A closer look at a post-consumer recycled polyolefin blend: chemical characterization at the nanoscale using tapping-mode AFM-IR

Abstract

The recycling of polyolefins such as polypropylene (PP) and polyethylene (PE) plays an important role in the reduction of the amount of improperly disposed plastic waste, as they among the most common polymers produced. Sorting is an important, but tedious step of the recycling process, where the polymer waste is separated into different polymer types to avoid contamination that leads to a deterioration of the final blend´s mechanical properties. Therefore, blends produced directly from heterogeneous waste streams that retain desirable properties and avoid thorough waste separation are potential cost-efficient alternatives to virgin materials. A blend´s nanoscale structure and chemical composition has great impact on its properties; however, current nanoscale characterization of polymers such as TEM, SEM, and AFM rely on staining, or previous knowledge of the sample´s chemical composition to obtain chemical information. These approaches may prove insufficient for the analysis of complex recyclates, where the composition is not as predictable as in virgin polymers AFM-IR (also known as PTIR), is a scanning probe-based nanoscale IR technique that combines the resolution of an AFM with the chemical information provided by IR spectroscopy. For the analysis of polymers, tapping mode AFM-IR is preferred over contact mode, due to its better resolution and smaller sensitivity to changes in the sample´s mechanical properties. In this work a PE/PP/rubber blend derived entirely from post-consumer waste collected at the municipal level is analysed using tapping mode AFM-IR. Several interesting features could be imaged and identified, namely sub-micron sized contaminant polymer particles (polyamide and polyurethane), and small (≈500 nm diameter) PP droplets present inside the PE phase. Additionally, the interface between PE and PP was analysed with the help of a gaussian mixture model applied to the IR maps, and identified as EPR rubber. To confirm this result, full-length AFM-IR spectra obtained in the same location were also analysed and the results were found to be in agreement with the IR images. Reference TEM measurements further confirmed the information provided by AFM-IR. We believe this work demonstrates the applicability and usefulness of AFM-IR to the routine analysis of complex polymers recyclates at the nanoscale.