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7.3 Adhesive selection Adhesives must be selected that are compatible with both adherends and meet perform- ance and design requirements. The choice of adhesive types is vast but for structural applications they can be broken down by chemistry group. 7.3.1 Epoxies and toughened epoxies These form the largest chemical family and can be supplied as: ■ Single component heat curing films ■ Paste adhesives, either one component heat activated (usually from 130 to 180 °C in 20 to 60 minutes) or 2 component room temperature curing. ■ Syntactic or foaming film adhesives for gap filling requirements Epoxy adhesives display: ■ High mechanical resistance (25 to 40 Mpa shear resistance). ■ Excellent adhesion to most metals, composites, many plastics, concrete, glass and wood ■ High chemical resistance ■ Long term durability (some airplane parts that were bonded with epoxy adhesives some 40 years ago are still in good condition). ■ High rigidity (except the toughened epoxies), so, therefore poor resistance to peel and cleavage. Epoxy adhesives are often used for aircraft manufacturing, for parts which require high modulus, high stiffness ( for instance flight control surfaces: flaps, rudders ), for leading and trailing edges, engine nacelles, bonding of sandwich panels, stringers and stiffeners on the fuselage, etc. They are also used for wind turbine blades, bonding of industrial and chemical equip- ment (pipes, tanks), electronic equipment and printed circuits, etc... 7.3.2 Polyurethanes There are semi structural PU adhesives (shear resistance from 6 to 20 MPa according to the formulations) in the form of pastes that provide excellent adhesion to composites, metals, plastics, glass, wood.... They are more flexible than most epoxies. These PU adhesives can be one component cured (with moisture) or 2 components cured (at room temperature or low temperatures). Prices are generally lower than epoxies. Current applications include the bonding of panels for mass transportation equipment (buses), bonding of hatches and doors for automotives, construction sandwich panels, and FRP boats. For example, trams, trucks and refrigerated truck bodies are now made with a structural steel or aluminium frame on which GRP exterior panels are bonded, usually with PU 2 components adhesives. 60 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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