The role of additives in the recycling of polymers

The main problems in post-consumer plastics recycling are due to the degradation undergone by the polymers during processing steps and by the products during their lifetime and, for heterogeneous recycling, to the incompatibility of different polymers. To reduce the negative effects of the recycling steps, two main ways can be adopted for homogeneous materials: restabilization during the recycling to avoid or at least to slow the degradation and addition of fillers and modifiers capable of improving the performance of thermoplastic polymers without increasing the final cost of the secondary material. In the case of mixed plastics, compatibilization is the necessary step to obtain secondary materials with acceptable properties.     

Multidisciplinary Team of Chemists and Geneticists Working to Reclaim Value Embedded in Plastics

This invited Team Profile was created by Clay C. C. Wang. He and his collaborators recently published an article on the conversion of polyethylenes to fungal secondary metabolites. First, the team employs an oxidative catalytic process, highly tolerant of impurities, to degrade post-consumer polyethylenes to carboxylic diacids. Then, they utilize engineered strains of the fungus Aspergillus nidulans to convert these diacids to structurally diverse and pharmacologically active secondary metabolites. “Conversion of Polyethylenes into Fungal Secondary Metabolites”, C. Rabot, Y. Chen, S. Bijlani, Y.-M. Chiang, C. E. Oakley, B. R. Oakley, T. J. Williams, C. C. C. Wang, Angew. Chem. Int. Ed. 2023, e202214609 ; Angew. Chem. 2023, e202214609 .     

Converting Waste Plastic to Liquid Organic Hydrogen Carriers

The accumulation of waste plastics in landfills and the environment, as well as the contribution of plastics manufacturing to global warming, call for the development of new technologies that would enable circularity for synthetic polymers. Thus far, emerging approaches for chemical recycling of plastics have largely focused on producing fuels, lubricants, and/or monomers. In a recent study, Junde Wei and colleagues demonstrated a new catalytic system capable of converting oxygen-containing aromatic plastic waste into liquid organic hydrogen carriers (LOHCs), which can be used for hydrogen storage. The authors utilized Ru−ReO_x/SiO₂ materials with zeolite HZSM-5 as a co-catalyst for the direct hydrodeoxygenation (HDO) of oxygen-containing aromatic plastic wastes that yield cycloalkanes as LOHCs with a theoretical hydrogen capacity of ≈5.74 wt % under mild reaction conditions. Subsequent efficiency and stability tests of cycloalkane dehydrogenation over Pt/Al₂O₃ validated that the HDO products can serve as LOHCs to generate H₂ gas. Overall, their approach not only opens doors to alleviating the severe burden of plastic waste globally, but also offers a way to generate clean energy and ease the challenges associated with hydrogen storage and transportation.     

Polymer Multi-Block and Multi-Block+ Strategies for the Upcycling of Mixed Polyolefins and Other Plastics

Due to a continued rise in the production and use of plastic products, their end-of-life pollution has become a pressing global issue. One of the biggest challenges in plastics recycling is the separation of different polymers. Multi-block copolymers (MBCPs) represent an efficient strategy for the upcycling of mixed plastics via induced compatibilization, but this approach is limited by difficulties associated with synthesis and structural modification. In this contribution, several synthetic strategies are explored to prepare MBCPs with tunable microstructures, which were then used as compatibilizer additives to upcycle mixtures of polyolefins with other plastics. A multi-block⁺ strategy based on a reactive telechelic block copolymer platform was introduced, which enabled block extension during the in situ melt blending of mixed plastics, leading to better compatibilizing properties as well as better 3D printing capability. This strategy was also applicable to more complex ternary plastic blends. The polymer multi-block strategy enabled by versatile MBCPs synthesis and the multi-block⁺ strategy enabled by in situ block extension show exciting opportunities for the upcycling of mixed plastics.     

