 10 and 200μl/min. The resulting coating thickness was correlated with the precursor flow rates. It was observed that for a given set of deposition conditions that an increase in precursor flow rate resulted in a more hydrophobic coating. This indicates a reduction in the oxidation efficiency of the He / O2 plasma at higher liquid precursor flow rates.