A problem with all air-carried weapons is the very high temperatures reached during high speed aircraft manoeuvres. This is caused by the aerodynamic heating of the structure as it moves through the air: of approximately 50°C at Mach 1, 200°C at Mach 2, 450°C at Mach 3 rising to over 800°C at Mach 3.5. The use of basic epoxy resins as adhesives becomes very difficult above Mach numbers of the order of 2. High temperature resistant adhesives are obviously required; a class of materials with such a high temperature resistance are the bismaleimides.
These materials are much easier to process than other high temperature resins such as polyimide. Condensation polyimides require great pressure to be applied when they are used as adhesives to prevent the water evolved during the cure forming voids. An inherent problem with all high temperature resins systems is the lack of toughness because of the high degree of cross-linking present in the materials. The Defence Research Agency (UK) has done much work to improve the mechanical properties of bismaleimide resins. Incorporating a second phase such a carboxyl terminated butadiene acrylonitrile rubber can toughen these materials. This rubber phase acts both to absorb energy itself and to promote other energy-absorbing mechanisms in the matrix resin. This improvement in properties can be attained without loss of other desirable qualities such as modulus and glass transition temperature. When used as adhesives the lap shear strength increases with the addition of rubber. Although the adhesive properties are not an improvement over toughened epoxies at room temperature they do maintain their properties over a larger temperature range as shown in figure 1. As would be expected, the materials containing the largest amount of rubber show the greatest change in strength with temperature.