Plastic production continues to increase and is expected to surpass more than 900 million tonnes in 2050. Plastic recycling has become necessary to provide correct disposal for this material to avoid landfill accumulation or, even worse, ending up in rivers and oceans. Plastic pollution is a global issue with further research and understanding needed to increase recycling rates. One essential approach to increase recycling rates is to evaluate the composition of outbound streams from current recycling facilities and what capacity they can be reused. In this study, the composition and quality of outbound bales containing #3-7 plastics destined to landfill or waste-to-energy facilities were assessed to understand the potential to increase recycling rates. Bales were sourced from three different Material Recovery Facilities (MRFs) located in the states of Iowa and Wisconsin and were manually sorted and analyzed using multiple plastic characterization techniques. Significant differences in bale composition were observed between MRFs, usually associated with the sophistication level or technology used in the sorting process with seasonal feedstock variability. Differences were substantial in residual levels of poly(ethylene terephthalate) (PET) and high-density polyethylene (HDPE), which are highly desired for mechanical recycling processes and not expected to be present in #3-7 plastics bales. Prior to physical, thermal, and molecular characterization on recovered PET, HDPE, low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) resins, traditional mechanical recycling processes were employed to include washing, extrusion, and injection molding of sorted material. Despite the differences in composition, some polymer properties presented similar values across MRFs. This research suggests that landfill-diverted mixed plastic waste can be utilized for novel and advanced recycling operations to recover unrecycled materials as processes can be designed to provide consistent polymer properties. This research also suggests the need for upgrading the sorting systems to prevent waste feedstocks that can be recycled with current technologies from ending up with low-value products or at landfills.
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Many highly porous materials with pore volumes greater than 90% are attractive due to their high specific surface area and tunable surface energy. These materials cannot be easily adopted in advanced applications due to their poor mechanical strength and low handling stress. The advent of 3D-printing methods comes handy in such cases. The porous materials can be 3D-printed directly as metamaterials that offer much higher elongation and compliance in response to extensional stress than the parent porous materials themselves. The porous materials can also be made an integral part of 3D-printed, load bearing scaffolds to take advantage of their multi-functional attributes in thermal insulation, liquid-liquid separation, and nanoparticle removal from air with performance close to those of HEPA filters. This talk will illustrate three such examples. In one example, polyurethane aerogel metamaterials show substantially high elongation due to strategic arrangements of the highly porous limbs. In the second example, high porosity polyimide domains are grown inside or on surfaces of solid polymer scaffolds to produce excellent thermal insulation or efficient removal of noxious dye molecules from aqueous streams. In last example, variable surface energy strands of syndiotactic polystyrene or polyimide are used in removing water droplets from a hydrocarbon oil. Some generalizations will be made that other researchers can use and expand the concepts presented in this talk to an array of materials systems.
In line with Europe's green deal, a new edition of the European action plan for a transition to a circular economy has been published in 2020. Amongst others, plastics have great potentials to achieve a high level of product circularity. In recent years, the plastics recycling industry has gained a great momentum to be one of the drivers towards a sustainable circular economy. However, there is still an abundance of challenges that need to be addressed and overcome in this sector. Therefore, a great focus in the new action plan is dedicated to plastics and plastic packaging products. Consequently, a set of mandatory or voluntary product requirements and regulations were reinforced or introduced as part of a new framework for eco-design and sustainable products. Furthermore, this legislative initiative also aims to enhance the traceability and the accessibility to product information through the implementation of certain digitalization tools, such as digital product passports (DPP). The main objective of this research is to provide a practical implementation of DPP of a pilot product made of recycled post-consumer plastic waste. It also aims to track the possible changes in the material property profile of a defined waste stream due to processing throughout the whole recycling process. High density polyethylene (PE-HD) bottle caps were selected as the targeted input waste stream. On the other end of the process, a frisbee (i.e., flying disc) was selected as the pilot product. Two collection methods were employed in this case study, namely informal and formal. The first fraction of bottle caps was collected by pupils and students (informal) over a period of two months in Upper Austria region with focus on PE-HD bottle caps. Whereas the other fraction was collected via the conventional methods (formal) and pre-sorted (1st sorting) to remove metal contaminants at the waste collection centers in Upper Austria. At the pilot plant, each fraction was hand-sorted (2nd sorting) individually to ensure a high purity of input materials. Afterwards, materials were shredded by an industrial shredder and then re-granulated using an industrial recycling extruder equipped with filtration and degassing systems. Thereafter, the resulting recyclates were injection molded into the finished frisbee. To characterize the material property profile of the different material states, several measurements including melt flow rate (MFR), differential scanning calorimetry (DSC), and mechanical tests were carried out. It was found that the informal collection led to a higher material purity as the other fraction had a more prominent melting peak of polypropylene (PP), which led to a slightly higher MFR value of this input fraction. However, no significant changes in the MFR values of the other materials were observed. In terms of the mechanical properties, the tensile stiffness and strength increased after processing. In contrast, the notched Charpy impact strength of the recyclates seemed to be slightly lower than that of both input streams.
