Keywords: hydrophobic coating, radome coating, rain attenuation,
rain fade, signal loss, super hydrophobic, superhydrophobic, super-hydrophobic,
improved satellite reception, Ku Band, Ka Band, Fluorothane.
Note: See Fluorothane or WX2100
for hydrophobic and superhydrophobic aircraft, marine and ground radome
coatings. See FluoroPel for fluoropolymers
and FluoroSyl for flluorosilanes.
Evaluation of Heavy Rain Performance for Various Hydrophobic Coatings
Following Long-Term UV Exposure
Purpose. This report attempts to address selected issues presented
in Army Regulation No. 70-38, which conforms to MIL-STD-210B and MIL-STD-810E.
These documents provide significant and interesting data about real-world
extremes in solar radiation and rain fall as well as establishing useful
test guidelines. A further aim is to present a comparison of various commercially
available hydrophobic and super-hydrophobic coatings following and during
conditions outlined in AR-70-38.
Solar Radiation Extremes. In hot, dry conditions found in the northern
deserts of Africa and Asia, the intensity of solar radiation can reach 1120
Watts/meter squared. The average of solar radiation in this region over
24 hours can exceed 386 W/m2. Seasonal daily minimums for these hot, dry
locations is 334 W/m2, and yearly averages are 360 W/m2. Total solar energy
reaching a test surface, at these locations is about (10)10 Joules/m2 per
year. Solar intensities in the hot, humid conditions at these latitudes
tend to be somewhat lower.
Damaging Radiation. The hard ultraviolet component of solar radiation
is primarily responsible for the degradation of hydrophobic coatings. The
most damaging spectral component is between 0.28 and 0.32 microns, amounting
to 0.45 percent of total radiance or about 5 W/m2. Total annual hard UV
reaching a test surface under extreme conditions would be 4.5 (10)7 Joules/m2
per year. The less damaging component between 0.32 and 0.40 microns represents
5.6 percent of the total or 63 W/m2. Little photo-chemical transformation
generally occurs in response to visible or infrared light.
Rainfall Extremes. The world's highest rainfall intensities occur
in Southeast Asia. Operational values during the rainiest periods are typically
5 inches of rain per hour at intermittent wind speeds of 15 meters per second.
Drop size is predominantly (2626 drops out of 3021) between 0.5 and 1.5
millimeters The highest intensities can reach 60 inches of rain per hour
for brief periods at wind speeds of 45 m/s.
Contact Angle and Rain Run-off: A test surface inclined at 45 degrees
will shed heavy rain in one of three modes, depending on its static contact
angle and smoothness. For static contact angles less than about 90 degrees,
heavy rain will form a sheet or film uniformly covering the surface. At
contact angles between about 90 and 130 degrees, rivulets form, allowing
water to run off as large drops or in streams. As surfaces become extremely
hydrophobic, with contact angles between 130 and 170 degrees, even extreme
rain tends to run off as very small drops.
Rain Testing After Hard UV Exposure. Since microwave antenna performance
has been shown to diminish as water filming increases, measurement of contact
angle throughout a heavy rain challenge seems important. Initial contact
angle data for unchallenged surfaces are of little benefit in preventing
rain attenuation unless those contact angles are maintained.
Rain and Radiation Simulation. An apparatus for simulating heavy
rain was developed to produce 10 to 60 inches of rain per hour with impact
speeds of 25 to 45 meters per second and drop sizes statistically conforming
to the Army data. To rapidly measure the effect of ultraviolet radiation,
a temperature controlled, low pressure mercury lamp chamber was constructed
to produce the UV equivalent of 4 years of Asian desert sun light per hour
at 120 degrees F. Although not measured, there is a substantial amount of
ozone in the UV chamber during a test, and resistance to ozone may indicate
further resistance to other industrial and natural corrosive agents.
The graph compares the contact angles during heavy rain exposure of Cytonix's
Fluorothane M-Series, FluoroPel WX, Boyd Coatings Research's "Liquid
Teflon", Channel Master's Jones-Blair coating, and Prodelin's "Super-Hydrophobic"
coating. Prodelin and Channel Master provided panels for testing, and the
Boyd coating was applied according to the manufacturer's specification.
Panels were exposed to the equivalent of ten years equitorial sun from a
400 watt low pressure mercury lamp with parabolic aluminum reflector at
a distance of 8 inches from the focal point. Panels were then exposed to
scattered droplets from a high velocity water stream impacting a smooth
plastic surface at 45 degrees. The scattered droplets have a drop size distribution
similar to heavy rain, reaching the test panels at a rate of 10 inches per
hour. The Prodelin, Channel Master and Boyd panels all had contact angles
below 90° and uniform water filming after the first few hours of the
rain challenge. Fluorothane M-Series and FluoroPel WX exhibited no significant
loss of contact angle after 240 hours and 2400 inches of rain. In subsequent
tests, Fluorothane M-Series and FluoroPel WX had 145° contact angles
after exposure to extreme rain at 60 inches per hour for one hour.
Cytonix Corporation
8000 Virginia Manor Road
Beltsville, MD 20705
phone: (888) CYTONIX or (301) 470-6267
fax: (301) 470-6269 or email: emailbox@cytonix.com
