Feedstock recycling of waste polymeric material

The aim of liquefaction is to yield gas and oil as a source of fuels and chemicals and waxes in the form of chemical compounds. Gasification, on the other hand, will be discussed in a separate section, since the aim of gasification is to yield gases such as hydrogen and carbon monoxide as a source for the production of chemicals or fuels, even though the processes involved are quite similar to those used in pyrolysis. The techniques employed in the production of activated carbon are very similar to those used in gasification. Steam, oxygen, and carbon dioxide are used for the reactive activation of char. Therefore, this process will be discussed together with gasification.

Furthermore, liquefaction can be conducted in the presence or absence of catalysts. While thermal decomposition is characterized by the radical fission of the polymer backbone, in the presence of a catalyst, charges are frequently involved that reduce the reaction temperature and alter the product distribution. Therefore, both processes are dealt with in separate sections.

The aim of gasification is to produce high calorific gases by the partial oxidation of organic feedstocks and coal by steam, oxygen, or carbon dioxide. The conventional feedstocks are fossil fuels. However, because of the need to reduce carbon dioxide emissions, waste plastic materials and biomass are considered appropriate replacement materials. The product gas consists mainly of hydrogen and carbon monoxide, in which hydrogen is of special interest because of the developing hydrogen energy cycle. Activated carbon and nanotubes are also accessible by the gasification processes.

Tsuji et al. [55, 56] proposed a two-step process for the gasification of polyolefins. In the first stage, polyolefins are pyrolyzed at 470°C in a melting vessel. The oil yield derived from the first stage reached about 90 wt%, while little coke was produced. In the second stage, the oil was gasified in a steam atmosphere using a tubular reactor at about 800°C in the presence of a Ni–Al₂O₃ as the gasification catalyst. Although the plastic-derived oil contained large amounts of high molecular aliphatic and aromatic hydrocarbons, the coking rate was low, and the hydrogen content of the synthesis gas was about 70 vol%. Similar results were obtained during the steam reforming of waste plastics when a fluidized bed reactor was used instead of a tubular reactor [57]. Tongamp et al. [58] obtained hydrogen gas by ball milling PE with Ca(OH)₂ and Ni(OH)₂, and subsequently heating the sample at a heating rate of 20 K min⁻¹ to a maximum temperature of 700°C. During this process, hydrogen was the main gas released. The highest purity of hydrogen gas was observed at a C/Ca/Ni ratio of 6:14:1, with a gas containing 95 vol% hydrogen and 5 vol% methane evolving between 400 and 500°C. Slapak et al. [59] calculated the cost of using a fluidized bed gasification plant to convert 50 kton PVC waste/year into synthesis gas and HCl. The aim was to use the synthesis gas for energy recovery. Half of the chlorine is recovered as pure HCl gas; the other half is CaCl₂ and has to be disposed of. The investment costs were calculated to be 10 million euros, resulting in an expected gate fee of 67 euros with a strong dependence on the CaCl₂ disposal costs. Yamamoto et al. [60] proposed a process combining the production of high calorific gases from MSW with the smelting of iron ore. The gas consisted mainly of carbon oxides and hydrogen. Dioxin emissions remained low, while volatile heavy metals were removed with the gas stream, assuring low concentrations of heavy metals in the slag. Kawamoto and Mabuchi [61] investigated the formation of polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) in the fly ash from a gasification process of MSW. PCDDs and PCDFs were formed at the surface of fly ash even in the absence of precursors and at a low carbon content of the fly ash. The highest concentrations were observed in a narrow temperature range at around 350°C.

Kamo et al. [62] investigated the gasification of dehydrochlorinated PVC and activated carbon in the presence of various hydroxides and carbonates using a batch reactor. The reaction was carried out under relatively mild conditions at temperatures between 560 and 660°C and a nitrogen pressure of 3.0 MPa. High yields of hydrogen were obtained with KOH and NaOH as reactants. The steam gasification of activated carbon in the presence of Li₂CO₃, Na₂CO₃, and K₂CO₃, respectively, resulted at 700°C in the formation of hydrogen and carbon dioxide [63]. The presence of carbonate and an increasing partial pressure of steam accelerated the hydrogen production significantly.