Polymer and its effect on environment

This is an era where plastic pollution is increasing hazardously. Plastics are spreading all over the environment due to this it's a big threat to the equilibrium of the environment and health of the human beings. Its not due to their properties but it is also a strong carrier of pesticides, poly aromatic hydrocarbons, diphenyl, pharmaceutical products etc. Majorly plastics are being used everywhere like in packaging, water bottles etc. We have about to reach the stage where we require to produce biodegradable or recyclable plastic. It reduces the usage of oil, CO₂ emission and reduces the quantity of waste to be disposed. Phthalates, BPA and others should be banned in plastic products which are in direct contact with food, children and bio-degradable plastics should be more used. Our study focused on varieties of plastics, its hazardous impact on the environment especially on the environment, its recycling strategies and use of biodegradable materials.     

High-Density Polyethylene with In-Chain Photolyzable and Hydrolyzable Groups Enabling Recycling and Degradation

Polyethylenes endowed with low densities of in-chain hydrolyzable and photocleavable groups can improve their circularity and potentially reduce their environmental persistency. We show with model polymers derived from acyclic diene metathesis polymerization that the simultaneous presence of both groups has no adverse effect on the polyethylene crystal structure and thermal properties. Post-polymerization Baeyer–Villiger oxidation of keto-polyethylenes from non-alternating catalytic ethylene-CO chain growth copolymerization yield high molecular weight in-chain keto-ester polyethylenes (M_n≈50.000 g mol⁻¹). Oxidation can proceed without chain scission and consequently the desirable materials properties of HDPE are retained. At the same time we demonstrate the suitability of the in-chain ester groups for chemical recycling by methanolysis, and show that photolytic degradation by extended exposure to simulated sunlight occurs via the keto groups.     

Insertion of Supramolecular Segments into Covalently Crosslinked Polyurethane Networks towards the Fabrication of Recyclable Elastomers

Thermoset plastics have become one of the most important chemical products in the world. The consequent problem is that although the thermosets possess excellent performance in mechanical strength, they cannot be reprocessed because of the internal permanent network structures. Optimizing the molecular design of thermosets is one of the most feasible ways to improve their recyclability. Here we present a facile and robust strategy to engineer the reprocessability of thermoset polyurethanes without compromising their mechanical toughness and chemical resistance via adding supramolecular additives during the polymer synthesis process. By using a multiple hydrogen bonding moiety as the model supramolecular additive, we demonstrate that the mechanical properties, recyclability, and chemical resistance of the crosslinked polyurethanes can be precisely controlled by adjusting the contents of the supramolecular additive. Systematic studies on the relations between molecular design and material properties are performed, and the optimized polyurethane network with a moderate amount of the supramolecular additive achieves the right balance between the robustness and recyclability. This work provides a cost-effective and practical way to chemically engineer thermoset plastics, aiming to enable the recycling of mechanically tough and chemically stable polymer materials.     

Biological recycling of plastics containing ester bonds

Biodegradable blend plastics such as polycaprolactone (PCL)-conventional plastics blends, PCL-polystyrene blend foams, PCL-poly-3-hydroxybutyrate (PHB) blends, PCL-raw cornstarch (CS) blends and PCL-CaCO₃ blends were developed. It was suggested that the thermophilic composting of biodegradable plastics containing PCL was one of powerful technologies for recycling of biodegradable plastics. Furthermore, we tried to get useful products from biodegradable plastics by microbial fermentation processes. Polypropylene (PP)-CS blend and PCL-CS blend plastics were aerobically converted into ethanol by Bacillus polymyxa. PHB and PP-CS blend and PCL-CS blend plastics were anaerobically converted into some organic acids and methane gas by mixed microbial cultures with a methanogenic bacterium.     

