Advances and approaches for chemical recycling of plastic waste

The global production and consumption of plastics has increased at an alarming rate over the last few decades. The accumulation of pervasive and persistent waste plastic has concomitantly increased in landfills and the environment. The societal, ecological, and economic problems of plastic waste/pollution demand immediate and decisive action. In 2015, only 9% of plastic waste was successfully recycled in the United States. The major current recycling processes focus on the mechanical recycling of plastic waste; however, even this process is limited by the sorting/pretreatment of plastic waste and degradation of plastics during the process. An alternative to mechanical processes is chemical recycling of plastic waste. Efficient chemical recycling would allow for the production of feedstocks for various uses including fuels and chemical feedstocks to replace petrochemicals. This review focuses on the most recent advances for the chemical recycling of three major polymers found in plastic waste: PET, PE, and PP. Commercial processes for recycling hydrolysable polymers like polyesters or polyamides, polyolefins, or mixed waste streams are also discussed.     

Biodegradable composite films based on waste gelatin

Gelatin, a naturally occurring polymer, is currently used in various applications comprising manufacturing of pharmaceutical products, x-ray and photographic films development and food processing. However, gelatin scraps generated in the different manufacturing processes may constitute a concern for the environment. Basically speaking, waste disposal deriving from plastics based on synthetic as well as semisynthetic polymeric materials, is becoming an increasingly difficult problem for their unfavorable volume-to-weight ratio and extremely wide variability of type, shape and composition of post consume plastic items that hinder the way to a general unique option for a simple and economically feasible management. As a partial solution to the global issue of plastic waste, in recent years much interest has been devoted to the formulation of environmentally degradable plastic items. Biodegradable mulching films were formulated from blends and composites based on waste gelatin and other natural waste such as sugarcane bagasse or synthetic materials such as PVA. Also, crosslinked films were produced using external or inherent crosslinker. The films were produced either by casting method or spraying on soil surface. The composites were submitted to biodegradation trials. The results showed that the films were biodegradable and the crosslinking could delay and predeterminate their biodegradation rate and extent.     

Synthesis, characterization and biodegradation of butanediamine-grafted poly(dl-lactic acid)

Novel butanediamine-grafted poly(dl-lactic acid) polymers (BDPLAs) were synthesized via a series of chemical bulk modifications in this study. Briefly, maleic anhydride (MAH) was first grafted onto the side chain of poly(dl-lactic acid) (PDLLA) molecules via melt free radical copolymerization using benzoyl peroxide (BPO) as initiator to get maleic anhydride-grafted PDLLA polymers (MPLAs); thereafter butanediamine (BDA) was immobilized onto grafted anhydride groups in MPLAs via N-acylation reaction to obtain the desired BDPLAs. Gel permeation chromatography with multi-angle laser light scattering (GPC-MALLS), FT-IR, ¹³C NMR and XPS were employed to qualitatively characterize these synthesized polymers. Rhodamine-carboxyl interaction method and ninhydrin reaction were further used to quantitatively determine the graft ratio of MAH (MAH%) in MPLAs and the graft ratio of BDA (BDA%) in BDPLAs, respectively. The degradations of BDPLAs, PDLLA and MPLAs were investigated by observation of the changes of the pH value of incubation medium, molecular weight and weight loss ratio for a time interval of 12 weeks in vitro, respectively. The results revealed that grafting butanediamine onto PDLLA has weakened or neutralized the acidity of PDLLA degradation products. A uniform degradation of BDPLAs was observed in comparison with an acidity-induced auto-accelerating degradation featured by PDLLA and MPLAs. The biodegradation behaviors of BDPLAs are tunable by controlling the content of BDA. BDPLAs might be a new derivative of PDLLA-based biodegradable materials for medical applications without acidity-caused irritations and acidity-induced auto-accelerating degradation behavior as that of PDLLA.     

Thermal degradation of polyethylene and polystyrene from the packaging industry over different catalysts into fuel-like feed stocks

