Sunlight May Degrade Plastics
Many plastics will degrade when exposed to the sun without the addition of light blockers, stabilizers, or UV absorbers. Therefore materials may discolor, crack or completely fall apart even though the UV degradation of a polymer in sunlight is usually only .05 deep in the material’s surface. And the highly brittle nature of some plastics themselves may add to complete failure of a component.
Folding Chair with UV Damage
Signs of UV Degradation
*Brittle outer layer (loss of tensile elongation)
*Reduction in molecular weight
*Loss in mechanical properties
*Change in chemical properties
*Discoloration, yellowing
*Fading of color
*Loss of clarity
*Formation of cracks
*Chalky appearance
Three Types of Ultraviolet Radiation
The electromagnetic spectrum is made up of seven sections. The sections are called gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves in order from highest energy to lowest, .
Ultraviolet radiation is mae up of three types: UVA, UVB, and UBC. UVA will make your skin tan, while UVB will cause it to burn. UVC is germicidal that kills microorganisms. The earth’s ozone layer blocks UVC light. However, the UV degradation of a polymer is most affected by UVB radiation.
How UV Light Degrades Plastics
Plastics are made up of polymer chains. When UV energy is absorbed by many plastics, it will excite photons in the material. 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 humidity, impurities, chemicals, mechanical load, air, temperature, or environmental pollutants can speed up this process.
Plastics That Degrade in UV Light
UV additives have been developed to better withstand photo-oxidation. 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
Nylon 6/6 is one of the most versatile engineering thermoplastic materials. This plastic is an excellent candidate for metal replacement applications since 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 engineering plastic often used for gears, chains, screws, clips, fuel pumps, luggage, lawn mower covers, etc. It is required to use 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 the air. UV protection is necessary for car parts as well as business machine components. UV exposure can lead to loss of gloss and can even cause cracks to form. For UV protection, a combination of the UV absorber benzotriazole and HALS is often used.
High-Density Polyethylene
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 containers, milk containers, trays, crates, and food packaging products. Carbon black is often used to make HDPE UV-resistant. A percentage of between 1-3 is recommended.
Polycarbonate
When polycarbonate is used in exterior applications, it will tend to be yellow. Also, it will lose mechanical properties, including impact strength. So UV stabilizers are required. A UV absorber of hydroxyphenyl-benzotriazole should be used. Note that hindered amine light stabilizers (HALS) are not recommended. The basic amine compounds speed up the hydrolysis of the material.
Polypropylene
The effects of Ultraviolet (UV) radiation greatly affect the properties of polypropylene. When exposed to direct sun for six months, it will cause severe loss of the strength of properties unless a UV inhibitor or high loading of carbon black pigment is used. Even with these added, life expectancy under severe UV conditions is limited. HALS and carbon black are often added to mitigate UV degradation.
Plastic That Is Naturally UV Resistant
However, some plastics do not degrade under UV radiation naturally. These include acrylic, Ultem as well as fluoropolymers, including PTFE, FEP, PFA, and PVDF. Hence these do not require additives and are useful in production parts for cars and even in parts of spacecraft exposed to long periods of potential UV degradation.
