Fluorescent Dye Staining as a Tool to Monitor and Quantify Changes in Plastic Surface Hydrophobicity Throughout Photodegradation (2022-04-01)

Abstract

The widespread use of plastics has resulted in the major environmental challenge of plastic pollution. One solution to minimizing the amount of plastic pollution is to improve reuse and recycling strategies, which are limited by the extent of plastic degradation. A significant pathway of plastic degradation is photo-oxidation resulting from UV light causing chemical changes in the plastic backbone. The incorporation of carbonyl groups into the plastic backbone from photooxidation can lead to chain cleavage with UV light as well as a decrease in surface hydrophobicity. Current techniques for monitoring plastic degradation are limited in their ability to observe the early stages of degradation. To fill this knowledge gap, this research seeks to develop a cheap, reproducible, and non-destructive technique for monitoring photooxidative degradation of polyethylene and polypropylene thin films using Nile Red as a fluorescent probe through measurements of surface hydrophobicity. Changes in Nile Red’s fluorescence spectra were be observed from stained, aged polyethylene and polypropylene samples due to Nile Red solvatochromism. As the surface hydrophobicity of the plastic decreases, the signal obtained from Nile Red’s fluorescence undergoes corresponding signal shifts. Trends with the fluorescent profiling were related to carbonyl index from FTIR measurements and contact angle goniometry, allowing for the correlation of fluorescence signals to current measurements of plastic hydrophobicity. Results demonstrate a linear trend in fluorescence spectra shifts as related to the chemical changes to the backbone, with trends dependent on polymer type but independent of polymer film thickness. Overall, this work establishes a new tool to better characterize the extent of plastics’ degradation, which may ultimately impact our ability to recover plastics and minimize plastic waste.