The Use of Radio-Frequency Activated Gas Treatment to Improve Bondability

Abstract

Abstract Two apparent contradictions are indicated in comparing recent work with previous work in the field of adhesion. The first contradiction concerns the significance of a weak boundary layer in adhesion failure. Bikerman and Hansen infer the presence of a weak boundary layer and imply that its removal is necessary to improve adhesion. Yet, neither its presence nor its removal are confirmed experimentally. Recent work with RTV silicone confirms experimentally the presence of a material meeting the definition of a weak boundary layer before and after the radio—frequency activated oxygen treatment, which improves adhesion. These results indicate that a possible bonding mechanism involves desorption or displacement of the weak bondary layer and preferential adsorption of the adhesive to form the bond. The other contradiction involves the report by Hansen and Schonhorn that the “CASING” treatment improves adhesion to poly (ethylene) without changing surface energy characteristics. Three laboratories report relatively large changes in contact angle on poly (ethylene) even after exposure to an activated gas for as little as 5 sec. Malpass and Bright report that, while there may be no change in contact angle immediately after removal from the activated inert gas environment, the contact angle rapidly decreases to very low values. This indicates that the low energy state is rather transitory in nature. These results along with those of Sowell and Westerdahl indicate that there may be no serious contradiction to the rule of energetics in bonding, i.e., the adhesive in the liquid state shall have a lower surface tension or surface free energy in order to wet, spread, and adhere to the higher surface energy adherend. Recent findings by Hudis, confirmed by Sowell, indicate that the ultraviolet emission from the radio—frequency gas plasma is the basic factor in the cross-linking of poly(ethylene). Sowell has shown that exposure to ultraviolet radiation also causes increase in critical surface energy and improvement in bondability of poly (ethylene).