In addition to reducing galvanic corrosion it is important that adhesive films when cured should not be brittle and should provide fracture toughness which means that a flaw in the film is protected against propagation. This is particularly important when the films are used for the bonding of substrates in vehicle manufacture to reduce or prevent fracture propagation in the event of accidents. It is also desirable that the cured films have an elongation to break to allow them to help absorb impact.
Thermosetting adhesive compositions are well known and many are based on epoxy resins and various methods have been proposed to improve the toughness of the adhesive bond formed and to control the glass transition temperature and accordingly reduce the brittleness of the cured adhesive. The adhesives are often used in highly automated processes such as in the production of components for automobiles, aerospace vehicles such as aircraft and rockets and in the construction and furniture industries. One desirable property is that the adhesive can be cured over a relatively short period of time and at temperatures conventionally used in the production of such components. In this way the curing of the adhesive will not unduly delay the component production process. It has also been proposed that the adhesives may be in the form of films.
The invention provides an adhesive film which satisfies these requirements.