Yasuda et al. [64] gasified a mixture of coal and PE at 800°C in a hydrogen atmosphere at a pressure of 7.1 MPa. The gasification of sole PE yielded 90% methane after 80 s. The mixing of coal and PE enhanced the methane formation, probably because of the compensation of the endothermic heat consumption of the coal hydrogenation by the exothermic heat release of the methane formation from PE. The addition of PE to coal in hydrogasification processes was suggested. Due to the low quality of Korean anthracite coal, Choi et al. [65] proposed the use of a mixture of anthracite and high calorific waste plastics in an oxygen blown steam reforming process. Van Kasteren [66] carried out steam reforming of a mixture of biomass and PE for the production of synthesis gas (carbon monoxide and hydrogen). The aim was the direct fermentation of the product gas in order to obtain ethanol. The highest efficiency was 42% for carbon monoxide and hydrogen, and it was achieved at 900°C with a feed containing a low PE content of 11 wt% (Fig. 4). Mun et al. [67] gasified woody biomass with air in order to obtain a high calorific product gas. At 800°C, the product gas reached a lower heating value of 10 MJ Nm [3] because of its high hydrogen and methane content. The presence of activated carbon in the reactor reduced the tar content of the gas to one sixth.

Uemura et al. [68] proposed a process for the decomposition of waste plastics into hydrogen and carbon. In the first step, plastic waste materials are pyrolyzed, giving mainly hydrocarbons. In the second step, the pyrolysis gas is decomposed into hydrogen and carbon using a nickel catalyst. The second step was investigated by degrading propene using various alumina-supported nickel catalysts in a spouted bed reactor at 770°C. Song et al. [69] obtained multiwalled carbon nanotubes from the reaction of PP over an organically modified montmorillonite and Ni₂O₃ at 700°C. The carbonization of PP was supported by the formation of Brønsted acid sites in the montmorillonite layers during the decomposition of the organic modifiers. The formation of carbenium ions resulted in the reduction of the PP molecular weight, and Ni₂O₃ catalyzed the growth of the carbon nanotubes.

The dehalogenation of waste plastic materials is of special importance because of the damage halogenated compounds can cause to recycling facilities, petrochemical plants, and combustion engines because of the corrosive effect of HCl. Moreover, the emission of halogenated compounds is a threat to the environment and human health. Polychlorinated dibenzodioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs) are listed as persistent organic pollutants (POPs). They are barely metabolized by organisms and remain in the environment for a long time, where they accumulate in fatty tissue and cause cancer if ingested. In order to prevent the release of these hazardous chlorinated compounds, a technique for the controlled degradation of halogen containing materials is essential.

The source of halogens in waste plastics can be divided into two main groups. Chlorinated polymers, such as PVC, polyvinylidene dichloride (PVDC), and polychloroprene, are the main sources of chlorine. Polybrominated compounds, such as tetrabromobisphenol-A (TBBPA), decabromo diphenylether (DPE), and decabromo diphenylethane (DDE), are still frequently used as flame retardants, even though halogen-free flame retardants are available. Since some electric appliances are used for a long time, the brominated flame retardants being processed now will still be present in the waste stream decades from now.

Over the years, three main strategies have been developed for the removal of halogens from waste plastics—thermal dehalogenation, the mechanochemical removal of halogens by a suitable absorbent, and wet dehalogenation in an appropriate basic solvent.

