The use of composite materials is on the rise. Consequently, this guide was developed to provide base-level Bioenvironmental Engineering (BE) personnel with a comprehensive baseline for identifying, evaluating, and controlling occupational and environmental hazards associated with composite fibers and materials.
The expectation from this guide was to assist commanders with risk management decisions when responding to composite materials in (1) aircraft repair and maintenance and (2) crash and recovery operations. BE personnel should be able to identify potential inhalation and dermal hazards, recommend personnel protection options and decontamination procedures, recommend environmental controls, and select the most suitable sampling strategies and methods for a given scenario.
Generally, the term “composite materials” refers to fibers bound with a resin in a polymer matrix. The fibers within composites are the load-bearing elements, while the resin molecules fill the voids and transfer the stress from fiber to fiber. Composite materials are “advanced” if the material has properties of high strength, high stiffness, low weight, corrosion resistance and, in some cases, special electrical properties.
Industry uses a variety of fibers and binders in the fabrication of composite materials. The most common composite fibers encountered in the Air Force are glass, boron, carbon/graphite, and aramid (commonly known as Kevlar®). Glass fibers can be bound together by polymer resin to form fiberglass composites. The terms “graphite fibers” and “carbon fibers” are often used interchangeably from one reference to another. This may be because both materials are made from the same base material – carbon.
The distinction between graphite and carbon fibers depends on the purity of the carbon contained in the fiber and in the manufacturing methods used to refine the carbon into a composite material. Additionally, there are composite materials that blend two or more basic fiber types into a blended hybrid material, such as “carbon-aramid-fiberglass” composite materials. The Air Force Research Laboratory continues to research new materials such as aluminum-carbon nanofiber composites and boron nitride nanotubes. Furthermore, an aircraft structure can be composed of numerous types of composites and metals as shown in the accompanying figure. This type of structure is known as a hybrid structure. While there is a variety of fibers and fiber blends used in the Air Force, health risks are primarily associated with the inhalation and the contact hazard exposure pathways.
Although some studies show composite fibers to be a potential health risk, other studies show the risk to be relatively less than that of asbestos or silica. To date, a limited number of studies on the toxicology of inhaled carbon fibers have been conducted. A few studies have been conducted that relate to exposure from fibers and dusts in the workplace. These studies concluded that no longterm health risks have been associated with exposure to raw carbon fibers under occupational conditions.
Some animal studies with raw carbon fibers and composite dust have also been conducted. It was concluded that carbon fiber and composite dust are significantly less toxic than crystalline silica dusts and fibers, such as asbestos, although more research was suggested to verify these findings. Additional research is needed to address the toxicity to composite fibers and their matrices.
Airborne composite materials and fibers have the potential to be hazardous to the respiratory system. The Mine Safety and Health Administration (MSHA) defines respirable dust as the fraction of airborne dust that passes a size-selecting device having the characteristics found in the following table.
Even though compliance agencies such as MSHA and the Occupational Safety and Health Administration (OSHA) specify a 50% cutpoint of 3.5 microns, the American Conference of Governmental Industrial Hygienists (ACGIH) specifies a 50% cutpoint of 4 microns. This is in accordance with the International Organization for Standardization/European Standardization Committee (ISO/CEN) protocol.
Generally, dust and fibers that meet the aerodynamic characteristic described above penetrate the airway beyond natural clearance mechanisms (cilia and mucous) and can become trapped within the alveoli of the lungs (sedimentation). Any time a foreign product is introduced into the respiratory tract, a risk exists of pulmonary scarring or other long-lasting respiratory damage. Because these particles enter where the gas exchange takes place within the lungs, other complications can arise as a result of exposure to the toxic products of combustion.
This work was done by Major Jon E. Black, Major Richard Yon, Captain Timothy Batten, Mr. David DeCamp, and Dr. Gregory Schoeppner for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) below. AFRL-0296