Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
Подождите немного. Документ загружается.
This Page Intentionally Left Blank
12
Sunlight, Ultraviolet, and Accelerated
Weathering
Patrick Brennan and Carol Fedor
The Q-Panel
Company,
Cleveland,
Ohio
1
.O
INTRODUCTION
Sunlight is an important cause
of
damage
to
coatings. Short wavelength ultraviolet light
has long been recognized as being responsible for most of this damage.'
Accelerated weathering testers use a wide variety of light sources to simulate sunlight
and the damage that it causes. Comparative spectroradiometric measurements of sunlight
and laboratory testers of various types show a wide variety of UV spectra. These measure-
ments highlight the advantages and disadvantages of the commonly used accelerated light
sources: enclosed carbon arc, sunshine carbon arc, xenon arc, and fluorescent UV. The
measurements suggest recommendations for the use of different light sources for different
applications.
2.0 SUNLIGHT
The electromagnetic energy from sunlight is normally divided into ultraviolet light, visible
light, and infrared energy. Figure
1
shows the spectral energy distribution
(SED)
of noon
midsummer sunlight. Infrared energy (not shown) consists
of
wavelengths longer than the
visible red wavelengths and starts above
760
nanometers (nm). Visible light is defined as
radiation between
400
and
760
nm. Ultraviolet light consists of radiation below
400
nm.
The International Commission on Illumination (CIE) further subdivides the UV portion
of the spectrum2 into UV-A, UV-B, and UV-C as shown in Figure
1.
The effects of the
various UV wavelength regions are summarized in Table
1.
2.1 Variability
of
Sunlight
Because UV is easily filtered by air masses, cloud cover, pollution, and other factors, the
amount and spectrum of natural UV exposure is extremely variable. Figure
2
compares
the UV regions
of
sunlight, measured at Cleveland, Ohio, at noon on:
103
104
2.0
1.5
5
2
1.0
8
cu'
E
C
U
m
.-
2
0.5
L
-
0.0
UV
Region
250
BRENNANANDFEDOR
350
450
550
650
Wavelength
(nanometers)
Figure
1
The sunlight spectrum
-The summer solstice
-The winter solstice
-The spring equinox
These measurements are in essential agreement with data reported by other investigators.4
Because the sun is lower in the sky during the winter months, it
is
filtered through
a greater air mass. This creates two important differences between summer and winter
sunlight: changes in the intensity
of
the light and in the spectrum. Most important, much
of
the damaging, short wavelength UV light is filtered out during winter. For example,
the intensity
of
UV at
320
nm changes about
8
to
l
from summer to winter.
In
addition,
the short wavelength
solar
cutoff shifts from about
295
nm
in
summer
to
about
310
nm
in winter. Consequently, materials sensitive to UV below
320
nm
would degrade only
slightly, if at all, during the winter months.
Table
1
Wavelength Regions
of
the
UV
Region Wavelength (nm) Characteristics
UV-A
400-3
15
Causes polymer damage
UV-B
315-280
Includes the shortest wavelengths found at the earth's
surface; responsible for severe polymer damage; absorbed
by window glass
germicidal
uv-c
280-
100
Found only in outer space; filtered out by earth's atmosphere;
Source:
Ref.
3.
SUNLIGHT, ULTRAVIOLET, AND ACCELERATED WEATHERING
105
1.2
1
.o
0.8
0.6
0.4
0.2
0.0
260 280 300 320 340
360
380 400
Wavelength (nanometers)
Figure
2
Seasonal variation
of
sunlight
UV.
3.0
ACCELERATED LIGHT SOURCES COMPARED TO SUNLIGHT
The following discussion
of
accelerated weathering light sources confines itself to the
question of UV spectrum. It does not address problems
of
light stability, the effects of
moisture and humidity, the effects of cycles, or the reproducibility of results. For simula-
tions
of
direct sunlight, artificial light sources should be compared to what we call the
Solar Maximum condition: global, noon sunlight, on the summer solstice, at normal inci-
dence. The Solar Maximum is the most severe condition met in outdoor service, and as
such it controls which materials will fail. It is misleading to compare light sources against
“average optimum sunlight,” which is simply an average
of
the much less damaging
March
21
and September
21
equinox readings. In this chapter, graphs labeled “sunlight”
refer to the Solar Maximum: noon, global, midsummer sunlight. Despite the inherent
variability of solar UV, our measurements show surprisingly little variation in the Solar
Maximum at different locations. Figure
3
shows measurements of the Solar Maximum at
three widely varied locations.
