Advanced materials for aircraft interiors: An addendum
By Staff | January 2007
Given the scale of the subject, our overview article on composites in aircraft interiors (see HPC November 2006, p. 24) left out, by necessity, some topics and passed over several key suppliers. One aspect not covered was the edge treatments and potted inserts in honeycomb-cored panels.
To prevent moisture ingress into the honeycomb and to provide a finished edge on cored panels, several options are available. One method surrounds the panel on all four sides with a “picture frame” of high-density foam, cut to the same thickness as the core itself. The panel’s prepreg skins overlap the core slightly so that the foam edges are covered, and the entire assembly is cocured. General Plastics Mfg. Co. (Tacoma, Wash.) has supplied Boeing-qualified high-density, high-temperature-resistant and flame-retardant polyurethane foam for edge closeouts since the 1960s, says the company’s market development manager Ted Hile. He reports that his company supplies foam sheets to fabricators, typically 18 lb/ft3 to 20 lb/ft3, with small narrow grooves in the surface. When foam is cut into the picture-frame strips and placed around the core, the grooves allow volatile gases to escape during cure of the phenolic resin in prepreg skins. Another supplier of polyurethane for edge treatments is Coastal Enterprises Co. (Orange, Calif.). Its foam, like that of other suppliers, meets The U.S. Federal Aviation Admin.’s (FAA) FAR Part 25.853 regulations as well as MIL P26514 rules for smoke, fire and toxicity, reports the company’s president Chuck Miller.
Alternative sealing methods in-clude using low-density syntactic epoxy compounds (pastes containing hollow microspheres) to “pot” the edge of a honeycomb panel, thus enhancing panel durability. Others include foam adhesive in sheet form, typically cut into strips of the same width as the honeycomb layer. When backing paper is peeled off, the film can be adhered to the core edge, allowing another panel to be joined to the first, a process called core splicing. During panel cure, the adhesive “foams” and expands to fill voids along the panel edges.
Closing honeycomb edges with paste syntactics significantly increases the compressive strength of panels and prevents edges from crushing under load. The syntactic materials also help prevent moisture incursion that can cause delamination, corrosion and weight gain in aircraft interior panels. Panels sealed using Huntsman Advanced Materials’ (The Woodlands, Texas) Epocast epoxy syntactics reportedly feature compressive strengths from 2,500 psi to 20,000 psi (17.24 MPa to 137.9 MPa), depending on which products are used, with service temperatures ranging from 150°F to 350°F (65.5°C to 177°C). Edge-sealing syntactics are formulated with densities as low as 0.50 g/cc.
