Environmentally durable lightning strike protection materials for composite structures

A fiber-reinforced thermosetting matrix resin prepreg/film adhesive/metal screen combination suitable for co-curing into lightning strike protected fiber-reinforced thermoset composite structures, comprising:

• a) one or more plies of a fiber-reinforced prepreg impregnated with a thermosettable matrix resin sufficient to prepare a composite structure of a desired thickness;
• b) a conductive metal screen having a nominal thickness of from 1 mil to 10 mils;
• and
• c) a thermosettable film adhesive;
• wherein the diffusion coefficient of the thermosettable prepreg matrix resin into the film adhesive satisfies the equation
• D<[T.sub.a ].sup.1.5 /V.sub.a .multidot.[T.sub.p /V.sub.p ]
• where D is the diffusion coefficient, T.sub.a is the dwell time of the adhesive resin at minimum viscosity V.sub.a is the minimum viscosity of the adhesive resin, T.sub.p is the dwell time of the prepreg matrix resin at minimum viscosity, and V.sub.p is the minimum viscosity of the prepreg matrix resin, T.sub.a, V.sub.a, T.sub.p and V.sub.p having been determined from the viscosity profile of the respective resin during cure at the same temperature/time ramp of 1.degree.-2.degree. C./min, and wherein D=0.1 when T.sub.a.sup.1.5 /V.sub.a <0.3;
• or where irrespective of D, T.sub.a.sup.1.5 /V.sub.a <0.3.