Corrosion of Paint
209
differences between summer and winter daylight hours. During the winter
months, much of the damaging short-wavelength UV light is filtered out.
For example, the intensity of UV light at 320 nm changes about 8 to 1 from
summer to winter. In addition, that short-wavelength solar cutoff shifts from
approximately 295 nm in summer to approximately 310 nm in winter. As a
result, materials sensitive to UV below 320 nm would degrade only slightly,
if at all, during the winter months.
Photochemical degradation is caused by photons or 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 a bond, but longer wavelengths of light cannot break it. Therefore, the short-wavelength cutoff of a
light source is of critical importance. If a particular polymer is sensitive only
to light below 295 nm (the solar cutoff point), it will never experience photochemical deterioration outdoors.
The ability to withstand weathering varies with the polymer type and
within grades of a particular resin. Most resin grades are available with
UV-absorbing additives to improve weatherability. However, the highermolecular-weight grades of a resin generally exhibit better weatherability
than the lower-molecular-weight grades with comparable additives. In addition, some colors tend to weather better than others.
Several artificial light sources have been developed to simulate direct sunlight. In the discussion of accelerated weathering light sources, the problems
of light stability, the effects of moisture and humidity, the effects of cycles,
or the reproducibility of results are not taken into account. Simulations of
direct sunlight should be compared to what is known as the solar maximum
condition — global moon sunlight on the summer solstice at normal incidence. The most severe condition that can be encountered in outdoor service
is the solar maximum, which controls the failure of materials. It is misleading to compare light sources against “average optimum sunlight,” which is
an average of the much less damaging March 21 and September 21 equinox
readings.
TabLE 7.6
UV Wavelength Region Characteristics
Region
Wavelength
(nm)
Characteristics
UV-A
400–315
Causes polymer damage
UV-B
315–200
Includes the shortest wavelengths found at the Earth’s surface
Causes severe polymer damage
Absorbed by window glass
UV-C
280–100
Filtered out by the Earth’s atmosphere
Found only in outer space
209
differences between summer and winter daylight hours. During the winter
months, much of the damaging short-wavelength UV light is filtered out.
For example, the intensity of UV light at 320 nm changes about 8 to 1 from
summer to winter. In addition, that short-wavelength solar cutoff shifts from
approximately 295 nm in summer to approximately 310 nm in winter. As a
result, materials sensitive to UV below 320 nm would degrade only slightly,
if at all, during the winter months.
Photochemical degradation is caused by photons or 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 a bond, but longer wavelengths of light cannot break it. Therefore, the short-wavelength cutoff of a
light source is of critical importance. If a particular polymer is sensitive only
to light below 295 nm (the solar cutoff point), it will never experience photochemical deterioration outdoors.
The ability to withstand weathering varies with the polymer type and
within grades of a particular resin. Most resin grades are available with
UV-absorbing additives to improve weatherability. However, the highermolecular-weight grades of a resin generally exhibit better weatherability
than the lower-molecular-weight grades with comparable additives. In addition, some colors tend to weather better than others.
Several artificial light sources have been developed to simulate direct sunlight. In the discussion of accelerated weathering light sources, the problems
of light stability, the effects of moisture and humidity, the effects of cycles,
or the reproducibility of results are not taken into account. Simulations of
direct sunlight should be compared to what is known as the solar maximum
condition — global moon sunlight on the summer solstice at normal incidence. The most severe condition that can be encountered in outdoor service
is the solar maximum, which controls the failure of materials. It is misleading to compare light sources against “average optimum sunlight,” which is
an average of the much less damaging March 21 and September 21 equinox
readings.
TabLE 7.6
UV Wavelength Region Characteristics
Region
Wavelength
(nm)
Characteristics
UV-A
400–315
Causes polymer damage
UV-B
315–200
Includes the shortest wavelengths found at the Earth’s surface
Causes severe polymer damage
Absorbed by window glass
UV-C
280–100
Filtered out by the Earth’s atmosphere
Found only in outer space