Elucidation of Biosynthetic Pathways of Natural Products

During the last decade, we have revealed biosynthetic pathways responsible for the formation of important and chemically complex natural products isolated from various organisms through genetic manipulation. Detailed in vivo and in vitro characterizations enabled elucidation of unexpected mechanisms of secondary metabolite biosynthesis. This personal account focuses on our recent efforts in identifying the genes responsible for the biosynthesis of spirotryprostatin, aspoquinolone, Sch 210972, pyranonigrin, fumagillin and pseurotin. We exploit heterologous reconstitution of biosynthetic pathways of interest in our study. In particular, extensive involvement of oxidation reactions is discussed. Heterologous hosts employed here are Saccharomyces cerevisiae, Aspergillus nidulans and A. niger that can also be used to prepare biosynthetic intermediates and product analogs by engineering the biosynthetic pathways using the knowledge obtained by detailed characterizations of the enzymes. (998 char.)     

Prediction of melt rheological properties from GPC molecular weights

The SCLAIR^® solution polymerization platform produces a wide variety of ethylene-α-olefin copolymers and polyethylene homopolymers. Commercial products exhibit density and melt index values ranging from about 0.920 to 0.962 g/cm³ and 0.3–75 g/10 min respectively. Polymer molecular weight distributions can be tailored to meet a broad selection of end-use requirements. In this study, we have used a chemometric analysis approach using The Unscrambler^® software to demonstrate statistical correlations between rheological properties and fundamental structural parameters for thirty-three commercial SCLAIR polyethylenes. We demonstrate that molten rheological properties such as melt index, stress exponent, zero-shear viscosity, characteristic relaxation time, cross-over modulus and frequency show good non-linear correlations with molecular weight characteristics of SCLAIR products as determined by gel permeation chromatography (GPC). We also show that, with the use of Partial Least Squares (PLS) regression techniques, most melt rheological properties can be accurately predicted on the basis of GPC data.     

Bicyclic Guanidine Promoted Mechanistically Divergent Depolymerization and Recycling of a Biobased Polycarbonate

We here report the organocatalytic and temperature-controlled depolymerization of biobased poly(limonene carbonate) providing access to its trans-configured cyclic carbonate as the major product. The base TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) offers a unique opportunity to break down polycarbonates via end-group activation or main chain scission pathways as supported by various controls and computational analysis. These energetically competitive processes represent an unprecedented divergent approach to polycarbonate recycling. The trans limonene carbonate can be converted back to its polycarbonate via ring-opening polymerization using the same organocatalyst in the presence of an alcohol initiator, offering thus a potential circular and practical route for polycarbonate recycling.     

Recovery of polyols from flexible polyurethane foam by “split-phase” glycolysis with new catalysts

Polyurethanes (PU) represent one of the most important groups of plastics, so the increasing quantity of wastes makes their recycling an urgent task. The general purpose of polyurethane chemical recycling is to recover constituent polyol, a valuable raw material. Among the suitable processes, glycolysis, specially in two phases, allows better quality products. In this study glycolysis reactions of flexible polyurethane foams were conducted in “split-phase” with different catalysts, in order to study their activity. Diethanolamine, titanium n-butoxide as well as octoate salts, which are novel compounds for this application, showed suitable catalytic activity. Reaction kinetics and glycolysis products were investigated. Times to reach complete conversion, chemical properties of the polyol phase and its purity depend on the catalyst employed. The novel catalysts developed have been probed to be a worthy and economic alternative to traditional catalysts.     

A Generic Platform for Upcycling Polystyrene to Aryl Ketones and Organosulfur Compounds

Polystyrene (PS) is one of the least recycled large-volume commodity plastics due to bulkiness of foam products and associated contaminants. PS recycling is also severely hampered by the lack of financial incentive, limited versatility, and poor selectivity of existing methods. To this end, herein we report a thermochemical recycling strategy of “degradation-upcycling” to synthesize a library of high-value aromatic chemicals from PS wastes with high versatility and selectivity. Two cascade reactions are selected to first degrade PS to benzene under mild temperatures, followed by the derivatization thereof utilizing a variety of acyl/alkyl and sulfinyl chloride additives. To demonstrate the versatility, nine ketones and sulfides of cosmetic and pharmaceutical relevance were prepared, including propiophenone, benzophenone, and diphenyl sulfide. The approach is also amenable to sophisticated upcycling reaction designs and can produce desired products stepwise. The facile and versatile approach will provide a scalable and profitable methodology for upcycling PS waste into value-added chemicals.     