Thermal degradation of waste polymers was carried out as a suitable technique for converting plastic polymers into liquid hydrocarbons, which could be used as feed stock materials. The catalytic degradation of waste plastics (polyethylene and polystyrene) was investigated in a batch reactor over different catalysts (FCC, ZSM-5 and clinoptillolite). The effects of catalysts and their average grain size on the properties of main degradation products (gases, gasoline, diesel oil) are discussed. The temperature range of 410-450 °C was used in the process. Both equilibrium FCC catalyst and natural clinoptilolite zeolite catalyst had good catalytic activity to produce light hydrocarbon liquids, and ZSM-5 catalyst produced the highest amount of gaseous products. Gases and liquids formed in cracking reactions were analyzed by gas chromatography. The liquid products consisted of a wide spectrum of hydrocarbons distributed within the C₅-C₂₈ carbon number range depending on the cracking parameters. The composition of hydrocarbons had linear non-branched structure in case of polyethylene, while from polystyrene more aromatics (ethyl-benzene, styrene, toluene, and benzene) were produced. The yields of volatile products increased with increasing degradation temperature. The olefin content of liquids was measured with an infrared technique and an olefin concentration of 50-60% was observed. The concentration of unsaturated compounds increased with decreasing temperature, and in the presence of catalysts. The activation energies were calculated on the basis of the composition of volatile products. The apparent activation energies were decreased by catalysts and catalyst caused both carbon-chain and double bond isomerisation.     

Controlled “Grafting-from” of poly[styrene-co-maleic anhydride] onto polydienes using nitroxide chemistry

High-throughput experimentation was used for the functionalization of polybutadiene and polyisoprene with several kinds of nitroxide moieties at 135 °C, as well as for kinetic investigations of the controlled free radical grafting of styrene and maleic anhydride onto the functionalized polydienes at 125 °C. The functionalized polymers were analyzed by GPC, ¹H NMR and FT-IR and the control of the grafting process was assessed by kinetic measurements and GPC analysis. The best control in the molecular weight of the grafts was observed for the polydienes functionalized with 4-oxo-TEMPO. TEMPO functionalization did not render sufficient nitroxide moieties to achieve controlled grafting, while polydienes functionalized with other 4-hydroxy TEMPO derivatives exhibited some level of grafting control at early reaction times, which was lost in later stages of the polymerization reaction. SG1 apparently decomposed at the tested reaction temperatures, as polydienes functionalized with SG1 did not show any level of control during the grafting process.     

Combined Chemical Activation and Fenton Degradation to Convert Waste Polyethylene into High‐Value Fine Chemicals

Plastic waste is a valuable organic resource. However, proper technologies to recover usable materials from plastic are still very rare. Although the conversion/cracking/degradation of certain plastics into chemicals has drawn much attention, effective and selective cracking of the major waste plastic polyethylene is extremely difficult, with degradation of C?C/C?H bonds identified as the bottleneck. Pyrolysis, for example, is a nonselective degradation method used to crack plastics, but it requires a very high energy input. To solve the current plastic pollution crisis, more effective technologies are needed for converting plastic waste into useful substances that can be fed into the energy cycle or used to produce fine chemicals for industry. In this study, we demonstrate a new and effective chemical approach by using the Fenton reaction to convert polyethylene plastic waste into carboxylic acids under ambient conditions. Understanding the fundamentals of this new chemical process provides a possible protocol to solve global plastic‐waste problems.     

Grafting of living poly(ethylene oxide) onto polystyrene via aromatic nucleophilic displacement of activated nitro groups

Polystyrene of M?_ω = 2.2 × 10⁴ was alkylated with 4-nitrophthalimidomethyl groups as grafting sites. Several backbone polymers with various degrees of grafting sites (G = 2–100%) were prepared and characterized by elemental analysis, IR, ¹H- and ¹³C-NMR, and viscosity measurements. “Living” poly(ethylene oxide) with narrow molecular-weight distribution was prepared in the presence of 15-crown-5, and grafted onto the 4-nitrophthalimidomethylated polystyrene. The nitro displacement reaction was fast and the grafting yield was quantitative (100%). The graft copolymers are highly soluble in water and in organic solvents. The intrinsic viscosities of the graft copolymers are higher than those of the backbone polymers. The intrinsic viscosities show an initial increase followed by a decrease as the degree of grafting increase.     

Grafting polymerization of styrene onto alternating terpolymers based on chlorotrifluoroethylene,hexafluoropropylene, and vinyl ethers,and their modification into ionomers bearing ammonium side‐groups

