Sunlight May Degrade Plastics
Many plastics will degrade in sunlight without the addition of light blockers, stabilizers or UV absorbers. Materials may discolor, crack or completely disintegrate. While the UV degradation of a polymer in sunlight is usually only .05 deep in the material’s surface the highly brittle nature of some plastics may lead to complete component failure.
Signs of UV Degradation
*Brittle outer layer (loss of tensile elongation)
*Reduction in molecular weight
*Loss in mechanical properties
*Change in chemical properties
*Fading of color
*Loss of clarity
*Formation of cracks
Three Types of Ultraviolet Radiation
The electromagnetic spectrum is made up of seven sections. In order from highest energy to lowest the sections are called gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, and radio waves.
Ultraviolet radiation is made up of three types: UVA, UVB and UBC. UVA will make your skin tan while UVB will cause it to burn. UVC is a germicidal that kills microorganisms. The earth’s ozone layer effectively blocks UVC light. It is UVB that has the most effect on the UV degradation of a polymer.
How UV Light Degrades Plastics
Plastics are made up of polymer chains. So when UV energy is absorbed by many plastics, it will excite photons in the material which then create free radicals. If oxygen is present the free radicals form oxygen hydroperoxides that 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 accelerate this process.
Plastics That Degrade in UV Light
Additives have been developed to better withstand photo-oxidation. These include carbon black, rutile titanium oxide, benzophenones, hydroxyphenyl-benzotriazole, hindered amine stabilizers (HALS), oxanilides, hydroxybenzophenone, enzotriazoles, hydroxyphenyltriazines, 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 frequently used, they are not the only ones applied in all applications.
Nylon 6/6 is one of the most versatile of the engineering thermoplastic materials. Because nylon 6/6 has excellent strength, ductility and heat resistance, it is an outstanding candidate for metal replacement applications. However, nylon does require UV resistant additives. A good choice is a three part combination of a phenolic antioxidant, with a 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 commonly used for gears, chains, screws, clips, fuel pumps, luggage, lawn mower covers etc. It is mandatory to use UV stabilizers when the material is used outdoors. Also 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 even the formation of cracks. For UV protection a combination of the UV absorber benzotriazole and HALS is often used.
High Density Polyethylene
High density polyethylene is commonly used in 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.
When polycarbonate is used in exterior applications it will tend to yellow. Also, it will it will loose mechanical properties including impact strength. So UV stabilizers become mandatory. A UV absorber of hydroxyphenyl-benzotriazole should be used. Note that hindered amine light stabilizers (HALS) are not recommended. The basic amine compounds accelerate the hydrolysis of the material.
The effects of Ultraviolet (UV) radiation greatly affects the properties of polyproylene. When exposed to strong, direct sunlight 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 the life expectancy under severe sunlight conditions is limited. HALS and carbon black are often added to mitigate UV degradation.
Plastic that are Naturally UV Resistant
However, some plastics do not deteriorate under UV radiation naturally. These include acrylic, Ultem as well as the 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.