Isobaric thermal expansivities of polyethylenes with various crystallinities over the pressure range from 0.1 to 300 MPa and over the temperature range from 303 to 393 K

A pressure-controlled scanning calorimeter (PCSC) has been applied for measuring the isobaric volume thermal expansivities (α_p) of crystalline polymers as a function of pressure up to 300 MPa at various temperatures. The measurements have been performed for several well-defined polyethylenes with various degrees of crystallinity at 302.6, 333.0, 362.6, and 393.0 K. The results are reported as values of coefficients in a correlation equation, which facilitates the use of reported data over large ranges of temperature and pressure. The general pressure-temperature behavior of α_p for all polyethylenes under study is such that α_p increases with temperature and decreases with pressure. The increase with temperature is smaller at high pressures and the isotherms of α_p have a tendency to converge at high pressures; α_p decreases linearly with the crystallinity of the polyethylene over the whole range of pressure and temperature under investigation. From the linear approximation of experimental data for polyethylenes with various crystallinities the estimated α_p for both crystal and amorphous phases of polyethylenes have been determined as a function of pressure up to 300 MPa at 302.6, 333.0, and 362.5 K. The obtained results have been compared with available literature crystallographic data and with the values derived from the Pastine theoretical equation of state for both crystalline and amorphous phases. © 1996 John Wiley & Sons, Inc.     

A pulsed field gradient NMR study of diffusion in semicrystalline polymers: self-diffusion of alkanes in polyethylenes

By means of the pulsed field gradient NMR technique the self-diffusion of six alkanes (from n-butane to n-pentadecane) in three low density polyethylenes and one high density polyethylene differently thermally treated was examined. The concentration dependence could be described very satisfactorily with the free volume theory in the form of Fujita (Adv. Polymer Sci. 3(1961) 1). The parameter B of the diffusants and the fractional free volumef ₂ of the polyethylenes were determined from the experimental data. The fractional free volumesf ₂ show a strong dependence on the type of polyethylene, the main influence results from the different content of CH₃ groups or short chain branches. The diffusion coefficient extrapolated to zero diffusant concentration is proportional to the eighth power of the amorphous content. This strong dependence shows that the free volumes of the amorphous parts of the polyethylenes are intimately connected with crystallinity, both determined by the different degrees of short chain branching. The pre-exponential factor in the free volume expression decreases with increasing amorphous content of the polyethylenes and increases with increasing length of the diffusants. It was found that the spherulite boundaries in the polyethylenes do not act as diffusion barriers.     

Morphology of CaCO3-filled polyethylenes

The morphology of one novel “homogeneous composite” and some conventional CaCO₃-particulate-filled polyethylenes (PEs) has been investigated by transmission and scanning electron microscopy. It is found that for three different linear polyethylenes (M _w = 37,000, 200,000, and 1.5 × 10⁶) studied, ordering on the scale of spherulites was absent in the CaCO₃-filled polymer. However, PE lamallae were observed growing normal to the filler interface. Changes in molecular weight of the polyethylene result in easily observed differences in morphology, while DSC studies indicate that crystallinity decreases in the presence of filler for the two lower-molecular-weight polyethylenes.     

Recycling Chiral Organocatalyst （4S）-Phenoxy-（S）-proline for Direct Asymmetric Aldol Reaction in Ionic Liquid （bmim）PF6

Room temperature ionic liquid (bmim)PF6 was evaluated for recycling an organocatalyst (4S)-phenoxy-(S)-proline for direct asymmetric aldol reactions. The desired aldol products were obtained with good yields up to 93.2% and enantioselectivities up to 88.5%, and isolation of the products by simple extraction allowed recycling the ionic liquid containing the immobilized catalyst in subsequent reactions without significant decrease of yields and enantioselectivities.     

