The exposure of plastics to ultraviolet radiation over time leads to their degradation. Photodegradation occurs when invisible ultraviolet radiation with a short wavelength breaks polymer chains. This phenomenon results in a loss of the piece's properties, such as reduced impact resistance, color change, cracking, decreased elongation to rupture, and tensile strength, as well as peeling of the surface of the material.

Ultraviolet radiation constitutes only 4.6% of sunlight, covering wavelengths from 280 to 400 nanometers. The harmful part of this radiation is between 280-315 nanometers. The amount of radiation also depends on the geographical area. Each polymer is sensitive to a specific wavelength; for instance, polypropylene is sensitive to wavelengths of 290-300, 330, and 370 nanometers, nylon is sensitive in the range of 290-315, and PVC is sensitive at 320 nanometers.

Photodegradation of polymers occurs when ultraviolet rays are absorbed by chemical groups in the polymer, which are called chromophores. The piece may contain additives such as halogenated flame retardants, fillers, or pigments. UV stabilizers have been designed and produced to prevent photodegradation by incorporating them into the polymer. There are three types of stabilizers, each of which will be explained below.

UV Absorbers

Absorbers are a type of stabilizer that competes with chromophores for the absorption of UV radiation. Absorbers convert harmful UV rays into infrared or heat radiation, which is dissipated through the polymer matrix. Carbon black is one of the most important, effective, and commonly used UV absorbers. Titanium dioxide (rutile) is another absorber that acts effectively in the wavelength range of 300-400 nanometers and is not suitable for shorter wavelengths.

Hydroxybenzophenone and hydroxyphenyl benzotriazole are recognized as UV stabilizers suitable for clear or colorless parts. However, hydroxyphenyl benzotriazole is not suitable for parts thinner than 100 microns. Other UV absorbers include oxanilides for polyamide, benzophenone for PVC, and benzotriazole and hydroxyphenyl triazine for polycarbonate. UV absorbers are low-cost but may only last a short time under environmental conditions.

Quenchers

Quenchers return chromophores from an excited state to a ground state by transferring energy. The energy transfer agent works by quenching the excited state of carbonyl groups formed during the photoxidation of plastic materials and through the degradation of hydroxy peroxides. This phenomenon prevents bond breakage and ultimately the formation of free radicals. Nickel quenchers are commonly used in agricultural films. However, these materials are not widely used because they contain heavy metals and result in browning or greening of the final piece.

HALS Amino Light Stabilizers

HALS is a long-lasting stabilizer that limits photodegradation by trapping free radicals formed during oxidation. The ability of HALS to scavenge radicals caused by UV absorption is explained by the formation of nitroxyl radicals through a process known as the Denisov Cycle.

There are various commercial grades of HALS available, but all share the common structure of 2,2,6,6-tetramethylpiperidine.

HALS is one of the most well-known UV stabilizers for a wide range of plastics. For example, HALS has activated the growth of polypropylene in the automotive industry. While HALS is also very effective in polyolefins, polyethylene, and polyurethane, they are not useful in PVC.

It is best to use each of the stabilizers in conjunction with UV absorbers. For instance, benzotriazole is usually combined with HALS to protect colored parts from fading or discoloration.

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