Over time, exposure to the UV radiation in sunlight will degrade plastics. Polymer photodegradation occurs when non-visible UV radiation made up of short wave lengths breaks down the polymer chains in plastics. This process results in the deterioration of physical properties such as loss of impact strength, changes in color, cracking, loss of elongation and tensile strength or chalking of the surface. For example, garden chairs lose their gloss and become brittle, the color of stadium seats appears chalky and some plastics will yellow and crack.
UV radiation accounts for only 4.6% of the solar spectrum. It is calculated in nanometers (nm) and ranges between 280 and 400, but the most aggressive part of the UVB range is the very short wavelengths between 280 and 315 nanometers. The amount of radiation energy exposure (irradiation) depends on where you are in the world. Irradiation is the radiation energy incident over a specific area over a given period of time. (Ly=Langley)
1Ly = 1 cal/cm2 = 4.184 E4Joule/m2
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.
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 light from the sun is absorbed by chemical groups in the polymer formation called chromophores. 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 the chromophores to absorb UV radiation. Absorbers change harmful UV radiation into harmless infrared radiation or heat that is dissipated through the polymer matrix. Carbon black is one of the most effective and commonly used light absorbers. Another UV absorber is rutile titanium oxide which is effective in the 300-400 nm range but is not very useful in the very short wavelength UVB range below 315. Hydroxybenzophenone and hydroxyphenylbenzotriazole are also well known UV stabilizers that have the advantage of being suitable for neutral or transparent applications. Hydroxyphenylbenzotriazole is not very useful in thin parts below 100 microns. Other UV absorbers include oxanilides for polyamides, benzophenones for PVC and benzotriazoles and hydroxyphenyltriazines for polycarbonate. UV absorbers have the benefit of low cost but may be useful only for short-term exposure
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 photo-oxidation 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 metal and contribute tan or green colors to the final product. Nickel quenchers are not as effective 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 photo-oxidation of a plastic material and thus limiting the photodegradation process. The ability of Hindered Amine Light Stabilizers to scavenge radicals created by UV absorption is explained by the formation of nitroxly 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
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 in PVC.
As all three function by different mechanisms, they are often combined into synergistic UV absorbing additives. For example, benzotriazoles are often combined with HALS to protect pigmented systems from fading and color changes.
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