Fukushima et al. [70] carried out the dehydrochlorination of MSW using a single screw extruder. At a temperature of about 360°C, the chlorine content was successfully reduced to less than 0.5 wt% in 2 min. This was comparable to the results obtained from a twin screw extruder. Kakuta et al. [71] found that plasticizers and fillers reduce the dehydrochlorination yield and prolong the reaction time at 230 and 260°C. Furthermore, they found that activated carbon with a surface area of about 1,700 m² g⁻¹ can be obtained from the carbonaceous residue after a preoxidation step at 300°C and subsequent activation with KOH at 800°C. Uddin et al. [72] degraded mixed plastics at 360 and 380°C in the presence of an alumina–silica catalyst and iron oxides as a chlorine absorber. The catalyst reduced the reaction time, and the iron oxide reduced the chlorine content of the oil. As a result, it was assumed that iron chloride was formed. Masuda et al. [73] investigated the influence of various metal oxides on the dehydrochlorination behavior of PVC at 400 and 800°C. ZnO, CaO, PbO, and La₂O₃ reduced the formation of chlorobenzene, while Fe₂O₃ and CeO₂ enhanced its formation. ZnO promoted the formation of carbonaceous residue, while the residue was reduced in the presence of CaO. Bhaskar et al. [74, 75] developed a chlorine sorbent consisting of 90 wt% CaCO₃ and 10 wt% phenol resin. This sorbent was successfully used in the dehalogenation of mixed plastics consisting of polyolefins, high-impact PS (HIPS) containing a brominated flame retardant, and PVC at 430°C. In the presence of the sorbent, halogen-free oil was obtained. However, the efficiency of the sorbent was reduced in the presence of PET, resulting in a halogen concentration of about 330 ppm in the oil. While the sorbent reduced the oil yield from the polyolefin mixed plastics, the presence of PET increased the oil yield. By employing the sorbent in a pilot plant using a batch reactor, 50 kg of MSW was successfully dehydrochlorinated, and an oil with a chlorine content of 100 ppm was obtained [76]. German et al. [77] investigated the interaction between PVC and polycarbonate and PET, respectively. It was found that various chlorinated compounds were formed. Small amounts of chlorobenzene and other chloroaryl and chloroalkylaryl compounds were obtained from polycarbonate. The degradation of PET resulted in the formation of significant amounts of chloroethylesters of benzoic and terephthalic acid (Fig. 5).

Park et al. [10] used the product gas from the dehydrochlorination of PVC at 250°C for the recovery of indium from liquid crystal display (LCD) powder containing In₂O₃. In₂O₃ formed in the presence of HCl volatile InCl₃. In this way, 67% of the indium was recovered at a chlorination temperature of 350°C.

Saeki et al. [78] ground PVC with a mixture of CaO, SiO₂, and Al₂O₃ to mechanochemically dehydrochlorinate it in a planetary mill. The composition was similar to that of slag and more efficient than CaO alone or CaCO₃. CaO acted as the dehydrochlorination agent, while SiO₂ and Al₂O₃ were grinding aids. Complete dehydrochlorination was achieved after 4 h of mechanochemical treatment. Tongamp et al. [79] used calcium sulfate as a dehydrochlorination agent during the mechanochemical treatment of PVC with a planetary mill. The dehydrochlorination ratio was 95% after 4 h when CaSO₄ was used, while only a dehydrochlorination ratio of 50% was observed in the presence of CaSO₄·2H₂O after 8 h of milling. Hydrogen sulfide was released during the treatment. The mill power consumption was calculated by the discrete element method (DEM) [9, 80]. It was found that the use of oyster shells (CaCO₃) was slightly more efficient than the use of CaO. The complete dehydrochlorination of PVC was able to be achieved within 2 or 3 h. After removing copper from automotive shredder residue (ASR), Endoh et al. [81] milled the PVC containing ASR in the presence of CaO and CaCO₃. After 8 h milling, the chlorine content of the material was reduced to just one tenth of its original value.

