Deposition of Hydrophobic Functional Groups on Wood Surfaces Using Atmospheric-Pressure Dielectric Barrier Discharge in Helium-Hexamethyldisiloxane Gas Mixtures

This work examines the functionalization of sugar maple (Acer saccharum) and black spruce (Picea mariana) wood surfaces using an atmospheric-pressure dielectric barrier discharge in He and He/HMDSO (hexamethyldisiloxane) gas mixtures. Wood samples were placed on one of the electrodes and the plasma was sustained by applying a 3.5 kV peak-to-peak voltage at 12 kHz. Analysis of the discharge stability through currentvoltage (IV) characteristics revealed a filamentary behaviour, in sharp contrast with the homogeneous He discharge obtained with a glass sample. Optical emission spectroscopy performed near the wood vicinity revealed strong N-2 and N-2(+) emissions, suggesting that wood outgassing plays an important role in the evolution of the discharge regime. Analysis of the surface wettability through water contact angle (WCA) measurements indicated that freshly sanded wood samples treated in He/HMDSO plasmas became more hydrophobic with WCAs in the 120 degrees 140 degrees range depending on treatment time and wood species. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy measurements on samples exposed to He/HMDSO plasmas revealed the deposition of hydrophobic Si(CH3)3-O-Si(CH3)2, Si(CH3)3 and Si(CH3)2 functional groups as well as an increase of the CH-to-OH band intensity ratio. For relatively thick coatings, the WCA following natural aging under uncontrolled conditions remained constant at 132 degrees +/- 3 degrees which highlights the stability of the plasma-deposited thin films, a very promising result for structural and decorative outdoor applications.