3.1
The Importance
of
Short Wavelength Cutoff
Photochemical degradation is caused by photons of light breaking chemical bonds. For
each type of chemical bond, there is a critical threshold wavelength of light with enough
energy to cause
a
reaction. Light
of
any wavelength shorter than the threshold can break
the bond, but longer wavelengths of light cannot break it regardless
of
their intensity
(brightness). Therefore, the short wavelength cutoff of
a
light source is of critical impor-
tance. For example, if
a
particular polymer is sensitive only to UV light below
295
nm
(the solar cutoff point), it will never experience photochemical deterioration outdoors. If
the same polymer is exposed to
a
laboratory light source that has a special cutoff
of 280
BRENNAN AND
FEDOR
1.2-
Kitt
Peak 6/86
.
Cleveland 6/86
b
Miami
6/87
AA
0.8
-
0.6
-1
0.4
l
o.2L 0.0
260 280 300 320 340 360 380
400
Wavelength (nanometers)
Figure
3
Solar
maximum
at
three
locations.
nm, it
will
deteriorate. Although light sources whose spectra include the shorter wave-
lengths produce faster tests, there's a possibility of anomalous results if a tester has a
wavelength cutoff too far below that of the material's end-use environment.
4.0
ARC-TYPE LIGHT SOURCES
4.1
Enclosed Carbon Arc (ASTM G-23)
The enclosed carbon arc has been used as a solar simulator in accelerated weathering and
lightfastness testers since 1918. Many test methods still specify its use. When the light
output of this apparatus is compared to sunlight, some deficiencies become obvious. Figure
4
compares the
UV
spectral energy distribution of summer sunlight (Solar Maximum) to
that of the enclosed carbon arc. The
UV
output of the enclosed carbon arc primarily
consists of two very large spikes of energy, with very little output below 350 nm. Since
the shortest
UV
wavelengths are the most damaging, the enclosed carbon arc gives very
slow tests on most materials and poor correlation on materials sensitive to short wavelength
uv.
4.2 Sunshine Carbon Arc (Open Flame Carbon Arc: ASTM G-23)
The introduction of the sunshine carbon arc in 1933 was an advantage over the enclosed
carbon arc. Figure
5
plots the
UV
SED
of summer sunlight against the
SED
of a sunshine
carbon arc (with Corex
D
filters). While the match with sunlight is superior to the enclosed
carbon arc, there is still a very large spike of energy, much greater than sunlight, at about
390 nm.
SUNLIGHT, ULTRAVIOLET, AND ACCELERATED WEATHERING
Enclosed Carbon Arc
\
Wavelength
(nm)
Figure
4
Enclosed carbon arc and sunlight.
5
4
107
Sunshine Carbon Arc
0
260 280 300 320
340 360
380
400
Wavelength (nrn)
Figure
5
Sunshine carbon arc
and
sunlight:
260-400
nm.
BRENNAN AND FEDOR
260
280 300
Wavelength (nm)
320
Figure
6
Sunshine carbon arc and
sunlight:
260-320
nm.
A
more serious problem with the spectrum of the sunshine carbon arc is found
in
the short wavelengths. To illustrate this, a charge
of
scale is necessary to expand the low
end of the graph. Figure
6
shows Solar Maximum versus sunshine carbon arc between
260 and 320 nm. The carbon arc emits
a
great deal
of
energy in the
UV-C
portion
of
the
spectrum, well below the normal solar cutoff point of 295 nm. Radiation
of
this type is
realistic for outer space, but is never found at the earth's surface. These short wavelengths
can cause unrealistic degradation when compared to natural exposures.
4.3
Xenon
Arc
(ASTM
G-26)
The xenon arc was adapted for accelerated weathering in Germany
in
1954.
Our under-
standing of the xenon arc spectra is complicated by two variables: the effect of filters and
the effect
of
irradiance settings.
Xenon arcs require a combination
of
filters to reduce unwanted radiation. Automo-
tive test methods that call for xenon arc exposure usually specify quartz inner and borosili-
cate outer filters. This filter combination allows severe, unrealistically short wavelength
UV
(as low as 270 nm).
The most widely used filter combination for the xenon arc is borosilicate inner and
outer filters (borohoro). This is a better simulation of sunlight than the quartzhoro filters,
because the cutoff wavelength of the borohoro is approximately 280 nm, somewhat closer
to the sunlight cutoff
of
295 nm. Figure
7
compares the xenon arc with the two filter
combinations to the Solar Maximum.
