exposure to ultraviolet radiation
Exposure to the sun's ultraviolet radiation can degrade plastic over time. Polymer photodegradation occurs when invisible ultraviolet radiation consisting of short wavelengths breaks down polymer chains in plastics. This is called the photodegradation process. It is caused by exposure to ultraviolet radiation and causes a decrease in physical properties. These include loss of impact strength, color change, cracking, loss of elongation and tensile strength, and surface chalking. For example, photodegradation caused by UV radiation can cause garden chairs to tarnish and become brittle. In addition, the color of the stadium seats will appear chalky, and some plastics will even yellow and crack.
Some Characteristics of Ultraviolet Radiation
Ultraviolet radiation makes up only 4.6 percent of the solar spectrum. It is measured in nanometers (nm) and ranges from 290 to 400. However, the most aggressive part of the UVB range is the very short wavelengths between 280 and 315 nanometers. Therefore, the amount of UV radiation energy exposure (irradiation) depends on where you are in the world.
what is irradiation
Irradiance is the amount of ultraviolet radiant energy incident on a specific area over a given period of time. 1Ly = 1 cal/cm 2 = 4.184 E 4 Joule/m 2 (Ly=Langley) Therefore, for one year of continuous outdoor use in Sudan, the energy transferred to the plastic parts is 220kcal/cm 2 /year, while in Sweden it is 70.
Every plastic is sensitive to certain wavelengths in the 290-400 nm UV region. Therefore, polypropylene has three maxima at 290-300, 330 and 370 nm. Nylon ranges from 290-315, while PVC homopolymer is 320.
Degradation of Polymers by UV Radiation
Polymer photodegradation occurs when ultraviolet radiation from the sun is absorbed by chemical groups in the polymer structure called chromophores. A chromophore is "an atom or group of atoms whose presence determines the color of a compound". Polymer formulations may include other additives such as halogenated flame retardants, fillers and pigments. Therefore, UV stabilizers were developed and added to polymers to suppress the photoinitiation process. The first three types are UV absorbers, quenchers, and hindered amine light stabilizers (HALS).
1) UV absorber
Absorbers are photostabilizers that work by competing with chromophores to absorb UV radiation. Thus, the absorber converts harmful UV radiation into harmless infrared radiation or heat that is dissipated through the polymer matrix. UV absorbers have the advantage of being low cost, but may only be suitable for short-term exposures. UV absorbers include:
Absorber
UV absorbers have the advantage of being low cost, but may only be suitable for short-term exposures. 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.
Hydroxybenzophenone
Hydroxyphenyl benzotriazole is also a well known UV stabilizer with the advantage of being suitable for neutral or clear applications. Hydroxyphenylbenzotriazoles are not very useful in thin parts below 100 microns.
Benzophenone for PVC
Anzotriazoles and hydroxyphenyltriazines for polycarbonates.
Oxalanilide for polyamides
2) Quencher
Quenchers return the excited state of the chromophore to the ground state through an energy transfer process. Energy transfer agents act by quenching the excited state of carbonyl groups formed during photooxidation of plastic materials and by decomposition of hydroperoxides. This prevents the breaking of bonds and ultimately the formation of free radicals.
Nickel quenching agent
Nickel quencher is the type commonly used in agricultural film production. These are not widely used because they contain heavy metals and give the final product a tan or green color. However, like the hindered amine light stabilizers discussed below, nickel quenchers are not effective in stabilizing UV radiation.
3) Hindered Amine Light Stabilizer (HALS)
HALS are long-lasting heat stabilizers that work by trapping free radicals formed during photooxidation of plastic materials. HALS thus limit the photodegradation process. The ability of HALS to scavenge free radicals arising from the absorption of UV radiation can be explained by the formation of nitroxyl groups in a process known as the Denisov cycle.
Although commercially available HALS products vary widely in structure, they all have a 2,2,6,6-tetramethylpiperidine ring structure. HALS are some of the most proficient UV radiation stabilizers.
UV Stabilizers for Various Plastics
For example, HALS has contributed to the development of polypropylene in the automotive industry. While HALS are also very effective in polyolefins, polyethylene and polyurethane, they cannot be used as stabilizers for UV radiation in PVC.
Combination of UV Stabilizers
Since all three functions work through different mechanisms, they are often combined into synergistic UV radiation absorbing additives. For example, benzotriazoles are often used in combination with HALS to prevent fading and discolouration in coloring systems.
