Environmental Engineering Reference
In-Depth Information
can absorb light effectively, but trace impurities in the plastic and some of
the additives in the material act as good chromophores. Plastics such as PS,
polycarbonate (PC), and polyethylene terephthalate (PET), have aromatic
functional groups that absorb UV radiation.
Absorbed solar UV-Visible radiation can result in several different types of
degradation reaction in a polymer.
1. Photolysis of the covalent bonds: The photon energy in solar radiation
(especially the UVR) exceeds the bond dissociation energy for common
covalent bonds in polymers. Despite this, direct photolysis of covalent
bonds in polymers is rare, especially with the predominant C-C or C-H
bonds in common polymers and only occurs in relatively weaker bonds
such as peroxy linkages {-O-O-} as seen in the oxidation of
polyolefins. Photolysis of polymer hydroperoxide ROOH results in
radical products that are able to initiate the oxidation reactions.
Photolysis is also involved in chain scission of (ethylene-carbon
monoxide) copolymers irradiated with UV radiation.
2. Photomodification of chemical structure without any chain scission:
Photoreaction may modify the main chain or the side chains of an
irradiated polymer. The average chain length (or DP) does not change
in the process even though
Mn
(g/mol) may be reduced. In the
light-induced dehydrochlorination of PVC (D'Aquino et al., 2012) for
instance, HCl is evolved without any significant main-chain scission or
cross-linking, introducing conjugated unsaturation into the main chain.
It is the absorption of blue wavelengths in the white light spectrum by
these polyene sequences that make degraded PVC surfaces appear
yellow. Surface discoloration of PVC cladding (siding) and window
frames exposed to sunlight is well known (Edge et al., 2010) and is a
result of this reaction.