The production of conventional cross-linked polymer networks and their composites, i.e., thermosets and thermoset composites, was estimated to consume more than 40 billion kg of polymer in 2020. Unfortunately, thermosets cannot be melt-reprocessed into moderate- to high-value products because permanent crosslinks prevent melt flow. Three of many examples include rubber tires, disposed at a rate of 300 million annually in the U.S. alone, polyurethane (PU) foam, and cross-linked polyethylene, with major economic and sustainability losses resulting because the spent materials are commonly landfilled or burned for energy. Here, I will report on research demonstrating the ability to employ simple one-step or two-step reactions to produce networks and network composites with dynamic covalent crosslinks that are robust at use conditions but allow for melt-state reprocessing at elevated temperature. Unique to our research group, we have developed several approaches that allow for melt-state reprocessing of addition-type polymer networks and network composites, including those synthesized directly from monomers containing carbon-carbon double bonds, such as those used in coatings and flooring, and those synthesized from polymer or combined polymer and monomer with both containing carbon-carbon double bonds, like materials used in tires and in cross-linked polyethylene. All approaches allow for full crosslink density recovery after multiple reprocessing steps. We have also demonstrated for the first time the ability to make PU and PU-like networks, e.g., polyhydroxyurethane and polythiourethane networks, reprocessable with full recovery of crosslink density. An "Achilles' heel" has been identified regarding the application of dynamic covalent networks, i.e., such networks are subject to creep at elevated or sometimes even room temperature, which is often highly undesirable. We have addressed this limitation in two ways. In one case, we add a fraction of permanent crosslinks to dynamic covalent networks. In a second class of systems, we employ dynamic chemistry with a sufficiently high activation energy, allowing for reprocessability at high temperature but with the dynamic chemistry essentially fully arrested well above room temperature, e.g., 70-80 degrees C. Implications of these studies for making major gains in the sustainability of polymer networks and network composites will be discussed.
Due to insufficient sorting and recycling, macroscopic contaminations remain in post-consumer polyolefin recyclates. It is known that these contaminations affect the mechanical properties of the recyclates, as they constitute defects and thus crack initiators. However, the influences of different types and amounts of macroscopic contaminants have not yet been analyzed systematically. In this study, to close this knowledge gap, virgin polypropylene (PP) was systematically contaminated with paper, aluminum, sand, wood, in-mold labels, jute fibers and long glass-fibers. Additionally, three commercially available post-consumer PP recyclates were investigated. In a two-stage process, all materials were injection-molded into plates and subsequently milled to specimens. The specimens underwent (i) tensile tests at 50 mm/min, (ii) intermediate-rate tensile tests at 2000 mm/min, and (iii) tensile impact tests. Further, optical microscopy was used to measure the dimensions of the defects on the fracture surfaces. First, the influences of various types and quantities of contamination were evaluated. No significant effects were detected, as the matrix material was very brittle. Compared to the virgin reference grade, most samples showed lower strain-at-break values, except for those with labels and long glass-fibers, for which strain values increased. All PP post-consumer recyclates exhibited a more pronounced ductile behavior, although the contaminations incorporated gave rise to relatively high standard deviations. Second, in a comparison of various testing speeds, a greater influence of contaminants was detected in test (iii). Samples taken from a position close to the sprue had better mechanical properties than samples taken from the opposite side of the plate, as contaminants tend to flow to the end of the produced part. Finally, a non-linear relationship between the energy needed for fracture in testing methods (ii) and (iii) and the dimensions of the contamination on the fracture surface was found. 2ff7e9595c
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