Shin et al. [82, 83] dehydrochlorinated flexible and rigid PCV using aqueous NaOH solutions at temperatures between 150 and 250°C. Complete dehydrochlorination was achieved at 250°C for flexible and rigid PVC after 3 and 5 h, respectively. The highest dehydrochlorination rate was observed in 5 and 3 M NaOH for flexible and rigid PVC, respectively. The treatment of the rigid PVC left a residue with small pores of 2 μm in size. After treatment, the flexible PVC pores were larger (16 μm) because of the high content of plasticizer that was dissolved. In both cases, the reaction proceeded by a first order kinetics. Since the disadvantage of using aqueous solutions for the dehydrochlorination of PVC is that high pressure is required to reach the necessary reaction temperature, Yoshioka et al. [84] proposed ethylene glycol to be used instead of water. Its high boiling point of 196°C made it possible to carry out the reaction at atmospheric pressure. The reaction was faster than in aqueous solution and reached a dehydrochlorination yield of 98% at 190°C. Elimination occurred during the reaction as well as the substitution of chlorine by hydroxy groups. Wu et al. [85] carried out the dehydrochlorination in polyethylene glycol (PEG) in the absence of a basic medium. From TG measurements, it was observed that the dehydrochlorination step of PVC occurred at lower temperatures in the presence of PEG. Dehydrochlorination in a batch reactor resulted in a dehydrochlorination yield of 74% after 1 h at a temperature of 210°C. Osada and Yana [2] used an aqueous NaOH solution under microwave radiation for the dehydrochlorination of flexible PVC. Microwave heating has the advantage of a direct heat transfer into the reaction medium (Fig. 6). Molecular vibrations are directly activated, resulting in a reduced reaction time. Using microwave heating, the plasticizer was removed after 30 min at 150°C, and complete dehydrochlorination was achieved at a temperature of 235°C for the same time using an 8 M NaOH solution. When ethylene glycol was used instead of water [3], the reaction temperature was reduced to 160°C, and the reaction time was cut to 10 min at an NaOH concentration of 1 M. A method based on the dehydrochlorination of PVC in ethylene glycol was proposed by Kameda et al. [86] for the partial substitution of chlorine. The highest substitution yield was observed for the azido group at 66%. Besides substitution, elimination was also observed. The substitution to elimination ratio followed the order hydroxy > azido = thiocyanato > phthalimido > iodo group. Yoshihara et al. [87] found that the reaction of PVC with Na₂S resulted in the elimination of 32% of the chloride and the substitution of 26% of it. The bivalent S²⁻ formed exclusively cross-linked sulfur bridges, which were mainly located close to the surface of the PVC particles, preventing the further penetration of the particles by the solvent.

Peng et al. [88] investigated the pyrolysis of TV housing plastics containing brominated flame retardants. It was found that the flame retardant decomposed between 280 and 350°C, while the degradation of HIPS took place between 350 and 450°C. The removal of 90% of flame retardant-related products was possible at temperatures just above 280°C using a vacuum below 5 kPa. At temperatures higher than 300°C, the degradation of the HIPS matrix has to be expected. The thermal degradation of flame-retarded HIPS was investigated by Jakab et al. [89]. The flame retardants DPE and DDE did not change the thermal stability of the polymer, while the synergist Sb₂O₃ reduced thermal stability by about 50 K. Various brominated diphenylethers and dibenzofurans were observed as degradation products. Bhaskar et al. [90] investigated the effect of the pyrolysis of HIPS containing DDE as a brominated flame retardant on the degradation of a plastic mixture consisting of PP, PE, and PS. The reaction was carried out with two heating steps at 330 and 430°C. Brominated compounds were mainly observed at 330°C. The presence of the synergist Sb₂O₃ led to a significant alteration of the product distribution. The copyrolysis of PET and flame-retarded HIPS resulted in a rise in brominated organic products. Even a small amount of PET increased the release of brominated compounds by several times. Mitan et al. [91] debrominated sole flame-retarded HIPS and acrylonitrile–butadiene–styrene copolymer and a blend of both polymers using various catalytic systems at 430°C. Considering the high oil and gas yield as well as low bromine content of the oil, the best results were obtained when mesoporous silica (FSM) was in contact with the liquid phase and a CaOH-composite (CaHC) was in contact with the gas phase. Brebu and Sakata [92] used ammonia for the debromination of flame-retarded HIPS at 450°C. Inorganic bromide was fixed as NH₄Br and recovered as a solid from the oil. Ammonia was not able to improve the removal of the organic bromine left in the oil. Luda et al. [93] found that the thermal degradation of TBBPA mainly took place in a temperature range between 270 and 340°C. HBr and brominated phenols and bisphenols were observed as the products, while large amounts of carbonaceous residue remained due to condensation reactions. Dibenzodioxin structures were found in the residue. However, they were not released. Futamura and Zhang [94] decomposed flame-retarded computer casings in tetralin between 380 and 400°C. Activated carbon acted as a catalyst, transferring hydrogen from tetralin to the plastic. No brominated organic compounds, such as polybrominated dibenzodioxins, were observed. The product oil mainly consisted of aromatic compounds.