Influence of initial polymer concentration in solution and weight-average molecular weight on the drawing behavior of polyethylenes

The influence of initial polymer concentration in solution (c), weight-average molecular weight (M_ω), and drawing temperature on the solid-state drawing behavior of linear polyethylenes was investigated. Optimum conditions, with respect to maximum attainable draw ratio, are observed in isothermal drawing experiments. Moreover, it is shown that high maximum attainable draw ratios can also be obtained upon multistage drawing of UHMW-PE (ultrahigh-molecular-weight polyethylene, M_ω > 10⁶ g/mol) gel films cast from concentrated solutions. The high maximum attainable draw ratio in combination with the high molecular weight (M_ω > 10⁶ g/mol) and polymer concentration (c = 10% w/v) is of particular interest because it results in tapes or fibers with a high Young's modulus (100 GPa) and tensile strength (2.5–3.5 GPa). It is also shown that the maximum attainable draw ratio of polyethylenes scales with the Bueche parameter (c · M_ω) to the ?0.5 power. This experimental observation indicates that intermolecular interactions not only dominate the rheological properties of polyethylene melts and concentrated solutions, but also strongly influence the solid-state drawing behavior of linear polyethylenes.     

Compatibilization and impact modification of mixed recycled plastics

In order to solve the problems related to the reuse and recycling of polymer materials, approaches must be developed to address the incompatibility of most plastics. In many cases, the complete separation of dissimilar plastics may not be economically feasible or technologically possible, so commingled plastics recycling may be the only option. Our work has concentrated on upgrading the performance of a blend of two incompatible polymers. For this, we have used a model composition of two plastics commonly found in business machine housings. Our approach concentrates on compatibilization of the blend with a tailored copolymer, in addition to impact modification of the different phases. Improvements in mechanical performance of the blends are elucidated through the use of transmission electron microscopy.     

Recycling technology of poly(ethylene terephthalate) materials

Poly(ethylene terephthalate) (PET) has become one of major post consumer plastics wastes, in addition to polyethylene (PE), polypropylene (PP), polystyrene (PS) and poly(vinyl chloride) (PVC). The challenge to large-volume plastics companies is to learn how to collect, separate, reprocess and market their low-cost products and make a profit, too. The effort of PET recycling, however, is the most successful story in the plastic recycling technology, including both reclaim and upgrade of PET waste. Beverage bottles made of PET are recycled more than 20% of the total production. The technology of today can reclaim the post-consumer PET bottles to produce high-quality granulated PET with better than 99% purity. A practical reclaim process for recycling PET bottles (including bottle, HDPE base cup, aluminum cap, liner, label and adhesive) is available by the Center for Plastics Recycling Research in USA. PET recycling process, like for other plastics, can be divided into three categories: incineration, physical recycling, and chemical recycling. To make the plastic recycling business pay requires more than simple recovery and marketing. Greatest profit potential is in upgraded and value-added reclaim products. Upgrading involves compounding with additives to make material more processable, adding reinforcement, or producing extrusions or finished parts from reclaim resins. For instance, a modified injection-moldable resin made from PET bottle scrap is claimed to provide high impact and processability at less cost than competitive materials. It is foreseen that chemical recycling of waste PET bottle becomes feasible if the price of raw material goes up. Three economical processes are involved in this technology: pyrolysis, hydrocracking, and hydrolysis. The hydrolysis process is presently employed to recover the raw material for unsaturated polyester resin manufacture or polyols for the production of polyurethane resin. It is reported in this presentation that polymer concrete could be a huge potential market for chemical reclaim of PET materials, especially for green or mixed-color PET, which are priced lower than colorless PET reclaim materials.