To reduce weight and increase the corrosion resistance of floor boards installed on its fleet of Boeing aircraft, United Airlines recently replaced metal-panel inserts with advanced Torlon(r)* polyamide-imide inserts. The flanged inserts that are used to secure floorboards in both passenger and cargo compartments are bonded in place in epoxy/fiberglass skin/Nomex(r)* honeycomb-core panels using a flexible polyurethane adhesive rather than the epoxy that is typically used for insert potting. United Airlines switched to the Uralane(r) 5774-A/B polyurethane, supplied by the Adhesives & Tooling Division of Vantico Inc., Los Angeles, after pull tests confirmed that the adhesive produced higher bond performance than epoxy on Torlon/honeycomb-panel reinforcement.

According to Rudy Sylvester, staff specialist engineer, Component Engineering -- Interiors at United’s San Francisco maintenance facility, “Our tests indicated that the Uralane polyurethane performed better than epoxy in both bolt pull-out strength and lap shear.” Uralane 5774-A/B is a tough, impact-resistant, semi-paste adhesive that is formulated for bonding engineering thermoplastics.

Dissimilar Substrates Prompt Change

United had historically installed metal inserts into aluminum/balsa-wood floorboards using Vantico self-extinguishing epoxy insert-potting compounds, including Epocast(r) 1618-A/B syntactic epoxy and Epibond(r) 1559-A/B fast-setting epoxy. When Boeing switched to epoxy/fiberglass honeycomb panels reinforced with Torlon inserts, the epoxy adhesives did not provide the needed bond strength between the dissimilar substrates. The airline then began investigating different adhesives for the project, including Vantico’s Uralane 5774-A/B polyurethane that is qualified to a BMS 5-105, Type 5 specification. It has a 1,525-minute work life, a room-temperature lap-shear strength of 2,000 psi on aluminum/aluminum bonds and a T-peel of 40 pli. The polyurethane provided the bond performance required to securely install the Torlon inserts in the composite floor panels.

Insert Potting

United technicians begin by installing an Accumix(tm) dual-barrel cartridge of Uralane 5774-A/B polyurethane into a dispensing gun along with a static-mixing nozzle. They then pull the trigger to mix resin and hardener components in the nozzle and dispense the fully blended polyurethane around the edges of a pre-drilled hole. Finally, the bottom half of the flanged insert is installed in the hole and allowed to cure at room temperature for six hours. Technicians then move to the next hole and follow the same procedure until each panel is completely reinforced.

“The polyurethane flows neatly and easily from the dispensing cartridge, forming a smooth bead around the holes in the panel,” Sylvester explains. “It provides us with the dependable performance we need,” he concludes.