High shear dispersion of tracers in polyolefins for improving their detection

Temperature Dependence of Kinetic Friction: A Handle for Plastics Sortation?

Sortation is a crucial step in mechanical recycling of post-consumer plastics (PCR) whereby properties such as density or spectral signature are used to separate plastics. However it is difficult to sort polyolefin flakes at high throughput by these properties. We ask whether the frictional properties of plastics as a function of temperature may be used as an alternate sorting property. However, fundamental studies of friction at temperatures near their melting points are limited. Here we measure the temperature dependence of kinetic friction for three common polyolefins (high and low den- sity polyethylene and polypropylene) as well as polyethylene terephthalate (PET), focusing on the softening/melting regime. The results are augmented by differential scanning calorimetry and temperature dependence measurements of both dynamic modulus and and probe tack. For the polyolefins, we find strong increases in the coefficients of kinetic friction during temperature ramps in the melting/softening regime. For the PET, we report a notable peak in the kinetic friction which we associate with the glass transition and cold-crystallization. We discuss the enhanced friction in the context of rub- ber friction, which exhibits comparable coefficients of kinetic frictions.

Polyethylenes and Polystyrenes with Carbazole Fluorescent Tags

The increasingly intense consumption of plastics and, above all, their improper disposal in the environment are causing serious environmental concerns. Great efforts have been made for the development of new methods aimed at facilitating and speeding up the identification and sorting of different materials in the plastic recycling process. In this field, new strategies based on fluorescent tagging have been developed. This work concerns the synthesis and characterization of new fluorescent copolymers of polyethylene (PE) and polystyrene (PS), which are among the most produced and consumed plastic materials. The synthesized copolymers are potentially suitable for use as fluorescent markers of PE and PS. Ethylene-co-N-pentenyl carbazole (P(E-co-PK)) and styrene-co-4-(N-carbazolyl)methyl styrene (P(S-co-SK)) copolymers were prepared by Ziegler–Natta and free radical polymerization, respectively. If excited at 300 nm, both P(E-co-PK)s and P(S-co-SK)s give fluorescence emissions resulting in them being optically active. Moreover, due to the low amount of fluorescent units, they show chemico-physical properties such as those of their corresponding homopolymers (PE and PS). P(E-co-PK)s and P(S-co-SK)s have been also tested as fluorescent markers of PE and PS. The experimental results demonstrate that from PE/P(E-co-PK) and PS/P(S-co-SK) blends prepared using only 1% by weight of fluorescent copolymer, distinguishable fluorescent emissions can be still detected.

Enabling the Polymer Circular Economy: Innovations in Photoluminescent Labeling of Plastic Waste for Enhanced Sorting

It is widely accepted that moving from a linear to circular economy for plastics will be beneficial to reduce plastic pollution in our environment and to prevent loss of material value. However, challenges within the sorting of plastic waste often lead to contaminated waste streams that can devalue recyclates and hinder reprocessing. Therefore, the improvement of the sorting of plastic waste can lead to dramatic improvements in recyclate quality and enable circularity for plastics. Here, we discuss current sorting methods for plastic waste and review labeling techniques to enable enhanced sorting of plastic recyclates. Photoluminescent-based labeling is discussed in detail, including UV-vis organic and inorganic photoluminescent markers, infrared up-conversion, and X-ray fluorescent markers. Methods of incorporating labels within packaging, such as extrusion, surface coatings, and incorporation within external labels are also discussed. Additionally, we highlight some practical models for implementing some of the sorting techniques and provide an outlook for this growing field of research.

Polymer Labelling with a Conjugated Polymer-Based Luminescence Probe for Recycling in the Circular Economy

In this paper, we present the use of a disubstituted polyacetylene with high thermal stability and quantum yield as a fluorescence label for the identification, tracing, recycling, and eventually anti-counterfeiting applications of thermoplastics. A new method was developed for the dispersion of poly[1-phenyl-2-[p-(trimethylsilyl)phenyl]acetylene] (PTMSDPA) into polymer blends. For such purposes, four representative commodity plastics were selected, i.e., polypropylene, low-density polyethylene, poly(methyl methacrylate), and polylactide. Polymer recycling was mimicked by two reprocessing cycles of the material, which imparted intensive luminescence to the labelled polymer blends when excited by proper illumination. The concentration of the labelling polymer in the matrices was approximately a few tens ppm by weight. Luminescence was visible to the naked eye and survived the simulated recycling successfully. In addition, luminescence emission maxima were correlated with polymer polarity and glass transition temperature, showing a marked blueshift in luminescence emission maxima with the increase in processing temperature and time. This blueshift results from the dispersion of the labelling polymer into the labelled polymer matrix. During processing, the polyacetylene chains disentangled, thereby suppressing their intermolecular interactions. Moreover, shear forces imposed during viscous polymer melt mixing enforced conformational changes, which shortened the average conjugation length of PTMSDPA chain segments. Combined, these two mechanisms shift the luminescence of the probe from a solid- to a more solution-like state. Thus, PTMSDPA can be used as a luminescent probe for dispersion quality, polymer blend homogeneity, and processing history, in addition to the identification, tracing, and recycling of thermoplastics.