Influence of ethanol vapor addition on the surface modification of polyethylene in a dielectric barrier discharge

Highlights: • Ethanol vapor addition to a medium pressure dielectric barrier discharge. • Investigation of the wettability and the chemical composition of LDPE surfaces. • Ethanol vapor in an air plasma results in better wettability and a rougher surface. • 2% ethanol vapor in a nitrogen plasma results in a 92% WCA reduction. • Polymerized layers with O/C and N/C ratios of 32% and 53% can be obtained. - Abstract: In this paper, ethanol vapor up to 50% is added to an argon, air or nitrogen dielectric barrier discharge at medium pressure to profoundly investigate the effect of ethanol addition on the surface modification of low density polyethylene (LDPE). Water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS) measurements show that the ethanol vapor addition effect on the LDPE surface depends on the used carrier gas. Adding ethanol to an argon plasma has no significant effect on the wettability nor on the chemical composition of LDPE compared to a pure argon plasma treatment. Ethanol addition does however slightly increase the LDPE surface roughness. Addition of small amounts of ethanol vapor to an air plasma makes it possible to incorporate additional nitrogen and oxygen groups on the LDPE surface, resulting in an extra decrease of 11% in WCA value. Moreover, the LDPE surface roughness is slightly increased due to the ethanol vapor addition. The most significant effect of ethanol addition is however observed when nitrogen is used as carrier gas. After an N₂/2% ethanol plasma treatment, an 85% reduction in WCA value to 8.5° is found compared to a pure N₂ plasma treatment. This very hydrophilic LDPE surface is obtained due to a significantly high incorporation of oxygen and nitrogen groups on the surface with an O/C and N/C ratio reaching 32% and 53% respectively. FTIR measurements also reveal that the observed extremely high wettability of LDPE is not the result of plasma activation but is due to plasma polymerization effects occurring on the surface resulting into the deposition of a plasma polymer containing ketones, amides as well as C=N groups. In addition, ageing studies have also been conducted and these studies reveal that for all carrier gases, ethanol addition to the discharge gas significantly suppresses the ageing effect. All the above mentioned conclusions therefore indicate that ethanol vapor based plasmas can be an excellent tool to increase the surface energy of polymers.