"Surface Characterisation of Siloxane Coatings Prepared using Atmospheric Pressure Plasma Liquid Deposition"

Lesley-Ann O’Hare1, Alan M. Hynes1, Morgan R Alexander2

1Dow Corning Plasma Solutions, Owenacurra Business Park, Midleton, Co Cork, Ireland
2School of Pharmacy, Boots Science Building, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

lesley.ann.ohare@dowcorning.com

Atmospheric pressure plasma liquid deposition - APPLD, combines atmospheric pressure plasma with injection of atomised liquid from the delivery nozzle directly into the plasma. As such, the liquid within each droplet is protected from the plasma, and is carried intact to the surface, where it spreads and polymerises to form a conformal, high-value functional coating, which retains the properties of the original precursor molecule.

The relationship between the chemistry of coatings deposited using this technique and the plasma deposition parameters has been investigated using a complementary suite of surface analytical techniques: contact angle analysis, X-ray photoelectron spectroscopy and atomic force microscopy.

A new methodology for curve-fitting of the XPS Si 2p and C 1s core levels of siloxane materials has been developed using siloxane standards. This method has been expanded to be used on siloxane coatings thinner than the depth of analysis of XPS. This enabled the chemistry of the coatings to be determined from XPS analysis without interference from the substrate.

The chemistry of siloxane coatings deposited on poly(ethylene terephthalate) film have been related to the plasma parameters utilised during deposition. Up to 95% polymeric siloxane [(CH3)3SiO1/2 and (CH3)2SiO2/2] have been retained for deposition of poly(dimethylsiloxane) precursor. The use of a poly(hydrogenmethylsiloxane) precursor led to a more oxidised coating, with up to 25% siloxane in (CH3)SiO3/2 environment. The trends in deposition rate for PDMS and PHMS were also found to be different.

Siloxane coatings durable to immersion in boiling water for 30 minutes have been achieved by plasma treatment of the substrate prior to deposition, and by plasma treatment of the coating following deposition.