Recent xenon arc models have
a
monitoring system to compensate for the inevitable
light output decay due to lamp aging. The most common irradiance settings are 0.35 or
0.55
Wlm' at
340
nm. Figure
8
shows how these two settings (with borohoro) filters
SUNLIGHT, ULTRAVIOLET, AND ACCELERATED WEATHERING
109
E
N'
E
C
2
a,
V
K
m
U
.-
e
-
L
Wavelength
(nm)
Figure
7
Xenon-effect
of
filters.
E
N'
E
c
3
a,
V
C
m
-0
._
e
-
L
260
280
300
320
340
360
380
400
Wavelength
(nm)
Figure
8
Xenon-effect of irradiance setting.
110
BRENNAN AND FEDOR
compare with the Solar Maximum. While
0.55
compares better with summer sunlight,
0.35
is more like winter sunlight. However, for practical reasons,
0.35
is the setting most
commonly used.
5.0
FLUORESCENT UV LAMPS
Since the
1970s,
fluorescent
UV
and condensation testers have come into wide use. There
are different lamps, with different spectra, for different exposure applications. The fluores-
cent
UV
testers and the arc testers use different approaches. The former do not attempt
to reproduce sunlight itself, just the damaging effects of sunlight. This approach is effective
because short wavelength
UV
causes almost all the damage to durable materials exposed
outdoors. Consequently, fluorescent
UV
testers confine their primary light emission to
the
UV
portion of the spectrum.
5.1 FS-40 Lamp (F40-UVB) (ASTM G-53)
In the early
1970s,
the
FS-40
became the first fluorescent
UV
lamp to achieve wide use.
This lamp has been specified in some automotive test methods, particularly for coatings.
Most
of
the
FS-40's
output is in the
UV-B
portion of the
UV
spectrum, along with some
UV-A.
This lamp has demonstrated good correlation to outdoor exposures for the
gloss
retention on coatings" and for the material integrity of plastics. However, the short wave-
length output below the solar cutoff can occasionally cause anomalous results, especially
for color retention of plastics and textile materials.'
5.2
UVB-313 Lamp (ASTM G-53)
Introduced in
1984,
the
UVB-313
is essentially a second-generation
FS-40.
It has the
same
SED
as the
FS-40,
but its output is higher and more stable. Figure
9
plots the Solar
1.2-
1
.o
-
0.8
-
0.6
1
0.4
0.0
1.2-
1
.o
-
0.8
-
0.6
-
0.4
-
0.2
-
0.0
260
280
300
320
340
360 380
400
Wavelength (nm)
260
280
300
320
340
360 380
400
Wavelength (nm)
Figure
9
UVB-313
and
FS-40.
SUNLIGHT, ULTRAVIOLET, AND ACCELERATED WEATHERING 111
Figure
10
UVA-340
and
sunlight.
Maximum against the UVB-3 l3 and the FS-40. Because of its higher output the
UVB-3 13 gives significantly greater acceleration over the FS-40 for most materials. With
the exception of the automotive industry, the UVB-313 is the most widely used light
source for the ASTM G-53 devices.
5.3 UVA-340 Lamp (ASTM G-53)
The UVA-340 was introduced in
1987
to enhance correlation
in
the G-53 devices. Figure
10
shows the UVA-340 compared to the Solar Maximum. This lamp is an excellent
simulation of sunlight in the critical short wavelength UV region, from about 365 nm,
down to the solar cutoff of
295
nm. Because the UVA-340 eliminates the short wavelength
output (i.e.,
5
output lower than sunlight), which can cause unnatural test results,
it
allows
more realistic testing than many
of
the other commonly used light sources. The UVA-340
has been tested on both plastics and coatings and greatly improves the correlation possible
with the fluorescent UV and condensation devices.'
6.0
CONCLUSIONS
The correlation between laboratory and natural exposures probably will always be contro-
versial. As Fischer has indicated,' test speed and test accuracy tend toward opposition.
Accelerated light sources with short wavelength UV give fast tests but may not always
be accurate, Where they are wrong, however, they usually err on the safe side if they are
too severe. Light sources that eliminate wavelengths below the solar cutoff of
295
nm
will give better, more accurate results, but the price for increased correlation is reduced
acceleration. Users must educate themselves to make this choice.