Chemistry
The Chemistry of Polymer Degradation by Photooxidation and Stabilizer Strategies
Quick fact
UV light from the sun provides enough energy to break carbon-carbon bonds in many polymers, initiating a chain reaction that can degrade materials even at low temperatures.
Why this is interesting
You leave a plastic chair in the garden, and after a few sunny summers it becomes chalky and brittle. What is happening at the molecular level?
Read the full explanation
Understanding The Chemistry of Polymer Degradation by Photooxidation and Stabilizer Strategies
Polymers are long chains of repeating units. When exposed to sunlight, the ultraviolet (UV) part of the spectrum carries enough energy to excite certain bonds, especially in carbonyl groups if present, and can also be absorbed by impurities or defects. The absorbed energy can lead to bond homolysis, creating free radicals. These radicals react with oxygen to form peroxy radicals, which abstract hydrogen atoms from adjacent polymer chains, generating new radicals and hydroperoxides. This initiates an autoxidation chain reaction, which can lead to chain scission (backbone breaking) or crosslinking (forming bridges between chains). Both change the polymer's mechanical properties: scission makes it brittle and weak, crosslinking makes it stiff and brittle as well. In crystalline regions, oxygen diffusion is slower, so degradation is slower. This explains why amorphous polymers like polyethylene tend to degrade faster at the surface first.
A deeper explanation
The primary photochemical reactions are Norrish Type I and Type II, which occur in carbonyl-containing polymers (e.g., polyesters, polyamides). Norrish Type I involves homolytic cleavage of the bond between the carbonyl carbon and the adjacent carbon, yielding two radicals. Norrish Type II involves an intramolecular hydrogen abstraction via a six-membered transition state, leading to chain scission with formation of an alkene and a carbonyl end group. For polymers without carbonyls, UV absorption may be weak, but impurities, catalysts, or oxidation products can act as chromophores. Once radicals form, the chain reaction proceeds: R• + O2 → ROO•, then ROO• + RH → ROOH + R•. Hydroperoxides are photolabile and decompose into alkoxy and hydroxy radicals under UV. This accelerates degradation. Stabilizer strategies aim to interrupt this cycle. UV absorbers (e.g., benzophenones) preferentially absorb UV photons, dissipating energy as heat. Hindered amine light stabilizers (HALS) act catalytically: they are oxidized to nitroxyl radicals, which scavenge alkyl radicals, regenerating themselves. Phenolic antioxidants (primary) donate a hydrogen atom to peroxy radicals, forming a stable radical, thus terminating the chain. These strategies are often combined to protect polymers during processing and service life.