Safety issues with advanced composite materials

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...


What is important to understand is that when advanced composite materials are machined or abraded, the resin matrix shatters liberating tiny filaments (fibers) of random length. Fiberglass and carbon/graphite filaments essentially maintain their original diameter during the machining process. Only aramid fibers show a mild tendency to shear down their length and generate sub-fibers, or "fibrils." Therefore, one can form some opinions as to what hazards are posed from the dust generated during composite machining operations based on the filament diameters. Carbon and graphite fibers tend to be about 7 microns in diameter, aramid fibers about 8 microns, and glass fibers range from 3 to 25 microns with the vast majority being 6 microns or larger. If the cut-off for fibers in the respirable category is 3 microns then, in theory, none of these materials poses a respiratory hazard. On the other hand we used to think asbestos was perfectly safe too.

The fact is that a small amount of the fibers and more of the pulverized resin dust can fall into the respirable category. The first question then becomes what is the permissible exposure limit (PEL). At the moment, it is generally recommended that dust from machining carbon, aramid, and glass fiber composites be controlled to standards for particulates not otherwise classified (NOC). The PEL for particulates NOC is 15 milligrams per cubic meter, 5 milligrams of which is allowed to be respirable. OSHA, the American Conference of Governmental Industrial Hygienists (ACGIH), and the U.S. military have recommended a somewhat lower PEL for carbon fiber of 10 milligrams per cubic meter (5 milligrams respirable).

Once we know what the exposure limit is, the second question becomes how much am I being exposed to? This is a question for your industrial hygienist and your safety officer to wrestle with. What is supposed to happen is this: An industrial hygienist conducts sampling of the air an employee is breathing during the machining and grinding operations that are typically conducted in your facility. The samples are analyzed to establish how much, and what kind of dust personnel are being exposed to. This information along with some recommendations is turned over to your safety officer in the form of a written report. It is the safety officer's job to compare the actual exposure to the regulated exposure limits (PEL and TLV) and decide what sort of engineering controls need to be in place and what personal protective equipment will be required for certain tasks.

Composite particulars
Every industry has its own set of unique safety issues to deal with and composites are no exception. Two common safety concerns that arise are uncontrolled exotherm and sensitization.

Uncontrolled exotherm
When we mix epoxies, molecules of the base resin cross-link to those of the hardener as it cures. A by-product of this reaction is heat. If the resin is spread out over a large surface area the heat dissipates naturally and the resin cures at a predictable rate. However, if a sufficient quantity of mixed resin is left in a container, the heat cannot escape and builds up in the resin. This by itself might not be a problem except that for every 18-degree (F) rise in temperature, the rate of cross-linking doubles. So as the heat builds within the resin mass it accelerates the cross-linking, which creates more heat, and so on. The resultant bubbling, smoking, fiery mess is called an uncontrolled exotherm. It is often called a runaway, or hot batch by shop personnel. The smoke from such an occurrence is toxic and inhalation should be avoided at all costs.

The best solution for the problem of uncontrolled exotherms is prevention. Never mix more resin than you really need for the job and never allow mixed resin to remain in a container longer than absolutely necessary. Get it out of the container and into the work as quickly as possible.

Sensitization
The phenomenon of sensitization has plagued the aviation industry for many years. Although it is more commonly associated with paints and coatings, it can also be caused by contact with matrix resin systems. Sensitization can best be likened to an allergic reaction that develops upon repeated exposure to a chemical. It can generally be classified in one of two ways. Dermal sensitization, which manifests itself as a skin rash, or respiratory sensitization, which elicits an asthmatic reaction and can be quite severe.

It is important to understand that with sensitization reactions, the severity or extent of the reaction does not necessarily relate to the degree of exposure. A person who is sensitized to a certain substance can react strongly when exposed to trace amounts of it. Some medical details about sensitization are still not fully understood, but this much we know for sure, your chances of becoming sensitized to a substance go up with every exposure. Once sensitized there is no "cure," and you'll be forced to find another line of work. Again, the simple solution is prevention. Always wear appropriate personal protective equipment (PPE) when working with resins. Minimum PPE would be the proper gloves, apron, and safety glasses or goggles when working with wet resins. Gloves alone will normally suffice when working with prepreg materials.

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