Surface Modification of Titanium and Polyimide Sheet for Adhesive Bonding

Abstract

Major industrial sectors like automotive, aerospace and others are increasingly using polymer composites in their structural parts. Polyimide sheet and adhesives, are high performance polymers. They are widely used in various engineering applications due to their excellent thermal, mechanical and chemical properties and their resistance to radiation and fire. Unfortunately, the surface of polyimide like any other polymer is inert and exhibits poor adhesion properties. It is established that for successful application of polymeric composite materials to form structural parts using adhesive bonding, they need to have special surface properties like hydrophilicity. The surface modification of polymers and metals play a predominant role in enhancing the surface energy for improved adhesion properties. The aerospace industry is giving special attention to surface modification of titanium which could enhance the adhesive bond durability. In general, surface modification of titanium is carried out by chromic acid anodization. In this thesis work, an alternative surface preparation technique of titanium is investigated which could be of the interest for future research. We propose to modify the surface of titanium by plasma ion implantation, This is clean and solvent free technique, which could possibly be of interest to the aerospace industry in terms of adhesive bond strength and durability. It is also noted that in search of the long term durability and efficient service performance in the context of the future generation of aerospace structures, there is an increasing need of metal-polymer composites. Therefore, work of particular relevance to this project has been on the improvement of high performance polymer-titanium composites through high performance adhesive bonding. Several methods may be employed for modifying the surfaces of polymers and metals, which range from wet chemical processes to dry physical processes. Dry processes, like electrical treatment by exposure to plasma have received special attention because of the uniformity of these surface modifications, its precise control and the absence of chemical hazards. It is now well known that the plasma treatment creates physical and chemical changes such as cross linking, degradation, formation of free radicals, oxygen ions functionalisation and etching. A high temperature resistant and thermally stable polyimide adhesive as supplied by Creative materials Inc. was selected. TGA, DSC and FTIR analyses of this high performance polyimide adhesive were performed. After analysis, the polyimide adhesive is used as the standard adhesive for joining and testing materials in this project. There are mainly two types of surface treatments i.e. mechanical treatment and atmospheric pressure plasma treatment, that are combined in the present research. The grit blasting and atmospheric pressure plasma treatment time is optimized to get optimum values of surface roughness and surface energy. When these surfaces with optimum values of surface roughness and surface energy are joined together using the PI adhesive, a remarkable increase in the value of the adhesive bond strength during lap-shear testing is obtained. The bond strength is increased by two and half times compared to the lap-shear test results of untreated surfaces. The characterization of untreated and treated titanium surfaces with XPS, SEM, a surface roughness profiler and a contact angle analyser is performed. The Lap-shear test results suggest that the bond strength increases with an increase in surface energy. The surface roughness profiler results show that the surface energy increases with an increase in surface roughness. FTIR results suggest that the surface energy increases with a decrease in carbon contents and an increase in oxygen contents in the top layers of the titanium surface. The characterisation of the polymer surface after exposure to plasma reveals that the polar component of the surface energy increases due to chain scission and oxidation, leading to generation of polar groups such as C-O, C=O & COO on the polymer surface. Therefore, the total surface energy also increases, leading to enhanced adhesion. AFM analysis of polyimide sheet indicates that there is an increase in surface energy with an increase in surface roughness. In the last part of this research work, the effects of the moisture and high temperature on the adhesive bond strength of the modified surfaces is investigated. At high temperature and relative humidity conditions, moisture ingression in the adhesive bond line takes place at a faster pace than at room temperature. The modified surface adhesive bonds retain their bond strength at high temperature, while heated in dry environment. In case of moisture pre–condition at elevated temperature before lap-shear testing, the bond loses the improved bond strength, possibly due to the attack of polar water molecules on the interface bonds of titanium and polyimide adhesive. The present work aims to contribute to the knowledge and understanding of several aspects of the surface treatment of metals and polymers for adhesive bonding in high temperature applications. Different problems related to the surface treatment and adhesive bonding for high temperature application has been discussed. Efforts are made to evaluate the performance of the adhesive bonds between modified titanium and polyimide at elevated moisture and temperature conditions. The present work has produced some encouraging results to perform further research on surface treatment methods for coating and high temperature adhesive material for different aerospace applications.