BLog

Uv Degradation of Plastics and Uv Resistant Additives

June 2, 2021
Light Flare

Sunlight May Degrade Plastics

Many plastics will degrade in sunlight without adding 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.

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. 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 comprises 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 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 from 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), 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 frequently 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. 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 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.

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 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.

Polypropylene

The effects of Ultraviolet (UV) radiation greatly affect the properties of polypropylene. 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, life expectancy under severe sunlight 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 deteriorate 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.

Click Here Graphic

Share:

Comments

Leave the first comment