The grafting polymerization of styrene initiated by the alkyl chloride groups of poly(CTFE‐alt‐VE) and poly[(CTFE‐alt‐VE)‐co‐(HFP‐alt‐VE] copolymers (where CTFE, HFP, and VE stand for chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), and vinyl ether (VE), respectively) followed by the chemical modification of the polystyrene grafts are presented. First, the fluorinated alternating copolymers were produced by radical copolymerization of CTFE (with HFP) and VE. Second, atom transfer radical polymerization enabled the grafting polymerization of styrene in the presence of the poly(CTFE‐alt‐VE)‐macroinitiator using the alkyl chloride group of CTFE as the initiation site. Kinetics of the styrene polymerization indicated that such a grafting had a certain controlled character. For the first time, grafting of polystyrene onto alternating fluorinated copolymers has been achieved. Differential scanning calorimetry thermograms of these graft copolymers exhibited two glass transition temperatures assigned to both amorphous domains of the polymeric fluorobackbone (ranging from ?20 to +56 °C) and the polystyrene grafts (ca. 95 °C). The thermostability of these copolymers increased on grafting. Thermal degradation temperatures at 5% weight loss were ranging from 193 to 305 °C when the polystyrene content varied from 81 to 27%. Third, chloromethylation of the polystyrene grafts followed by the cationization of the chloromethyl dangling groups led to original ammonium‐containing graft copolymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Elaboration of corona functionalized regular unimolecular hyperbranched polystyrene nanoparticles

Functional arborescent graft polystyrenes prepared by the “graft-on-graft” technique, involving the iterative grafting of end functional polymer chains onto reactive polymer backbones were synthesized. The zero-generation comb polymers and then the first generation hyperbranched structures were obtained by the coupling reaction of living α-acetal polystyryllithium onto linear or comb chains of poly(chloroethyl vinyl ether) (PCEVE) of controlled D̄P̄_n and structure. Both the PS grafts and the PCEVE reactive backbones were synthesized individually by living polymerization techniques. Initiation of styrene polymerization from acetal functionalized lithium derivatives yield the ω-functionalization of all external polystyrene branches. Derivatization of these acetal branch termini allowed the generation of aldehyde, hydroxyl and carboxyl groups as well as the introduction of functional organic molecules at the periphery of the nanoparticles.     

Design of new polymer architectures by combination of anionic and cationic living polymerization techniques

The grafting of polystyryl lithium onto poly(chloroethyl vinyl ether) chains has been investigated. The reaction proceeds cleanly and quantitatively thus allowing the synthesis of comblike polymers. Since the dimensions of the polystyrene branches and of the poly(chloroethyl vinyl ether) backbone can be controlled by living polymerizations, both the length and the number of branches of the graft copolymers can be tuned. The latter behave as star polymers. The possibility to initiate a new cationic polymerization of chloroethyl vinyl ether from polystyrene branches bearing acetal termini in order to prepare the corresponding stars with poly(chloroethyl vinyl ether-b- styrene) branches is also examined. Finally access to hyperbranched polymers of controlled architecture and dimensions by deactivation of a second amount of polystyryl lithium onto the last blocks of poly(chloroethyl vinyl ether) is also reported.     

Abiotic and biotic degradation of oxo-biodegradable polyethylenes

Conventional polymeric materials accumulate in the environment due to their low biodegradability. However, an increase in the biodegradation rate of these polymers may be obtained with the addition of pro-degrading substances. This study aimed to evaluate abiotic and biotic degradation of polyethylenes (PEs) using plastic bags of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) formulated with pro-oxidant additives as test materials. These packaging materials were exposed to natural weathering and periodically analyzed with respect to changes in mechanical and structural properties. After a year of exposure, residue samples of the bags were incubated in substrates (compost of urban solid waste, perlite and soil) at 58 °C and at 50% humidity. The biodegradation of the materials was estimated by their mineralization to CO₂. The molar mass of the pro-oxidant-activated PE decreased and oxygen incorporation into the chains increased significantly during natural weathering. These samples showed a mineralization level of 12.4% after three months of incubation with compost. Higher extents of mineralization were obtained for saturated humidity than for natural humidity. The growth of fungi of the genera Aspergillus and Penicillium was observed on PE films containing pro-oxidant additives exposed to natural weathering for one year or longer. Conventional PE films exposed to natural weathering showed small biodegradation.     

Comparison of extraction methods for sampling of low molecular compounds in polymers degraded during recycling

