Safety issues with advanced composite materials
By Greg Mellema
The last 25 years has seen a dramatic rise in the use of advanced composite materials on aircraft. Since these materials aren't as old and well-sorted-out as the metal technology traditionally used to build aircraft, companies often struggle with how to effectively manage composite materials, not only from a repair standpoint, but a safety standpoint as well.
Getting a firm grasp of the safety issues associated with composite materials is a difficult task at best. Accurate information concerning the real hazards posed by composite materials is difficult to come by and even more difficult to stay abreast of. The following is a discussion of the hazards generally associated with advanced composite materials. It is by no means all-inclusive, but it will give you a basic understanding of the kind of safety issues you need to be on the lookout for.
The hazards associated with the repair of advanced composite structures can be divided into three basic categories; matrix resins, reinforcement fibers, and a short list of miscellaneous issues peculiar to composite materials.
There are literally hundreds of different resin matrix systems. Some of the more common thermoset resins include bismaleimide (BMI), polyimide, and phenolic, but the single most common resin system used in aerospace today is epoxy. Epoxies are well suited for use as matrix materials; they have excellent adhesive properties and come in a wide range of strength, durability, and processing options.
Unmixed epoxy resins typically come in liquid form and in two parts, the base resin (component A) and the hardener (component B). When these are mixed together in the correct proportions they cross-link, or cure, to create a durable solid material. While there are certainly exceptions, component A is generally considered to have a relatively low order of toxicity. Component B, however usually contains aromatic and/or aliphatic amine compounds. These corrosive amines are skin, eye, and often, respiratory irritants. Many modern aircraft call for the use of prepreg materials to affect structural repairs. Prepreg is fabric that has previously been impregnated with a mixed resin. The resin is then "B-staged," or partially cross-linked until it no longer flows at room temperature. While these materials are less messy and more convenient to use, the resins still contain amines and other compounds, so contact with the skin or eyes must be avoided.
The most common reinforcement fibers found in the aerospace industry are carbon and graphite fiber, Aramid fiber (Kevlar®), and fiberglass. These materials are chemically inert and pose no health risk while in their dry fabric form or when cured in a resin matrix. However, machining a cured laminate liberates short fibers and allows them to become airborne. It is these airborne fibers that are a potential concern. The long, thin shape of such a fiber may cause it to become trapped deep in the lung and damage lung tissue.
At the end of the bronchial passages in the lung are tiny sacks called alveoli. Through the walls of the alveoli gas exchange takes place. Your blood gives up carbon dioxide and takes on oxygen to be distributed throughout the body. If a fiber is inhaled and manages to penetrate the alveoli, it typically becomes lodged inside. Ultimately this fiber scars the tissue and renders the alveoli incapable of gas exchange.
OSHA uses the term "respirable" to describe fibers of this type and respiratory protection must be used to limit their inhalation. In order to qualify as respirable a particle must have an aerodynamic diameter of less than 3 microns. Aerodynamic diameter is the most important determinant of the respirability of a fiber; it is different from its actual geometric diameter. The aerodynamic diameter of a fiber takes into account the fiber's density and aspect ratio as well as its geometric diameter. Fibers larger than this are deposited in the nose, or on the tracheal or bronchial walls where they are disposed of by normal lung clearance mechanisms.
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