Chemistry
Mechanisms of Photodegradation of Polymers and Stabilization Strategies
Quick fact
Ultraviolet light with wavelengths between 290–400 nm carries enough energy to break the weakest carbon–carbon and carbon–hydrogen bonds in most polymers. The resulting radical chain reactions can cause a polyethylene film to lose 90% of its elongation at break within just a few months of outdoor exposure.
Why this is interesting
You may have noticed that an old plastic lawn chair left in the sun becomes brittle and faded. Why does sunlight, which gives life to plants, also destroy plastics?
Read the full explanation
Understanding Mechanisms of Photodegradation of Polymers and Stabilization Strategies
Polymers are long chains of repeating units held together by covalent bonds. When exposed to ultraviolet (UV) light, the photon energy can be absorbed by the polymer's chromophore groups, exciting them to a higher electronic state. This can lead to bond cleavage, generating free radicals—atoms or groups with unpaired electrons. Most polymers do not absorb UV strongly, but impurities, additives, or trace catalyst residues can act as photosensitizers. The free radicals initiate a chain reaction with oxygen, producing peroxides and hydroperoxides that further fragment the chain. This process, called photooxidation, leads to chain scission (breakage of the backbone) and crosslinking (formation of new bonds between chains). The loss of mechanical properties such as tensile strength and flexibility is a direct result of these structural changes.
A deeper explanation
The mechanism of photodegradation proceeds through several steps: initiation, propagation, and termination. In initiation, an excited chromophore either directly dissociates to form radicals (Norrish type I) or decomposes into non-radical products (Norrish type II). For example, carbonyl groups, often present as impurities or from oxidation, undergo Norrish type I reactions to produce an acyl and an alkyl radical. These radicals rapidly react with oxygen (O2) to form peroxy radicals (ROO•), which can abstract a hydrogen atom from a nearby polymer chain, yielding a hydroperoxide (ROOH) and a new alkyl radical (R•). The hydroperoxide is unstable under UV and decomposes into two radicals, amplifying the cycle. This autocatalytic process leads to rapid degradation once initiated. Crosslinking occurs when two macroradicals recombine, creating a network that makes the polymer brittle. Stabilization strategies aim to interrupt this cycle: UV absorbers (e.g., benzotriazoles) act as a sunscreen, absorbing harmful UV and dissipating the energy as heat; hindered amine light stabilizers (HALS) scavenge free radicals through a regenerative cycle involving nitroxyl radicals, effectively breaking the autocatalytic chain; and antioxidants (e.g., hindered phenols) quench peroxides and hydroperoxides before they can initiate new radical cycles. By combining these strategies, manufacturers can significantly enhance the durability of polymers exposed to sunlight. Understanding these mechanisms is crucial for selecting appropriate stabilizers and for predicting the service lifetime of plastic products in outdoor environments.