Exposure to UV Radiation
Over time, exposure to UV radiation from sunlight will degrade plastics. Polymer photo-degradation occurs when non-visible UV radiation made up of short wavelengths breaks down the polymer chains in plastics. This is called the photo-degradation process. It is caused by exposure to UV radiation and results in the deterioration of physical properties. These include loss of impact strength, changes in color, cracking, loss of elongation and tensile strength as well as chalking of the surface. For example, photo-degradation from UV radiation causes garden chairs to lose their gloss and become brittle. Also, the color of stadium seats appears chalky, and some plastics will even become yellow and crack.
Some Characteristics of UV Radiation
UV radiation accounts for only 4.6% of the solar spectrum. It is calculated in nanometers (nm) and ranges between 290 and 400. However, the most aggressive part of the UVB range is the very short wavelengths between 280 and 315 nanometers. The amount of UV radiation energy exposure (irradiation) depends on where you are in the world.
What Is Irradiation
Irradiation is the UV radiation energy incident over a specific area over a given period of time. 1Ly = 1 cal/cm2 = 4.184 E4Joule/m2 (Ly=Langley) Thus the amount of energy transmitted to a plastic part in one year of continuous outdoor use is 220kcal/cm2/year in Sudan while in Sweden it is 70.
Each plastic is sensitive to certain wavelengths within the 290-400 nm UV region. Polypropylene has three maxima at 290-300, 330, and 370 nm. The range for nylon is 290-315, and PVC homopolymer is 320.
Uv Wavelength Sensitivity of Polymers (NM)
|Material||Activation spectra maxima|
|Styrene Acrylonitrile||290, 310-330|
|Polypropylene||290-300, 330, 370|
|PVC copolymer||330, 370|
Polymer photodegradation occurs when UV radiation from the sun is absorbed by chemical groups in the polymer formation called chromophores. Chromophores are “an atom or group of atoms whose presence is responsible for the color of a compound”. The polymer formula may include other additives such as halogenated flame retardants, fillers, and pigments. UV stabilizers have been developed and are added to a polymer to inhibit the photoinitiation processes. The top three types are Ultraviolet Absorbers, Quenchers, and Hindered Amine Light Stabilizers (HALS).
1) Ultraviolet Absorbers
Absorbers are a type of light stabilizer that functions by competing with chromophores to absorb UV radiation. Absorbers change harmful UV radiation into harmless infrared radiation or heat that is dissipated through the polymer matrix. UV absorbers have the benefit of low cost but may be useful only for short-term exposure. UV absorbers include:
UV absorbers have the benefit of low cost but may be useful only for short-term exposure. UV absorbers include:
- Carbon black is one of the most effective and commonly used UV radiation absorbers.
- Another UV absorber is rutile titanium oxide which is effective in the 300-400 nm range. However, it is not very useful in the very short wavelength UVB range below 315.
- Hydroxyphenylbenzotriazole is also a well-known UV stabilizer that has the advantage of being suitable for neutral or transparent applications. Hydroxyphenylbenzotriazole is not very useful in thin parts below 100 microns.
- Benzophenones for PVC
- Enzotriazoles and hydroxyphenyl triazines for polycarbonate.
- Oxanilides for polyamides
Quenchers return excited states of the chromophores to ground states by an energy transfer process. The energy transfer agent functions by quenching the excited state of a carbonyl group formed during the photooxidation of a plastic material and through the decomposition of hydroperoxides. This prevents bond cleavage and ultimately the formation of free radicals.
Nickel quenchers are a common type used in agricultural film production. These are not widely used as they contain heavy metals and contribute tan or green colors to the final product. However, nickel quenchers do not effectively stabilize UV radiation as the Hindered Amine Light Stabilizers discussed next.
3) hindered amine light stabilizers (HALS)
HALS are long-term thermal stabilizers that act by trapping free radicals formed during the photooxidation of a plastic material. Therefore HALS limits the photodegradation process. The ability of Hindered Amine Light Stabilizers to scavenge radicals created by the absorption of UV radiation is explained by the formation of nitroxyl radicals through a process known as the Denisov Cycle.
Although there are wide structural differences in the HALS products commercially available, all share the 2,2,6,6-tetramethylpiperidine ring structure. HALS are some of the most proficient stabilizers for UV radiation.
UV Stabilizers for a Wide Range of Plastics
For example, HALS has enabled the growth of polypropylene in the automotive industry. While HALS are also very effective in polyolefins, polyethylene, and polyurethane, they are not useful as stabilizers for UV radiation in PVC.
Combinations of UV Stabilizers
As all three functions by different mechanisms, they are often combined into synergistic UV radiation-absorbing additives. For example, benzotriazoles are often combined with HALS to protect pigmented systems from fading and color changes.
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