The use of sol-gel technology for adhesive and structural durability applications.

The use of adhesives for joining metallic and non-metallic substrates is widely accepted in various structural engineering applications, such as aerospace, automobiles. In order to meet the industry requirements, the ability to achieve good mechanical properties under a wide variety of conditions should be considered. A major advantage of adhesive bonding over conventional mechanical fastening such as bolts and rivets is that it enables dissimilar materials to be joined. However, these materials (i.e. cured epoxy resins) are brittle and have poor resistance to cracks which indicate low capability to absorb the external energy therefore limiting their application in fields requiring high adhesive and impact strength. The incorporation of secondary components or modifiers into the epoxy system can overcome these issues. Sol-gel technology is finding increasing applications, for example, as hydrophobic selfcleaning and decorative colour coatings, formation of low-temperature cure high purity optical components and biomedical applications. The basic advantage of the sol-gel process is its ability to form inorganic structures and hybrid organic and inorganic network structures at relatively low temperatures using conventional coating techniques such as dip coat, spin coat or spraying. The present work is based upon the use of sol-gel technology to produce an adhesive which can be used to bond aluminum 2024-T3 and mild steel substrates. A novel adhesive material is produced from the combination of a hybrid silica sol-gel system, nano-inorganic particles and an epoxy polymer. The adhesive strength was investigated using a universal tensile test machine. The percentage of the bond strength of the material formed through the hybrid silica-base sol-gel is found to be related to the condensation reaction of sol-gel or via a formation of Si-O-Si networks in the adhesive. The effects of adding different concentrations of organic and inorganic materials on sol-gel adhesive strength were studied. For example, by doping a DGEBA epoxy resin, TiO[2], gamma-Al[2]O[3] MWCNT and mixture of PANI and gamma-Al[2]O[3] into the sol-gel, an increase in bond strength via the enhancement of the cross-linking among a hybrid sol-gel system was observed. The effects of cure process were investigated and it was observed that an increase in cure temperature/time led to an increase of the adhesive strength. In addition, the adhesive strength was increased as the specimen surface roughness increased due to a mechanical keying effect. Lap shear strength was also increased as bond geometry of the joint increased. The strength of the sol-gel adhesive joint has found to decrease on increasing the test temperature, being related to a softening of the sol-gel adhesive. Furthermore the adhesive strength decreased as the immersion time in 3.5%NaCI increased as a result of water absorption and the formation of corrosion products along the interfacial bond line. A study of sol-gel adhesive performance subjected to cyclic loading showed fatigue behaviour, as reported in the literature, notably increasing fatigue life with decreasing fatigue load. No significant effects in the fatigue behaviour of the lap joints were noted for different loading frequencies. FTIR, Raman and XPS analysis confirmed that improvements in the strength of the hybrid epoxy/sol-gel materials were related to the formation of different covalent bonds in the adhesive matrix, i.e. Si-O-Si, Si-O-Al and Si-O-C.