SUNLIGHT MAY DEGRADE PLASTICS
UV light damages polymers when they are exposed to the sun can. Many plastics experience UV light damage without the addition of light blockers, stabilizers, or UV absorbers. Therefore materials may fade or change, crack or completely fall apart if left in the light of the sun. This is true even though the sun only gets through .05 deep into the surface of the material. And the highly brittle nature of some polymers themselves may add to the complete failure of a part.
SIGNS OF UV DEGRADATION
*Brittle outer layer (loss of tensile elongation)
*Loss of molecular weight
*Loss in mechanical properties
*Change in chemical properties
*Discoloration, turning yellow
*Fading of color
*Loss of clarity
*Cracks will form
THREE TYPES OF ULTRAVIOLET RADIATION
There are seven sections to the electromagnetic spectrum. The sections in order from highest to lowest are gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves.
UVA, UVB, and UBC are the three types of ultraviolet light.. UVA will make your skin tan, while UVB will cause it to burn. UVC is germicidal and kills microorganisms. The earth’s ozone layer blocks UVC light. However, the UV damage to a polymer is most affected by UVB radiation.
HOW UV LIGHT DAMAGES POLYMERS
Polymer chains are the building blocks of plastics. Photons in many of these are excited by UV energy. Then this creates free radicals if oxygen is present. The free radicals from oxygen hydroperoxides break the polymer chains (chain scission. ) This process is referred to as photo-oxidation. In addition, the presence of moisture, impurities, chemicals, mechanical load, air, temperature, or pollutants can speed up this process.
PLASTICS THAT DEGRADE IN UV LIGHT
To better stand up to photo-oxidation UV additives were developed. These include carbon black, rutile titanium oxide, benzophenones, hydroxyphenyl-benzotriazole, hindered amine stabilizers (HALS), oxynitrides, hydroxy benzophenone, enzotriazoles, hydroxyphenyl triazines, nickel quenchers, and others. And each plastic may use one or more of these to obtain the best possible UV resistance for the polymer and application. So although the UV-resistant additives shown below are often used, they are not the only ones applied in all applications.
Nylon 6/6 is one of the most versatile plastics used by engineers. It has very good strength, ductility, and heat resistance. However, nylon does require UV resistant additives. A good choice is a three part combination of a phenolic antioxidant, with phosphite and a hindered amine light stabilizer (HALS.) The best UV light stabilizing effect is to add a UV absorber on top of the HALS.
ABS (POLYOXYMETHYLENE OR POM)
ABS is an plastic used by engineers in gears, chains, screws, clips, fuel pumps, luggage, lawn mower covers, etc. It is required to use a UV stabilizers when ABS is used in the sun as well as when it is exposed to rain, fluorescent lamps (such as in bathrooms), and in the air. UV protection is necessary for parts of cars as well as business machine parts. If a plastic absorbs UV light it may suffer from loss of gloss and can even cause cracks to form. A combination of the UV absorber benzotriazole and HALS is often used for UV protection..
High-density polyethylene is used for garbage bags, grocery bags, insulation for wires and cables, agricultural mulch, bottles, and housewares. It is also used in fruit juice and milk cartons, trays, crates, and other food packaging products. Carbon black is often used to make HDPE UV-resistant. A percentage of between 1-3 is recommended.
Polycarbonate will tend to yellow in applications where the sun’s rays are present. Also, it will lose mechanical properties, including impact strength. So the use of UV stabilizers is required. A UV absorber of hydroxyphenyl-benzotriazole should be added. However, note that hindered amine light stabilizers (HALS) are not thought to be appropriate. The basic amine compounds speed up the hydrolysis of the material.
The effect of Ultraviolet (UV) radiation greatly affects polypropylene. When exposed to direct sun for six months, it will cause severe loss of tis strength unless a UV inhibitor or high loading of carbon black pigment is used. However, even with these are added, the life of this plastic under severe UV conditions is limited. Thus HALS and carbon black are often added to control UV degradation to a material.
POLYMERS THAT IS NATURALLY UV RESISTANT
However, some plastics do not suffer UV damage naturally . These include acrylic, Ultem as well as the fluoropolymers; PTFE, FEP, PFA, and PVDF. Hence these do not require added chemicals to prevent UV damage. So they are useful in production parts for cars and even in parts of space vehicles exposed to long periods of UV light.
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