Many polymeric materials can be treated with solvent systems, which fulfill several tasks. The solvent provides better heat and mass transport properties compared with thermal degradation by itself. Solvents might also function as reactants during hydrolysis, methanolysis, and glycolysis. Moreover, at higher temperatures water exhibits excellent solvation properties, making it possible to dissolve materials that have low solubility at lower temperatures.

The general term for processes in which the solvent also acts as a reactant is solvolysis. Notably, plastics containing reactive groups can be depolymerized by solvolytic processes. Polycarbonate and PET possess ester bonds that undergo hydrolysis and transesterification in the presence of water and alcohols, respectively. Other polymers decompose in inert solvents at high temperatures by thermal degradation without the fission of reactive groups.

The advantage of solvolysis is the possibility to recover monomers at a relatively low temperature, thus allowing them to be chemically modified in some cases, depending on the solvent used. However, fillers and additives might have a negative impact on the process, making it difficult to use the solvent for a long time without regeneration.

The classical field for solvolysis is the depolymerization of PET. Methanolysis was carried out by Goto et al. [95]. The reaction was carried out in a batch reactor with supercritical methanol at a temperature of 300°C and a pressure of 20 MPa. The molecular weight of the PET decreased with time and reached 1,000 Da after 10 min. The highest yield of dimethylterephthalate (DMT) (80%) was obtained after 2 h. Genta et al. [96] proposed a kinetic model in which PET undergoes at first a random scission in the heterogeneous phase, which results in the formation of oligomers. After that, the oligomers react in the homogeneous phase with methanol from the chain ends, forming DMT. Lopez-Fonseca et al. [97] carried out the glycolysis of PET with an excess of ethylene glycol at 196°C using various catalysts. After 8 h, bis(2-hydroxyethyl) terephthalate with a yield of about 70% was obtained in the presence of zinc acetate and sodium carbonate. The use of sodium carbonate was suggested as an eco-friendly catalyst for the glycolysis of PET. Oku et al. [98] used ethylene glycol as a solvent for the alkaline hydrolysis with NaOH at temperatures between 150 and 180°C. PET was quantitatively converted into disodium terephthalate after 80 and 15 min at 150 and 180°C, respectively. The reaction was significantly enhanced by the addition of ethers, such as dioxan, tetrahydrofuran, and dimethoxyethan. Spychaj et al. [99] depolymerized PET in the presence of various polyamins between 200 and 210°C. The resulting polyamine can be used as a component in the production of reactive resins, i.e., epoxy resins. The use of triethanolamine for the depolymerization of PET resulted in a polyol that can be used for the production of polyurethanes (Fig. 7). Mohd-Adnan et al. [100] investigated the hydrolysis of poly(l-lactic acid) in pressurized steam between 100 and 130°C in an autoclave. It was found that the initial homogeneous random degradation process changed at 130 ºC after about 45 min and molecular weight average of about 5,000 g mol⁻¹ to a heterogeneous degradation process. The activation energy increased with the weight loss.

Fiber-reinforced plastics (FRPs) consist mainly of glass or carbon fibers embedded in a matrix of unsaturated polyesters cross-linked by styrene. Iwaya et al. [101] depolymerized FRP in subcritical benzyl alcohol and diethyleneglycol monomethylether between 190 and 350°C in the presence of K₃PO₄ as a catalyst. Benzaldehyde and benzoic acid were identified as polyester-related products, styrene derivatives as products from PS. The fibers were successfully recovered for reuse. Kamimura et al. [102‐105] used supercritical methanol for the degradation of FRP between between 250 and 275°C. N,N-dimethylaminopyridine was found to be an efficient catalyst. The main products of the methanolysis were dimethylterephthalate and propylene glycol. The products obtained from the depolymerization were used for the synthesis of new FRP, which exhibited similar properties to commercial FRP. The product quality was improved by the reduction of the amount of catalyst and the purification of the methanolysis products by activated carbon. The preparation of FRP from the recovered monomers using calcium acetate and titanium tetrabutyrate as catalysts provided a product with mechanical properties comparable to FRP obtained from new monomers.

