Light, strong composite ductwork keys aircraft comfort

While passengers never see them, ventilation ducts are an important part of any airliner. Their fabrication and performance specifications are more complicated than one might suspect. Complex shapes are required to fit within the curved fuselage, yet they must withstand high temperatures and some pressure cycling without leaking and must meet U.S. Federal Aviation Admin. (FAA) fire and smoke requirements as well. The Aerospace Components group of Saint-Gobain Performance Plastics (Seattle, Wash.) manufactures ducts for newer planes and replacement ducting for heritage systems as well. While table-rolled fiberglass prepreg is used in the newer systems, fiberglass/rigid polyurethane foam ductwork is still produced for Boeing's 747, 767-200 and C-17 aircraft.

These lightweight, self-insulating replacement parts are hand cast in round, square or curved shapes as long as 156 inches/4m and 20 inches/0.5m in diameter. Two-part polyurethane is mixed in a milkshake-mixer-type device and poured into metal tools that form each duct half. Tools are lined with dry, woven fiberglass cloth. The foam penetrates and wets out the cloth, which forms a skin over the foam, giving the parts sufficient strength for installation above ceilings and below floors in aircraft passenger cabins.

To bond the duct halves together or attach end fittings, technicians butt-weld the edges of adjacent parts with strips of the same fiberglass cloth, wet out with Epibond 1544-A82/D epoxy adhesive, supplied by Huntsman Advanced Composites (The Woodlands, Texas). The semipaste epoxy does not sag on vertical surfaces, has a 20-minute gel time and is formulated with a low 1.18 g/cm2 density to add as little weight as possible to the ducts. Using a mix ratio of 100 parts resin to 13 parts hardener, technicians apply the wet adhesive to the bond surfaces with a brush or trowel. Dry strips of fiberglass are placed over the adhesive bead and smoothed to eliminate voids and thoroughly wet out the strips. Parts can be handled after a four-hour room-temperature cure.

Once cured, the Epibond adhesive demonstrates compressive strength of 8,000 psi/55.16 MPa and aluminum/aluminum lap shear strength of 2,600 psi/17.93 MPa (after 24 hours at 77°F/25°C). Service temperature is 200°F/93°C.