The demand for mechanical recycling of plastic waste results in an increasing amount of recycled polymeric materials available for development of new products. In order for recycled materials to find their way into the material market, high quality is demanded. Thereby, a complete and closed loop of polymeric materials can be achieved successfully. The concept of high quality for recycled plastics imply that besides a pure fraction of e.g. polyethylene (PE) or polypropylene (PP), containing only minor trace amount of foreign plastics, knowledge is required about the type and amount of low molecular weight (LMW) compounds. During long-term use (service-life), products made of polymeric materials will undergo an often very slow degradation where a series of degradation products are formed, in parallel, additives incorporated in the matrix may also degrade. These compounds migrate at various rates to the surrounding environment. The release rate of LMW products from plastics depends on the initiation time of degradation and the degradation mechanisms. For polymers the formation of degradation products may be initiated already during processing, and subsequent use will add products coming from the surrounding environment, e.g. fragrance and aroma compounds from packaging. During recycling of plastics, emissions which contain a series of different LMW compounds may reach the environment leading to unwanted exposure to additives and their degradation residues as well as degradation products of polymers.Several extraction techniques are available for sampling of LMW compounds in polymers before chromatographic analysis. This paper reviews and compares polymer dissolution, accelerated solvent extraction (ASE), microwave assisted extraction (MAE), ultrasound assisted extraction (UAE), super critical fluid extraction (SFE), soxhlet extraction, head-space extraction (HS), head-space solid phase micro extraction (HS-SPME), and head-space stir bar sorptive extraction (HSSE) as appropriate sampling methods for LMW compounds in recycled polymers. Appropriate internal standards useful for these kinds of matrices were selected, which improved the possibility for later quantification. Based on the review of extraction methods, the most promising techniques were tested with industrially recycled samples of HDPE and PP and virgin HDPE and PP for method comparison.     

GRAFTING OF POLYSTYRENE ONTO A NANOMETER SILICA SURFACE BY MICROEMULSION POLYMERIZATION

The grafting of polystyrene onto a nanometer silica surface by microemulsion polymerization is described. Silicawas functionalized with 3-methacryloxypropyltrimethoxysilane coupling agent before polymerization. A mixture of ionic andnon-ionic surfactants as well as water-soluble and oil-soluble initiators were used. The effect of the amount of silica and ionicsurfactant on the graft polymerization was studied. The graft polymerization procedure for styrene was also applied to methylmethacrylate. Composite particles with a core-shell structure were obtained and the yield and grafting efficiency of monomerwere high.     

Emulsion procedures for thermally reversible covalent crosslinking of polymers

Quaternization and dequaternization of tertiary amine compounds were employed to obtain thermally reversible ionene networks from aqueous colloidal polymer dispersions prepared via emulsion polymerization. Chlorine‐functionalized polymers prepared via the emulsion copolymerization of styrene (St), butylacrylate (BA), or both with chloromethylstyrene, and amino‐functionalized polymers prepared via the emulsion copolymerization of St, BA, or both with 2‐(dimethylamino)ethylacrylate or 4‐vinylpyridine, were reacted without polymer separation, with a ditertiaryamine crosslinker and a dihalide crosslinker, respectively, to obtain crosslinked polymers. Crosslinked polymers were also obtained via the reaction of a chlorine‐functionalized polymer dispersion with an amino‐functionalized polymer dispersion or via the drying of the polymer blend prepared from the two kinds of dispersions. Reactive solubility experiments, flowability investigations (by thermocompression at ca. 215 °C), IR, and ¹H NMR analyses of the obtained crosslinked polymers indicated that the generated ionene bridges dequaternized on heating and requaternized on cooling. In comparison with solution crosslinking, no organic solvent was employed, and simple procedures were required for the preparation of the thermally reversible covalent crosslinked polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4373–4384, 2000     

Antistatic thermoplastic blend of polyaniline and polystyrene prepared in a double-screw extruder

One of the major aims of research on intrinsically conducting polymers (ICP) is the production of blends combining the processing properties of thermoplastic polymers with the conductivity of conducting polymers. The main problem in applying ICP on a large scale in the plastic industry is the impossibility of plasticizing these polymers under heat and shear. However, the use of functionalized acids improves the thermal stability and processability of conductive polymers. In this work the doping process was carried out during processing, also denoted as “reactive processing”. This procedure reduces the number of steps to obtain the final product, PS/SBS/PAni. Blending of polystyrene with dodecylbenzenosulfonic acid doped polyaniline was carried in a double-screw extruder using the block copolymer of styrene and butadiene, SBS, as compatibilizer. A conductive thermoplastic (σ = 10⁻⁶-10⁻² S cm⁻¹) was obtained in the form of ribbons, which were used to evaluate the thermal, mechanical, morphological and electrical properties. We used SBS as compatibilizer and different formulations were tested according to a statistical response surface method. The mechanical and electrical properties of these thermoplastic blends are adequate for antistatic applications.     