Hu et al. [106] used various unpolar solvents as additives for the methanolysis of polycarbonate. While the reaction of polycarbonate in methanol and in the presence of a catalytic amount of NaOH yielded only 7% BPA at 60°C after 330 min, the same reaction was completed after 70 min with the addition of toluene. The reaction also yielded 100% of the dimethylcarbonate. A similar reaction with glycerol and KOH as a catalyst in dry dioxane at 100°C resulted in high yields of 4-(hydroxymethyl)-1,3-dioxolan-2-one and BPA after 25 min [107]. The reaction of glucose with polycarbonate in pyridine resulted in glucose dicarbonate with a yield of 46%. Kim et al. [108] carried out the depolymerization of polycarbonate in ethylene glycol at 220°C for 85 min. High yields of BPA and ethylene glycol carbonate were obtained with an ethylene glycol/polycarbonate ration of 4. It was found that the glycolysis of polycarbonate proceeded much in the same way as the methanolysis of PET [96]. Margon et al. [109, 110] investigated the equilibrium conditions of the polycarbonate decomposition products diphenylcarbonate (DPC) and BPA after the degradation of polycarbonate in phenol, as well as the extraction of these products in supercritical carbon dioxide. The Peng–Robinson equation of state (PR EOS) binary interaction parameters of the systems DPC–carbon dioxide, BPA–carbon dioxide, phenol–DPC, phenol–PBA, and DPC–BPA were used to develop a model of the tertiary system phenol–DPC–BPA–carbon dioxide. This model allowed the extraction behavior of the different compounds in this system to be predicted. Sato et al. [111] carried out the thermal degradation of polycarbonate in tetralin, decalin, and cyclohexanone under pressure. The thermal degradation resulted at 440°C in high yields of phenol and isopropenylphenol. Between 300 and 350°C, high yields of BPA were recovered. The addition of CaCO₃ was necessary in order to maintain the reaction.

Molero et al. [112, 113] obtained mainly oligomeric polyols from the glycolysis of polyurethane at 189°C. The reaction was carried out in “split phase,” involving two phases that are formed during the reaction. The produced polyols remain separated in the upper layer of the reaction mixture. Potassium octoate was found to behave as a catalyst with high activity, leading to the complete dopolymerization of polyurethane within 2 h. This process also showed a high selectivity for glycolysis and resulted in little hydrolysis. Troev et al. [114] used triethyl phosphate for the depolymerization of polyurethane at 190°C. The resulting oligomers contained about 8 wt% phosphor.

Tagaya et al. [115] used several model compounds in order to investigate the mechanism of the degradation of phenol resins in sub- and supercritical water. The addition of tetralin had a positive effect on the degradation of bis(o-hydroxyphenyl)-methane. Alkali salts acted as catalysts. Water acted not only as the reaction medium, but also as a reactant, causing the oxidation of methylene groups. Shibasaki et al. [116] degraded aromatic ethers in sub- and supercritical water between 250 and 430°C. Na₂CO₃ was present as a catalyst. The main products obtained were phenolic compounds from aromatic ethers and benzyl aldehyde and benzyl alcohol from dibenzylether. The water content and temperature had a significant influence on the reaction.

Moriya and Enomoto [117] investigated the degradation of PE in supercritical water. The reaction proceeded more slowly in the initial stage than during thermal degradation. However, less char was produced during the hydrothermal treatment. The products obtained were mainly alcohols and ketones. Aguado et al. [118] degraded HDPE in decalin at 400°C and 2 MPa in nitrogen atmosphere. Due to an improved heat and mass transfer, more gas and oil and less solid were obtained compared with the reaction without decalin. Decalin enhanced the production of α-olefins and n-parafins.

Chen et al. [119] conducted the depolymerization of nylon-6 in subcritical water between 280 and 330°C. The use of phosphotungsten heteropoly acid as a catalyst resulted in high yields of ε-caprolactam with 6-aminocaproic acid and oligomers as minor products. The highest yield of 78% of ε-caprolactam was achieved at 300°C after a reaction time of 85 min.

