One of the pathways of degradation of polythene is said to lead via different oxidative mechanisms to carbonyl functions within the polymeric chains. The oxidized chain would then be able to convert further, eg. via chain cleavage through Norrish type reactions to shorter chain alkenes and aldehydes which can transform oxidatively further to carboxylic acids and derivatives. At macro.scale, the oxidative degradation causes embrittlement of the material. One would expect that oxidation of a polythene chain would cause a change of interaction with neighboring chains leading to a different near-order.
My question is how the oxidation proceeds further, deeper into the material. As oxygen is necessary for the oxidation, does the permeability of oxygen through the upper layers change upon oxidation of the uppermost layers? As polymer layers deeper within the material are no longer exposed to light, is it known which excited oxgen species are responsible for oxidation within the material? Is it known how deep the oxidized zone needs to be (in % of the whole) for the macroscopic embrittlement to be noticed? Have mass spectrometric analyses been carried, where the polythene material is ablated (eg., laser ablated) and the oxidized fragments have been measured as a function of the distance of the measured material to the surface layer?
Dear Thies Thiemann ,
could you enrich your interesting question with (some of your) initial data (and/or bibliographic references) ?
To Ioannis Samaras
From our own studies we have seen that the intensity of the carbonyl oxidation band of oxidatively degraded polythene and polypropylene is a good indication of the brittleness of the plastic particle, predicting how easily the plastic piece breaks. Our research in this area mostly centers around the degradation of bottle tops.
M. al Kaabi, V. Poulose, T. Thiemann, J. Environ. Protect. (JEP), 2025, 16(2), 66-86.
Oxidative degradation of plastic bottle tops in an arid, terrestrial environment - Identifying oxidative degradation by infrared spectroscopy
B. Selem, A. Alkarbi, A. Alhosani, F. Ahmad, F., M. Elemam, S.H. Iftikhar, A. H. Mourad, T. Thiemann, In Proceedings of the 18th International Conference on Environmental Science and Technology, Athens, Greece, 2023. https://cms.gnest.org/sites/ default/files/Proceedings/cest2023_00485/cest2023_00485.pdf
Plastic to microplastic in the United Arab Emirates – plastic bottle caps in a hot, arid environment
However, we have never been able to pinpoint the number of units in the polymer as a function of the intensity of the carbonyl band - nor have we a good indication of how deep in layers into the material really goes. From our experiments, we do see that the more layers we take off the plastic piece, the less oxidation there is.
Also, we understand that the carbonyl (and partially carboxyl) band in the infrared of an oxidatively degraded PE or PP particle is a composite of a number of bands. We do at the beginning of the oxidation certain bands (two) appear where then with time one overtakes the other. However, a number of further bands (CO, CO-O) are in evidence. This had already been discussed by Yaqoubi et al:
for instance: Yagoubi, W., Abdelhafidi, A., Sebaa, M. and Chabira, S.F. (2015) Identification of Carbonyl Species of Weathered LDPE Films by Curve Fitting and Derivative Analysis of IR Spectra. Polymer Testing, 44, 37-48. https://doi.org/10.1016/j.polymertesting.2015.03.008
Now in a longer polythene chain, only the oxidation of the first, second, and third carbons would lead to bands of different frequencies. Bands at other carbons would all show the same wave number. The remainder of other oxidation bands need to come from other species such as carboxyl and ester groups that have formed, where the formation of an ester function from a polymer hydrocarbon chain is not straightforward.
Also, should polythene or polypropylene oxidize one should expect a weight increase. However, Norrish type fragmentations of the oxidized plastic would lead to a weight decrease. We have never carried out controlled lab experiments with bottle tops over long periods of time (but only outdoor experiments), so that no conclusive trend in weight (upwards or downwards) could be established. In addition, small molecule additives could potentially leave the plastic over time, also leading to a diminishing of the weight. A trend (downwards) was only established when the plastic particle started shedding microplastic particles.
So, there are the following questions:
a.) how many carbons (eg., what percentage) need to be oxidized to change the near- and far order in a polymer (different PE and PP types most likely would lead to different answers)
b.) how does the oxidation of outer 'layers' affect the oxidation of polymeric carbons deeper in the material?
Both of these questions could be answered by mass spectroscopy if the plastic is laser ablated.
c.) Additionally, it would be interesting to understand if air oxygen can be transformed photochemically into an excited form (eg., singlet oxygen), where sensitization by the plastic surface (with all its additives) and/or the soil/surface the plastic lies on plays a role. Our experiments were carried out on soil in hot, arid UAE. Oxidation of PE and PP was very rapid as measured by IR.
Best wishes,
Thies
Dera Thies Thiemann
It seems this paper can give you some replies to you questions : Study of Factors Affecting UV-Induced Photo-Degradation in Different Types of Polyethylene Sheets, https://www.mdpi.com/2073-4360/16/19/2709
In addition, Confocal micro-Raman resonance spectrometry is the useful method to check functional groups (including oxygen-containing ones, I think) inside the subsurface layers of the polymer films.
Best wishes
Alexander
Alexander Pud Thank you, Dr. Alexander Pud, for this. It is an article that I had not yet seen. It does have interesting data on oxygen permeability of PE and oxidized PE sheets. Still, some questions are left unanswered
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