Side‐chain terpyridine polymers through atom transfer radical polymerization and their ruthenium complexes

Polymers containing side‐chain terpyridine ligands of well‐defined architectures and controllable molecular weights and molecular weight distributions are reported. These polymers were synthesized by the atom transfer radical polymerization (ATRP) of a newly synthesized terpyridine monomer with three functional initiators. The obtained polymers were characterized with ¹H NMR and gel permeation chromatography techniques. The efficiency of the ATRP technique and the overall control of the molecular characteristics of the polymers were demonstrated by a kinetic study of the polymerization reaction. Subsequently, the ruthenium(III)/ruthenium(II) complexation chemistry was employed for the attachment of bis(dodecyloxy)‐functionalized terpyridine moieties onto each side 2,2′:6′,2″‐terpyridine unit of the main polymeric backbone. Thus, the grafting approach was successfully combined with the metal–ligand coordination chemistry for the preparation of highly soluble polymeric complexes. The resulting complexes were fully characterized by means of ¹H NMR, gel permeation chromatography, and ultraviolet–visible spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4838–4848, 2005     

Graft polymerization of methyl methacrylate: New macroinitiators containing β-diketonate moieties

Polymers containing side β-diketonate moieties have been synthesized through the polymerization of styrene initiated by cobalt(II) 5-methyl-5-hexene-2,4-dionate. The polymers thus prepared have been used as macroinitiators for grafting of methyl methacrylate onto a polystyrene matrix. The grafting density and the mean size of grafted chains have been estimated. It has been shown that the graft polymerization may be governed via variation of the concentration of the macroinitiator and the content of active centers in it.     

Micro-Raman investigations of PVDF-based proton-conducting membranes

Proton-conducting membranes of poly(vinylidene fluoride), PVDF, grafted with styrene and thereafter sulfonated, were investigated by Raman spectroscopy. The depth and surface distribution of polystyrene grafts and crosslinker, as well as the sulfonation efficiency, were determined by using confocal micro-Raman spectroscopy. Highly grafted samples show homogeneous distribution of grafted material and homogeneous sulfonation. Depth profiles reveal uneven graft distribution for samples with low and intermediate degrees of grafting. In crosslinked samples, the crosslinker concentration in the interior of the film is found to be roughly 50% of the surface concentration. In contrast to what has previously been reported, the sulfonation efficiency is poor at low graft levels and is further inhibited by the presence of divinyl benzene, DVB, as crosslinker. At degrees of grafting above ∼ 60%, the crosslinker does not affect the sulfonation efficiency and a level of 70–90% sulfonation is reached. The matrix polymer structure is found to be largely retained after irradiation, grafting, and sulfonation. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3317–3327, 1999     

Chemical structure of poly(ethylene terephthalate)–styrene and nylon–styrene graft copolymers prepared by radiation techniques

The graft copolymerizations of styrene onto poly(ethylene terephthalate) (PET) and nylon fibers were carried out by the mutual irradiation and preirradiation methods. True graft copolymers were isolated from the products by extraction and characterized by hydrolysis and osmometry. Among the swelling agents employed, methanol was most effective for increasing the extent of grafting onto PET. In both methods of the grafting, the molecular weight of polystyrene formed in the substrate matrix was higher than one million if no chain-transfer agent was added to the monomer solution. Similar to the case of radiation grafting onto poly(vinyl alcohol) and cellulose, the isolated graft copolymer carried only one branch per copolymer molecule in both cases. Of great interest is the particularly low extent of grafting in the case of PET–styrene. This should be attributed to the low sensitivity of PET to radiation. The grafting site on the mother polymer molecule is discussed on the basis of the solution behavior of the branch polymers separated from the backbone.     

Immobilization of β-galactosidase on Polystyrene Microspheres Attached L-Alanine

This study investigated the properties of immobilized β-galactosidase on polymeric beads having Schiff base. Polystyrene microspheres attached L-Alanine (FMPS-Ala) was synthesized from (4-formyl-3-methoxyphenoxymethyl)polystyrene (FMPS) and L-alanine by condensation. A coordinasyon polymer involving Ni²⁺(FMPS-Ala-Ni) was produced with the template method and characterized. β-galactosidase was immobilized onto the (FMPS), (FMPS-Ala) and (FMPS-Ala-Ni) complexes via covalent bonds. The Km/Vmax values were calculated as 0.343 mM/0.0259 mM min^?1for free β-galactosidase and 0.104 mM/0.0126 mM min^?1, 0.0617 mM/0.0417 mM min^?1and 0.210 mM/0.0287 mM min^?1for β-galactosidase immobilized to the (FMPS), (FMPS-Ala) and (FMPS-Ala-Ni) supports, respectively. The storage stability of (FMPS-Ala-Ni) was determined to be higher than that of the (FMPS) and (FMPS-Ala) polymers.