Adhesively bonded polymeric composite materials are frequently replacing metallic materials in many applications due to their excellent mechanical, thermal, cryogenic, and chemical properties and are being extensively used in automotive and aerospace industries not just for their ease in manufacturing and low unit cost but also for their potentially excellent tribological performance in engineered forms.
The automotive industry faces increasing pressure to maximize performance while minimizing weight and cost. Additional considerations include noise abatement, new safety standards, and protecting the environment from pollution. The recent rise in oil prices has made these technological advances important for competitiveness in the global automobile market (Toyota 2005; Fuyuno 2005). One way of improving vehicle efficiency is the reduction of vehicle weight as light-weighting of vehicles not only enhances fuel efficiency but also lowers vehicle emissions thereby improving driving performance. Safety of passengers has always been a major concern for automobile engineers. Materials which have high absorption of energy offer much resistance during crash/accidents. This is measured in terms of specific energy absorption (S.E.A) (Sheshadri 2006).
It is given by

where W = total energy absorption, V = volume of crushed material, ρ = density of the material.
The SEA of different materials is compared in Fig. 5. From the above figure it is clear that a structure built with a polymer composite fabricated by adhesive bonding is six to eight times safer than a structure built with metals. In event of a crash, the energy is absorbed by the structure made of polymer composite through plastic buckling. As a result, the impact of the crash is reduced at the other end of the structure.
Many studies have been carried out to explore the possibilities of using adhesive bonding in the exterior body panels, frameworks/chassis, bumpers, drive shafts, suspension systems, wheels, steering wheel columns, and instrument panels of automotive vehicles. Both thermoplastic and thermosetting polymers are being used as composites fabricated by adhesive bonding, and they find wide scale applications in automotive sector as given in Table 3.

The most common thermosetting polymers that are being used are epoxy, crosslinked polyurethanes, unsaturated polyester, and phenol formaldehyde (Al-Zahrani et al. 2009). Among thermoplastics, polyethylene (PE), polypropylene (PP), and polyvinylchloride (PVC) are commonly used in both plastic and composite auto parts.
Thermoplastic resins offer a number of advantages over conventional thermosetting resins like lower cycle time, high service temperature, excellent chemical and impact resistance, low coefficient of thermal expansion, excellent fire, smoke and toxicity performance, good fatigue performance, low wastage, and recyclability (Patel 2009). Their low level of moisture uptake results in less degradation of mechanical properties under hot and wet conditions (Al-Zahrani et al. 2009). They can be remelted and remolded, therefore, can be recycled which is most unlikely for thermosets. They have the ability to create more complex shapes both online and in subsequent forming operations. Most thermoset resins are relatively brittle, and most thermoplastics are extremely ductile. This ductility gives the final composite greater impact resistance and damage tolerance.
The structural polymer composites made from epoxy and polyester thermosetting although have many beneficial properties such as low density, good mechanical properties combined with good insulation, and environmental resistance; they suffered from chemical instability, as the impregnated intermediate or prepreg has a limited shelf life (Vodicka 1996). Due to these properties thermoplastic polymers are preferred over thermosets for high-performance applications (Vodicka 1996).
Thermoplastics are further classified in to three categories: commodity thermoplastics, engineering thermoplastics (ETP), and high-performance thermoplastics (HPTP). A distinguishing feature of HPTP is that their heat-deflection temperatures are above 200o C, which is 50–100 % higher than standard engineering thermoplastics. High-performance thermoplastics (HPTPs) are used in specialized applications that require a combination of extraordinary properties. HPTPs show superior short- and long-term thermal stability (higher melting point, glass transition temperature, heat-deflection temperature, and continuous use temperature), chemical and radiation resistance, resistance to burning, and improved mechanical properties (stiffness, strength, toughness, creep, wear, and fatigue).
Most high-performance thermoplastics are semicrystalline in nature as their levels of crystallinity never exceed above 90 %. Crystallinity in high-performance polymers is an important factor and shows strong influence on chemical and mechanical properties. In broad terms, crystallinity tends to increase the stiffness and tensile strength, while amorphous areas are more effective in absorbing impact energy (Vodicka 1996).
High-performance engineering thermoplastics that are commonly used in various applications are polyphenylene sulfide (PPS), polyether ether ketone (PEEK), polyetherimide (PEI), and several formulations of polyamide, commonly called nylon (Amin and Amin 2011).
Polyether ether ketone (PEEK) (service temperature 250o C to +300o C, tensile strength: 120 MPa) is a high-performance thermoplastic polymer which is gaining significant interest in aerospace and automotive industries. PEEK is a lightweight polymer and offers excellent mechanical performance at high temperatures. It can successfully replace metals and other polymers due to its unique combination of outstanding wear performance, processing flexibility, and excellent chemical resistance (Thomas and Visakh 2011). It is evident from Fig. 5 that the carbon/PEEK composite demonstrates the highest specific energy absorption (SEA). Thus, composites made from PEEK are a better option for applications in automotive industry.
Aerospace engineers typically specify those materials that can withstand harsh environments and reduce manufacturing costs and offer processing flexibility. The changeover from the use of aluminum materials to polymers in the aerospace industry came from the availability of lightweight polymer composites based on reinforcements.
The use of lightweight composite parts fabricated by adhesive bonding have numerous application in aviation such as doors, rudders, elevators, ailerons, spoilers, flaps, fairings, etc., in passenger aircrafts such as in Boeing 747 and 767 (Mazumdar 2002).
Commercial aircrafts such as Boeing 757 and 767 contain less than 5 % advanced composites by weight, while military aircraft like GA-18A contains 1–20 %. New aircraft designs such as the Boeing Dreamliner 787 and Airbus A350 use more composite materials and place a major premium on weight reduction consequently significant application of adhesive bonding as such.
Epoxy adhesives are widely used in structural aerospace applications due to their ability to withstand temperature as high as ~250o C. Epoxies are superior to polyester and vinyl ester resins in their resistance to moisture and other environmental influences (Lubin and Peters 1998).
Bismaleimide adhesives, like epoxies, are easy to handle and process but are more resistant to fluctuations in hot/wet conditions as compared to epoxies. Bismaleimides do not generate volatiles during cure, thus resulting in less porosity (da Silva and Adams 2005). These materials have high temperature resistance and withstand temperatures of 300o C, but they are very brittle and are prone to microcracking (Zhuang 1998).
Polyimide adhesive exhibits better performance than bismaleimide in response to hot/wet temperature extremes. They are widely used for high temperature applications as some of their grades can sustain temperature over 300o C. The cured materials typically exhibit high glass transition temperatures, good chemical resistance, and high modulus and creep resistance (Zhuang 1998). The limitation of polyimide is that it is very brittle and release volatiles during curing, which produces voids in the resulting composite.
Adhesively bonded polymers have successfully replaced various metallic components in automotive and aviation industries due to their light weight. However, the use of adhesives and polymeric materials in primary structures of an automotive and aerospace is still limited due to high temperatures encountered by combustion gases in automotive and aerodynamic heating especially in the case of aerospace. This limitation can be overcome by the application of high temperature resistant adhesives, i.e., dispersing thermally stable nanofillers in to an adhesive.
The inclusion of nanofillers could increase the decomposition temperature of adhesives indicating that the thermal decomposition of the matrix is retarded by the presence of the nanoreinforcement. Therefore, dispersion of appropriate nanofillers would be of great interest for automotive and aerospace industries as per as application of adhesive bonding is concerned.