Photo-oxidation of polymers

Effect of UV exposure on polypropylene rope

Photo-oxidation is the degradation of a polymer surface in the presence of oxygen or ozone. The effect is facilitated by radiant energy such as UV or artificial light. This process is the most significant factor in weathering of polymers. Photo-oxidation is a chemical change that reduces the polymer's molecular weight. As a consequence of this change the material becomes more brittle, with a reduction in its tensile, impact and elongation strength. Discoloration and loss of surface smoothness accompany photo-oxidation. High temperature and localized stress concentrations are factors that significantly increase the effect of photo-oxidation.

Chemical mechanism

Aldehydes, ketones and carboxylic acids along or at the end of polymer chains are generated by oxygenated species in photolysis of photo-oxidation. The initiation of photo-oxidation reactions is due to the existence of chromophoric groups in the macromolecules. Photo-oxidation can occur simultaneously with thermal degradation and each of these effects can accelerate the other.

The photo-oxidation reactions include chain scission, cross linking and secondary oxidative reactions. The following process steps can be considered:[1] initial step, chain propagation step, chain branching and termination step. In the initial step, free radicals are formed by photon absorption. In the chain propagation step, a free radical reacts with oxygen to produce a polymer peroxy radical (POO•). This reacts with a polymer molecule to generate polymer hydroperoxide (POOH) and a new polymer alkyl radical (P•). With chain branching, polymer oxy radicals (PO•) and hydroxy radicals (HO•) are formed by photolysis. The termination step is cross linking which is a result of the reaction of different free radicals with each other.

Initial step
${\displaystyle {\ce {Polymer->P\bullet +\ P\bullet }}}$
Chain propagation
${\displaystyle {\ce {P\bullet +\ O2->POO\bullet }}}$
${\displaystyle {\ce {POO\bullet +\ PH->{POOH}+\ P\bullet }}}$
Chain branching
${\displaystyle {\ce {POOH->PO\bullet +\ OH\bullet }}}$
${\displaystyle {\ce {{PH}+OH\bullet ->P\bullet +\ H2O}}}$
${\displaystyle {\ce {PO\bullet ->Chain\ scission\ reactions}}}$
Termination
${\displaystyle {\ce {POO\bullet +\ POO\bullet ->cross\ linking\ reaction\ to\ non-radical\ product}}}$
${\displaystyle {\ce {POO\bullet +\ P\bullet ->cross\ linking\ reaction\ to\ non-radical\ product}}}$
${\displaystyle {\ce {P\bullet +\ P\bullet ->cross\ linking\ reaction\ to\ non-radical\ product}}}$

where[1] PH = Polymer

POOH = Polymer hydroperoxide

Effects of dyes/pigments

Adding pigment light absorbers and photostabilizers (UV absorbers) is one way to minimise photo-oxidation in polymers. Antioxidants are used to inhibit the formation of hydroperoxides in the photo-oxidation process.[2]

Dyes and pigments are used in polymer materials to provide color changing properties. These additives can reduce the rate of polymer degradation. Cu-phthalocyanine dye can help stabilize against degradation, but in other situations such as photochemical aging can actually accelerate degradation. The excited Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in the PC, which increase the formation of free radicals. This acts as the starting points for the sequential photo-oxidation reactions leading to the degradation of the PC.[3]

Electron transfer sensitization is a mechanism where the excited Cu-phthalocyanine abstracts electrons from PC to form Cu-Ph radical anion and PC radical cations. These species in the presence of oxygen can cause oxidation of the aromatic ring.[4]

Photo-oxidation protection

Poly(ethylene-naphthalate) (PEN) can be protected by applying a zinc oxide coating, which acts as protective film reducing the diffusion of oxygen.[5] Zinc oxide can also be used on polycarbonate (PC) to decrease the oxidation and photo-yellowing rate caused by solar radiation.[6]

Photo-catalytic oxidation of polymers

Single-use plastic products after their service life often end up in urban areas and in the environment. Most likely, plastic incorrectly disposed of finds its way to lakes, rivers and finally oceans, consisting of danger for marine and terrestrial flora and fauna. The tendency of the plastic to be subjected to photo-oxidation can be positively used and enhanced thanks to the addition of a catalyst. In fact, the plastic with the addition of a catalyst is subject to a fast and aggressive photo-oxidation that degrades the macro and microparticles in fewer harmful sub-products such as low molecular weight compounds (hydroperoxides, peroxides, and carbonyl saturated and unsaturated group)[7].

References

1. ^ a b Rabek, JF 1990, Photostabilization of Polymers:Principles and Application, ELSEVIER SCIENCE PUBLISHER LTD, England
2. ^ "Photo-oxidisation of electroluminescent polymers studied by core level photoabsorption specttroscopy" (PDF). American institute of physics 1996. Retrieved 9 February 2011.
3. ^ "THE PHOTO-OXIDATION OF POLYMERS - A comparison with low molecular weight compounds" (PDF). Pergamon Press Ltd. 1979 - Pure & Appi. Chem., Vol. 51, pp.233—240. Retrieved 9 February 2011.
4. ^ Clodoaldo Saron, Fabio Zulli, Marco Giordano, Maria Isabel Felisberti, Influence of copper-phthalocyanine on the photodegradation of polycarbonate, Polymer Degradation and Stability, Volume 91, Issue 12, December 2006, Pages 3301-3311
5. ^ L. Guedri-Knani, J. L. Gardette, M. Jacquet, A. Rivaton, Photoprotection of poly(ethylene-naphthalate) by zinc oxide coating, Surface and Coatings Technology, Volumes 180-181, 1 March 2004, Pages 71-75
6. ^ A. Moustaghfir, E. Tomasella, A. Rivaton, B. Mailhot, M. Jacquet, J. L. Gardette, J. Cellier, Sputtered zinc oxide coatings: structural study and application to the photoprotection of the polycarbonate, Surface and Coatings Technology, Volumes 180-181, 1 March 2004, Pages 642-645.
7. ^ Tofa, Tajkia Syeed (2019). "Visible light photocatalytic degradation of microplastic residues with zinc oxide nanorods". Environmental Chemistry Letters. 17 (3): 1341-1346. doi:10.1007/s10311-019-00859-z.