An adhesive or glue is a mixture in a liquid or semiliquid state that adheres or bonds items together. Adhesives cure (harden) by either evaporating a solvent or by chemical reactions that occur between two or more constituents (Todd et al. 1994; Lau et al. 2002). The adhesives are classified into different types. A detailed description of the different types of adhesives has been given in Table 2.
Choice of correct adhesive is critical in order to produce strong and durable bonds (Vodicka 1996). Unlike thermoplastics, thermosetting adhesives do not melt or flow on heating but become rubbery and lose its strength. The molecular chains present in thermosetting adhesives undergo irreversible cross-linking during curing (Mays et al. 1992). The most familiar thermosetting adhesives are the family of epoxies. Limitations of the above adhesives for joining of different materials have been solved by epoxy adhesives, and presently these adhesives are widely used.

It is evident from literature that thermoplastic resins can withstand temperatures comparable to many aerospace graphite/epoxy prepregs (177o C/350o F). Adhesives that have been used successfully in aerospace are typically epoxy-based adhesives with the brands such as FM300 for both PPS and PEEK, FM 37, and FM 87. Kinloch AJ (1986) and Kinloch described the use of a wide range of epoxy- and acrylic-based adhesives for bonding applications: Hysol 9309.3; FM73M; Permabond F241, F245, F246, and V501; and Bostik M890 and M896. An extensive review of adhesives for PEEK/graphite composite bonding is also given in, and it was found that FM300 recorded a close second highest lap shear strength value of 23 MPa to AF-163-2 K adhesive (24.5 MPa) when tested at 298 K.
It is to be noted that aviation and space industries require special types of adhesives as one of the main problems with the application of adhesive for spacecraft and supersonic and hypersonic aircraft is high temperatures encountered by them in space. This is caused by the aerodynamic friction heating of the structure as it moves through the air. The temperatures vary from approximately 50o C at Mach 1, 200o C at Mach 2, 450o C at Mach 3, and to above 800o C at Mach 3.5 (Shaw and Tod 1994). Therefore, the use of basic organic resins as adhesives become very difficult above Mach numbers of 2 (Shaw and Tod 1994), and consequently, high temperature-resistant adhesives are obviously required.
An epoxy adhesive, Duralco 4703 is one such option which can withstand harsh conditions. The adhesives mixed with hardener are employed on the substrate surface in liquid form (Kinloch 1986). Their low viscosity, good wettability, penetrating ability, and the presence of highly polar groups in the molecule help the liquid to spread over the substrate leading to good adhesive joint especially with metals, ceramics, glass, etc.
Epoxy resin is the most commonly used polymer matrix for advanced composite materials. Many attempts have been made to modify the properties of epoxy by the addition of either rubber particles so that the matrix-dominated composite properties are improved. The addition of rubber particles improves the fracture toughness of epoxy, but decreases its modulus and strength. The addition of fillers, on the other hand, improves the modulus and strength of epoxy, but decreases its fracture toughness. The heat-deflection temperature of epoxy is also improved by the addition of fillers. In recent years, micro- and nano-scaled particles have been considered as filler material for epoxy to produce high-performance nano composites ith enhanced properties.