The Heat Is Off: Advanced Resins are Revolutionizing the Way We Repair Aircraft
Aircraft design has progressed rapidly over the last 110 years. The simple wooden planes that were used mostly as scouts in World War I gave way to the riveted metal bombers and fighter planes of World War II. By the 1980s, computer-aided design and manufacturing revolutionized the entire process. Today, innovative new composite materials are replacing steel, titanium, and aluminum in a wide array of aerospace components from access panels to fuselages. Composites have improved strength-to-weight ratios, excellent resistance to corrosion, and the potential for quick and expedient repairs. The current two-component, thermally-accelerated resins used in composite repair add many steps to the repair process that make them inefficient and costly to use on aerospace structures. Switching to an ambient temperature cure, one-part resin can greatly streamline aircraft repairs, saving money and man-hours and significantly increase weapon system readiness.
Aerospace composite components consist of a woven fiber matrix, often fiberglass or carbon fiber, that is impregnated with a strong resin. Usually, a two-part, thermally-accelerated epoxy resin is used. However, thermally-accelerated resins need to be heated to 250 °F to properly cure with the desired structural characteristics. Because of this cure process, two-part epoxy resins are not thermally stable and require refrigeration until use to obtain optimal results. This requirement is a significant issue for smaller, more rural outposts with fewer resources, especially those that may be required to perform in-field repairs. Additionally, the two-part system begins its cure process as soon as the components are mixed, leaving repair technicians with a short window, or “pot-life,” in which the viscosity of the resin is low enough for it to be applied to an aircraft component repair. During the application process, the two components must be mixed, worked thoroughly into the fiber sheets, and degassed under vacuum to remove any entrained gases unable to escape from the resin and to form voids upon curing. These voids can act as failure initiation sites. The short window for the repair leaves no room for error and can result in wasted time and materials if a patch needs to be removed and reapplied. After the resin sets, it needs to be heated uniformly to achieve proper structural properties. However, components that are thermally connected to the repair (such as the wing or fuselage skin, interior components, etc.) act as heat sinks. This makes heating the entire patched area to a proper steady temperature a complicated operation requiring specialized heaters that can become more art than science. The entire process necessitates using large amounts of equipment (as shown in Figure 1) and extra man-hours that could be eliminated if the U.S. Navy instead switched to a one-component resin system.
One option is photocurable acrylic resins, both on the market and in development, which are cured using ultraviolet (UV) photo initiators. These resins cure rapidly when exposed to UV wavelength photons but are stable at ambient temperatures, allowing them to be stored for a long time without refrigeration. UV acrylic resins are one-part systems, eliminating the need to measure and mix two components on site. This saves time and eliminates concerns about the short pot life. Because UV-curable acrylic resins do not need to be stored as separate components, they can even be pre-impregnated into the fiber matrix, leaving technicians with easy-to-apply, pre-made patches capable of greatly simplifying repairs. UV systems require no heating elements and are activated solely by the UV rays from the sun, minimizing equipment required for repairs. Because heating can cause the off-gassing of residual volatiles, composites cured without external heating often have fewer voids than traditionally cured epoxies and result in repairs that have fewer potential weaknesses. These UV resins have been shown to cure faster than thermally activated epoxies, reducing repair times by 50% and helping decrease costs and minimize aircraft downtime.
UV-cured resins do have some limitations. Typically, they are restricted to use with fiberglass, quartz, and other light-colored fiber reinforcements. This is because carbon fiber matrices are dark and absorb a significant amount of the UV light exposed to the repair, causing uneven curing toward the bottom in thicker sections of the patch. For this reason, UV-curable composites are also limited in their thickness by necessity. Despite these limitations, UV resins still provide great advantages and potential when compared to the current deployed two-part system.
There is an even more innovative type of resin in development—an anaerobically-cured epoxy acrylate with greater potential. This resin replaces UV initiators with anaerobic curing agents and results in a very stable resin capable of being stored at ambient temperatures for months, but with the ability to cure very quickly with a vacuum. Tested anaerobic resins have cured even faster than their UV curing counterparts, resulting in a start-to-finish repair time that can be as little as two hours (examples shown in Figures 2 and 3).
Component systems greatly simplify the composite repair processes, eliminating process steps and the need for cumbersome, expensive equipment. Their ease of use greatly increases weapon system readiness for damaged aircraft and decreases the cost and man-hours associated with the required repairs. Their stability at ambient temperatures makes them ideal for in-field repairs. Tests of these repairs show strength comparable to and sometimes exceeding traditional epoxy patches, ensuring the repair does not compromise the integrity of the aircraft or the safety of the personnel aboard.