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Fundamentals of Corrosion
5.1 Radiation
Polymeric materials in outdoor applications are exposed to weather
extremes that can be extremely deleterious to the material, the most harmful of which is exposure to ultraviolet (UV) radiation, which can cause
embrittlement, fading, surface cracking, and chalking. Most plastics, after
being exposed to direct sunlight for a period of years, exhibit reduced
impact resistance, lower overall mechanical performance, and a change
in appearance.
The electromagnetic energy from sunlight is normally divided into UV
light, visible light, and infrared energy. Infrared energy consists of wavelengths longer than the visible red wavelengths, and starts above 760 nm.
Visible light is defined as radiation between 400 and 760 nm. UV light consists of radiation below 400 nm. The UV portion of the spectrum is further
subdivided into UV-A, UB-B, and UV-C. The effects of the various wavelength regions are shown below:
Ultraviolet Wavelength Regions
Region
Wavelength
(nm)
Characteristics
UV-A
400–315
Causes polymer damage
UV-B
315–280
Includes the shortest wavelengths at the Earth’s surface
Causes severe polymer damage
Absorbed by window glass
UV-C
280–100
Filtered by the Earth’s atmosphere
Found only in outer space
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. 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. During the winter months, much of the damaging
shortwavelength UV light is filtered out. For example, the intensity of UV at
320 nm changes about 8 to 1 from summer to winter. In addition, the shortwavelength solar cut-off shifts from about 295 nm in summer to about 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 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
Fundamentals of Corrosion
5.1 Radiation
Polymeric materials in outdoor applications are exposed to weather
extremes that can be extremely deleterious to the material, the most harmful of which is exposure to ultraviolet (UV) radiation, which can cause
embrittlement, fading, surface cracking, and chalking. Most plastics, after
being exposed to direct sunlight for a period of years, exhibit reduced
impact resistance, lower overall mechanical performance, and a change
in appearance.
The electromagnetic energy from sunlight is normally divided into UV
light, visible light, and infrared energy. Infrared energy consists of wavelengths longer than the visible red wavelengths, and starts above 760 nm.
Visible light is defined as radiation between 400 and 760 nm. UV light consists of radiation below 400 nm. The UV portion of the spectrum is further
subdivided into UV-A, UB-B, and UV-C. The effects of the various wavelength regions are shown below:
Ultraviolet Wavelength Regions
Region
Wavelength
(nm)
Characteristics
UV-A
400–315
Causes polymer damage
UV-B
315–280
Includes the shortest wavelengths at the Earth’s surface
Causes severe polymer damage
Absorbed by window glass
UV-C
280–100
Filtered by the Earth’s atmosphere
Found only in outer space
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. 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. During the winter months, much of the damaging
shortwavelength UV light is filtered out. For example, the intensity of UV at
320 nm changes about 8 to 1 from summer to winter. In addition, the shortwavelength solar cut-off shifts from about 295 nm in summer to about 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 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