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500°F. As the JP-8 flame temperature penetrates the
composite, the epoxy decomposes in a matter of seconds.
If the composite laminate is not very thin, epoxy
will begin to smolder after the flame ceases at much lower
temperature than the flame temperature. The smoldering
is described as a slow, flameless form of combustion
emitting toxic compounds. It is difficult to detect epoxy
smoldering because little or no visible smoke is produced
(smoke is being produced, it is just not detectable in the
visible range). Smoldering epoxy is not sensitive to wind
and does not spread to areas that didn’t previously experience
an increase in temperature. Smoldering composites
are dangerous because the condition can go undetected
and can easily transition to flaming combustion.
Carbon fiber combustion starts to occur when most or all
resin has burned off and the external heat is around
1000°F (fiber type dependent). This is the stage when
carbon fiber oxidizes and decomposes creating fibrils.
Adequate airflow is needed to supply sufficient oxygen
to self-sustain. A red glow is visible at higher temperatures
(~1400°F). Smoldering epoxy doesn’t generate
enough heat to cause carbon fiber combustion.
(b) COMBUSTION PRODUCTS. The final combustion
product for all organic material is carbon, carbon dioxide
(CO2) and water. Complete combustion is never reached.
Carbon monoxide (CO) and many other products of incomplete
combustion will be produced. See Table 3.5-2
for a list of possibilities.
1 JP-8 FUEL. JP-8 fuel consists of aliphatic and
aromatic hydrocarbons with small amounts of proprietary
additives. The plume at an aircraft mishap is a dense
black sooty smoke, majority of it coming from the burning
of JP-8. The smoke contribution from the composite
materials is minimal in comparison to the quantity of fuel
burning. Soot is carbon particles and products of incomplete
combustion. The soot has a very small particle
size, therefore will rise in the thermal column, become
diluted and disperse downwind.
2 MISCELLANEOUS AIRCRAFT MATERIALS. In addition
to carbon and hydrogen resins, adhesives, plastics,
core material and coatings contain other chemical
elements. Elements like oxygen, nitrogen, chlorine, bromine,
fluorine, and metal compounds. These elements
contribute to the increase in the amount or number of
toxic gases and irritants generated. Toxic products released
from the combustion of coatings and the composite
are carried with the JP-8 smoke.
3 FIBERS. Melting glass fibers can fuse, which destroys
the fiber shape. If melting glass is released during
a flaming combustion stage, glass beads will form
and do not remain airborne.
Burning Kevlar® fiber decomposes releasing toxic combustion
products similar to burning resin, plastic or wool.
When the ignition source is removed Kevlar® tape and
fabrics are not expected to continue to burn. Kevlar®
pulp in core material may smolder.
Carbon fiber is ~ 92-98% carbon, the remaining is nitrogen
and trace processing contaminates. Burning carbon
fiber releases nitrogen. Oxidation erodes the fiber
causing a change in the original diameter and length of
the fiber. Smaller size fibers can become airborne and
remain airborne during a flaming combustion condition
(PHOTO 32 and 33). Unidirectional carbon fiber tape
engulfed in the flame for a period of time forms clusters,
decomposed carbon fibers, and fiber ash. Clusters form
first, then the decomposing fiber and finally the ash forms.
TO 00-105E-9
3-25
Fiber clusters are lightweight but will not remain suspended
in air. If created, clusters will be found all around
the site. Clusters do not cause puncture wounds. The
matrix is needed to support the fiber to provide the stiffness
necessary to cause a puncture. Small size fibers
will travel with the plume but starts to settle out while the
plume continues downwind. Due to surface winds,
plumes are almost always tilted. Higher wind velocities
increase the plumes angle of tilt that will result in a limited
plume rise and the potential for an increased ground
level concentration. Single carbon fibers are hard to see
floating in the air. They may linger around the burnt debris
after the fire is out but will settle out. Re-suspension
potential is greatest right after the initial deposit (24 to 48
hours). With weathering the fibers become incorporated
in the environment (into the soil) and the chance of resuspension
drops quickly. Dry areas with little vegetation
will increase the potential of re-suspension.
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