Du et al. [120] used a system of PEG and NaOH for the denitrogenation of acrylonitrile–butadiene–styrene (ABS), dissolved in tetrahydrofurane, dioxane, or toluene. After 2 h of reaction time at 160°C, 93% of the initial nitrogen was removed from the ABS material when tetrahydrofuran was used as a solvent.

Wahyudiono et al. [121] decomposed lignin in supercritical water between 380 and 400°C. Catechol, phenol, and o-cresol were the main products of the guaiacol degradation with catechol reaching more than 40 wt%. Long reaction times caused the production of char. Increasing water density accelerated the product formation. Kamo et al. [122] obtained tar from the reaction of wood from Japanese cedar with benzyl alcohol and NaOH or cresol and sulfuric acid at temperatures between 250 and 350°C. Various plastic materials were solubilized in the resulting tar. Epoxy resin from printed circuit boards was almost completely solubilized at temperatures between 250 and 300°C. Tanaka et al. [123] extracted dietary fibers from the juice processing residue of Citrus junos. This material, high in pectin, hemicelluloses, and cellulose, was treated with subcritical water between 160 and 320°C. About 78% of the pectin and 80% of the cellulose were extracted at 160 and 200°C, respectively.

The majority of contributions for the ISFR were dedicated to the chemical recycling of plastics. Over the course of time, the ISFR gradually widened its field of interest to areas not directly connected with the classical process of feedstock recycling resulting in petrochemical oils and monomers for the chemical industry. Mechanical recycling, including material separation, blending, etc., was given more space.

The most obvious difference from chemical recycling is that mechanical recycling attempts to conserve the chemical structure of the polymer. The aim is to remove the additives and fillers without destroying the polymer backbone. In order to obtain a useful recyclate, it is necessary to provide a sorted polymer fraction. For this reason, various separation techniques are applied. Since the properties of recyclates differ in general from those of the initial polymer, they are often blended with virgin materials in order to meet certain specifications.

Various separation techniques were introduced during ISFR symposia. Gondal and Siddiqui [124] applied laser-induced breakdown spectroscopy (LIBS) for the identification of various polymers. Laser-produced plasma emissions allowed the identification of the major plastics HDPE, LDPE, PP, PS, PET, and PVC by their carbon/hydrogen ratio. LIBS allows these plastic to be identified in real time. Reddy et al. [8] used froth flotation for the separation of PVC and PET from other plastics. After employing surface ozonation, 90% of PVC and PET were floating at 40°C. At a mixing velocity of 180–200 rpm, the PVC settled, while PET floated at the surface. Weiß et al. [125] used high-pressure water jets for the removal of latex binders and other materials from textiles. This method was specifically designed with the treatment of waste carpets and automotive interior materials in mind. It is applicable to materials consisting of fibers embedded in a matrix with a lower critical threshold pressure than the fibers themselves.

Garcia et al. [126] used various solvents for the dissolution of extruded PS in order to reduce the volume of the material. It was found that the similar polarity of structurally similar terpenoids provided them with the best dissolution properties. Their solubility increased with the rise in temperature. However, at 60°C, a slight degradation of the PS was observed. Furgiuele et al. [127] reduced the volume of PP/PS blends by solid state shear pulverization. Below melt and glass transition temperature, a fine powder was obtained. The high shear rates were suspected of resulting in the fission of the polymer backbone, resulting in block copolymers. The successful blending of both polymers was confirmed by the reduction of the glass transition temperature of the PS-rich phase. Inagaki et al. [128] sulfonated PS from PS foam and TV cabinets in order to obtain a water-soluble polymer that could be used as a flocculant in waste water treatment. It was found that sulfonated PS (PSS) with a medium viscosity was efficient in the treatment of industrial inorganic waste waster, while highly viscous PSS was suitable for use in the dewatering of domestic waste water (Fig. 8). Garcia et al. [129] investigated the properties of blends between PVC recyclate and virgin and recycled styrenic polymers, styrene acrylonitrile (SAN) and acrylonitrile–butadiene–styrene (ABS). PVC–SAN blends showed improved resistant mechanical properties and reduced ductile properties, while PVC–ABS blends had improved ductility.

