MIL-PRF-81904C(AS)
4.6.2 Condition in container. While stirring the material with a hand paddle, the
material shall be observed for smoothness and homogeneity. There shall be no evidence
of pigment flotation, coarse particles, or settling. The condition of the coating in the
container, when examined, shall meet the requirements of 3.4.
4.6.3 Working properties.
4.6.3.1 Cure time. When the coating is applied to a thickness of 0.155 ±0.020 in.
(4 ±0.5 mm) on the prepared surface of a test panel, it shall be examined for conformance
to 3.5.1. Shore D hardness shall be tested in accordance with ASTM D2240.
4.6.4 Coating properties.
4.6.4.1 Thermal efficiency test (TET). This test exposes the coating compound to
a fire environment with a total heat flux of approximately 17.6 BTU/ft2-sec (200 kW/m2),
as measured by Schmidt-Boelter heat flux gauges. The coating compound shall be
prepared and applied to three test panels in accordance with 4.5.1.2.
4.6.4.1.1 Test setup. Prior to testing, five K-type thermocouples shall be welded
to the back of each of three test panels in an X-pattern, with one in the center, and one
approximately 1 inch from each corner. The sides and back of the test panel shall be
insulated with refractory fiber insulation, nominally 6 lbs/ft3 (96 Kg/m3). A holder shall
be used to mount the panel in a vertical plane and shall incorporate one Schmidt-Boelter
heat flux gauge directly above the centerline of the top surface of the panel, with the heat
flux gauge face in the same plane as the exposed surface of the panel. A propane-oxygen
torch shall be used to expose the test sample to a heat flux of 17.6 BTU/ft2-sec ± 1.8
BTU/ft2-sec (200 kW/m2+20 kW/m2). The torch shall be oriented with its face in a
vertical plane to burn the flame horizontally. The elevation of the torch relative to the
center of the test sample and the exact distance from the torch to the surface of the
sample shall be determined by the calibration run, specified in 4.6.4.1.2; however the
location shall ensure that flames impinge on the surface of the sample. A data acquisition
system, with a minimum sample rate of 1 sample per second (1 hertz), shall be used to
record the time, temperature, and heat flux measurements.
4.6.4.1.2 Calibration. Prior to each TET, a calibration run shall be conducted
using a blank plate with a Schmidt-Boelter heat flux gauge mounted in the center. The
face of the heat flux gauge shall be in the same plane as the exposed surface of the blank
plate. The average heat flux measured by the gauge shall be 17.6 BTU/ft2-sec ± 1.8
BTU/ft2-sec (200 kW/m2+20 kW/m2), once the gas flow has stabilized. The flow of gas
shall then be terminated, but the location of the torch shall be maintained. The heat flux
gauge specified in 4.6.4.1.1 is in addition to the gauge used for calibration and shall also
be in position during the calibration run the heat flux measured by this permanent heat
flux gauge shall be monitored during the TET to confirm expected torch performance.
4.6.4.1.3 Test procedure. Upon completion of the calibration run and
confirmation that the sample holder temperature is no more than 90º F (32º C), the blank
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