Exposure to ultraviolet (UV) radiation causes plastics to degrade over time. Photodegradation occurs when invisible, short-wavelength UV radiation breaks polymer chains. This leads to deterioration in the properties of the part, including reduced impact resistance, discoloration, cracking, reduced elongation at break and tensile strength, and surface flaking.
UV radiation accounts for only 4.6% of solar radiation and covers wavelengths from 280 to 400 nm. The damaging portion of this radiation lies between 280 and 315 nm. Radiation intensity also depends on geographical location. Each polymer is sensitive to specific wavelengths. For example, polypropylene is sensitive to 290–300, 330, and 370 nm, nylon to 290–315 nm, and PVC to 320 nm.
Polymer photodegradation occurs when UV radiation is absorbed by chemical groups in the polymer known as chromophores. The part may contain additives such as halogenated flame retardants, fillers, or pigments. UV stabilizers have been developed to prevent photodegradation when added to polymers. There are three types of stabilizers, each discussed below.
UV Absorbers
UV absorbers are stabilizers that compete with chromophores for the absorption of UV radiation. They convert harmful UV radiation into infrared radiation or heat, which is dissipated through the polymer matrix.
Carbon black is one of the most important, effective, and widely used UV absorbers. Rutile titanium dioxide is another absorber that works effectively in the 300–400 nm wavelength range but is unsuitable for shorter wavelengths.
Hydroxybenzophenones and hydroxyphenyl benzotriazoles are UV stabilizers suitable for transparent or colorless parts. However, hydroxyphenyl benzotriazoles are unsuitable for parts thinner than 100 µm. Other UV absorbers include oxanilides for polyamides, benzophenones for PVC, and benzotriazoles and hydroxyphenyl triazines for polycarbonate. UV absorbers are inexpensive but may withstand environmental conditions for only a short period.
Quenchers
Quenchers return chromophores from an excited state to their ground state through energy transfer. These energy-transfer agents work by quenching the excited states of carbonyl groups formed during the photo-oxidation of plastics and by decomposing hydroperoxides. This prevents bond cleavage and the subsequent formation of free radicals. Nickel quenchers are a common type used in agricultural films. However, their use is limited because they contain heavy metals and impart a brown or green color to the final product.
Hindered Amine Light Stabilizers (HALS)
HALS provide long-term stabilization by trapping free radicals formed during oxidation, thereby limiting photodegradation.
The ability of HALS to scavenge radicals generated by UV absorption is explained by the formation of nitroxide radicals through a process known as the Denisov cycle.
Various commercial grades of HALS are available, but they all share the 2,2,6,6-tetramethylpiperidine ring structure.
HALS are among the best-known UV stabilizers for a wide range of plastics. For example, they have enabled the increased use of polypropylene in the automotive industry. While HALS are also highly effective in polyolefins, polyethylene, and polyurethane, they are not effective in PVC.
Each type of stabilizer is best used in combination with UV absorbers. For example, benzotriazoles are commonly combined with HALS to protect colored parts against fading or discoloration.