Zhang et al. [130, 131] devulcanized ground tire rubber mechanochemically by grinding with a self-designed pan-mill. The partly devulcanized rubber was blended with natural rubber in various ratios. The blends showed excellent tensile properties. Ground tire rubber was blended with LDPE using solid-state shear milling and azodicarbonamide as a chemical blowing agent. The differences in the glass transition temperature of the soft elastomer segment and the hard thermoplast segment became smaller, indicating that rubber and LDPE were more compatible after milling.

Silva Spinacé and de Paoli [132] investigated the impact of repeated processing with a single screw extruder. The mechanical properties and the crystallinity degree changed drastically during the first three cycles. The melt flow index and the number of carboxylic end groups increased, indicating mechanochemical degradation. However, no changes in the thermal stability were observed after five cycles.

Chong et al. [133] used oyster shells coated with cetyltriammonium bromide as a flame retardant for recycled PE. The flame retardancy was attributed to the decomposition of CaCO₃ at temperatures of more than 800°C. The mechanical properties of the composite material improved with the addition of the oyster shells. Recycled PE with an oyster shell content of more than 30 wt% fulfilled the classification of V-0, according to the UL 94 specification. Sridhar et al. [134] synthesized fly ash reinforced thermoplastic vulcanizates from waste tire powder. The thermal stability increased with the fly ash content. The mechanical properties were investigated as well.

Over the last decade, progress has been made in many fields of plastic recycling. The aim has always been to improve the quality of the products obtained from waste materials. The larger the effort for the treatment of waste, the higher the recovered product has to be in price in order for the effort to be considered economically feasible. The lowest effort possible is that required for improving mechanical recycling techniques. However, mechanical recycling sets high standards for the material to be recycled, making it viable only for a few applications. Downcycling is an effect often observed during the mechanical recycling of plastics. The newly gained possibility of producing food grade recycled PET is promising, however, as is the Vinyloop process for the recycling of PVC [135].

More effort is necessary for the chemical recycling of polymers. The recovery of monomers is only feasible for pure polymers, which must be gathered in large quantities or are otherwise expensive. The combination of the separated collection of PET with the bottle-to-bottle glycolysis AIES process as it is realized in Japan is a good example for future recycling systems. The liquefaction of MSW is still struggling because of the effort required to provide acceptable oil qualities. The removal of heteroatoms such as halogens, sulfur, and nitrogen makes it expensive to produce oil from waste plastics. The competitive production of synthetic crude oil is hindered by the limitation posed by the size of the processing facilities, at just one tenth the size of a typical petrochemical plant. This can be attributed to the lack of availability of waste plastics—only 6% of the crude oil production goes into the chemical sector. Therefore, research on the possibilities of processing mixtures of plastics with high boiling fractions from the crude oil rectification [24] is an important way of finding new processing strategies. It must be remembered, however, that the emission of hazardous substances from plastic waste treatment is an environmental concern that can threaten the acceptance of such processes among the population [136, 137].

For all these reasons, regulative measures have been taken in order to come to a sustainable use of resources [138]. Sustainability means that resources are used in a way that the environmental system is able to recover from our interference. Our world is ruled by the economic interests of the individual, which do not include the quality of life and other intangible values. Sustainability, however, requires financial support without the opportunity of directly profiting from this investment, and therefore, it is uneconomical. On the other hand, sustainability has an impact on the quality of life of whole populations, since it reduces the environmental burden derived from products consumed and put to use in our daily lives, and therefore, sustainability is highly desirable. This can only be reached with the implementation of regulations and provision of financial incentives. Limitations for land filling, the dismantling of end-of-life-vehicles [139], and extended producer responsibility [138] are important steps for a recycling-oriented society. A new approach was seen recently by the EU Commissioner for Fisheries, Maria Damanaki, when she suggested paying the fishing industry for fishing the debris that pollutes the world’s oceans. This does not result in financial gain for those who pay these actions, but it makes life better.

The bottom line is that waste material contains many valuable materials: gold, silver, platinum, copper, and other rare metals are present in WEEE; and PMMA is a monomer, which has been recovered for decades by depolymerization [31]. The task in the coming decade is to add to the list of materials and substances considered worthy of being recovered.