Recycling of Waste from Polymer Materials: an Overview of the Recent Works

Polymer Degradation and Stability 98 (2013) 2801e2812

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Polymer Degradation and Stability

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Review article Recycling of waste from polymer materials: An overview of the recent works

Kotiba Hamad a,b,*, Mosab Kaseem b, Fawaz Deri b a Plasticity Control and Mechanical Modeling Laboratory, School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, South Korea b Laboratory of Materials Rheology (LMR), Faculty of Science, Department of Chemistry, University of Damascus, Damascus, Syria article info abstract

Article history: Polymer recycling is a way to reduce environmental problems caused by polymeric waste accumulation Received 13 August 2013 generated from day-to-day applications of polymer materials such packaging and construction. The Received in revised form recycling of polymeric waste helps to conserve natural resource because the most of polymer materials 24 September 2013 are made from oil and gas. This paper reviews the recent progress on recycling of polymeric waste form Accepted 25 September 2013 some traditional polymers and their systems (blends and composites) such as polyethylene (PE), poly- Available online 4 October 2013 propylene (PP), and polystyrene (PS), and introduces the mechanical and chemical recycling concepts. In addition, the effect of mechanical recycling on properties including the mechanical, thermal, rheological Keywords: Polymer materials and processing properties of the recycled materials is highlighted in the present paper. Ó Waste 2013 Elsevier Ltd. All rights reserved. Recycling Properties

1. Introduction to plastic packaging e plastic products in general tend to be discarded after first use. However, there are examples of reuse in During last decades, the great population increase worldwide the marketplace. For example, a number of detergent manu- together with the need of people to adopt improved conditions of facturers market refill sachets for bottled washing liquids and living led to a dramatically increase of the consumption of poly- fabric softeners. Consumers can refill and hence reuse their mers (mainly plastics). Materials appear interwoven with our plastic bottles at home, but in all of these cases the reusing of the consuming society where it would be hard to imagine a modern plastic bottles and containers do not continue for long time society today without plastics which have found a myriad of uses in epically in the food applications. fields as diverse as household appliances, packaging, construction, Mechanical recycling: also known as physical recycling. The medicine, electronics, and automotive and aerospace components. plastic is ground down and then reprocessed and compounded A continued increase in the use of plastics has led to increase the to produce a new component that may or may not be the same amount of plastics ending up in the waste stream, which motivated as its original use [1]. to more interest in the plastic recycling and reusing. This review Chemical recycling: the polymer waste is turned back into its focuses on the reclamation and recycling of plastics. There are oil/hydrocarbon component in the cases of polyolefin’s and several options for how this can be done: reuse, mechanical recy- monomers in the case of polyesters and polyamides, which can cling, and chemical recycling: be used as raw materials for new polymer production and petrochemical industry, or into the pure polymers using suitable Reuse: the most common examples of reuse are with glass chemical solvents [2]. containers, where milk and drinks bottles are returned to be cleaned and used again. Reuse is not widely practiced in relation A review of the works reported polymers recycling revealed that the mechanical recycling and chemical recycling are the most widely practiced of these methods and the most of the studies * Corresponding author. Plasticity Control and Mechanical Modeling Laboratory, discussed in the present paper are focused on the two methods. School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712- However, from industrial point of view, the mechanical recycling is þ 749, South Korea. Tel.: 82 1030448258. ’ E-mail address: [email protected] (K. Hamad). the most suitable because it s low cost and reliability. The aim of

0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.09.025 2802 K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 this paper is to discuss the most recent works reported the me- of the blend decreased nearly by a factor of 0.15e0.3 due to the chanical and chemical recycling of some traditional polymers and reduction of the molecular weights with the processing cycles, their systems including blends and composites. these results are not consistent with earlier experimental obser- vations reported for the recycling of pure PLA [8] where it was 2. Discussion found that the zero viscosity of pure PLA decreased by a factor of 0.82 just after one processing cycle, and this difference was 2.1. Polylactic acid (PLA) and its systems attributed to the good thermal stability of PS compared to PLA. In addition, the reduction of the molecular weight with the PLA is one of the most important biodegradable polyesters processing cycle resulted in increasing the flow activation energy of derived from renewable sources (mainly starch and sugar). Until the samples. The relationship between the flow activation energy the last decade, the main uses of PLA have been limited to and molecular weight was reported by Collins and Metzger [10], biomedical and pharmaceutical applications such as implant de- where they found that as the molecular weight of the polymer vices, tissue scaffolds, and internal sutures, because of its high cost decreases, the influence of the temperature on the melt viscosity and low molecular weight. Since, the existence of both hydroxyl (flow activation energy) increases. The mechanical results showed and a carboxyl group in lactic acid enables it to be converted that stress at break and strain at break of the sample decreased directly into polyesters via a polycondensation reaction; a consid- sharply after each processing cycle (Fig. 2) due to the lower cohe- erable interest has been paid to the academic research associated sion in the blend resulted from the molecular weights decreasing with PLA polymer and its copolymers [3e5]. Although PLA is a after the processing cycles. The same behavior was noted from biodegradable material, which would significantly reduce envi- Young’s modulus measurements but in this case the reduction of ronmental pollution associated with its waste, the knowledge the mechanical property (Young’s modulus) was not sharp after about the material recycling and changes in the properties of PLA each processing cycle. upon its multiple processing is a very important subject [6]. 2.1.2. Chemical recycling 2.1.1. Mechanical recycling Chemical recycling of PLA based polymer blends were reported Duigou et al. [7] studied the effect of recycling on mechanical by Tsuneizumi et al. [11] on polylactic acid/polyethylene (PLA/PE) behavior of biocompostable flax/poly(L-lactide) composites which and polylactic acid/poly (butylene succinate) (PLA/PBS) polymer were used as an alternative to glass fiber-reinforced petrochemical blends. Two routes associated with the chemical recycling of PLA/ polymers. The composites were fabricated using a single screw PE blend were performed: extruder and then molded using an injection machine. In order to investigate the recyclability of the material, the fabricated com- Direct separation of PLA and PE first by their different solu- posites were subjected to six injection cycles and the effect of the bilities in toluene, followed by the chemical recycling of PLA number of the injection cycle on the rheological, mechanical and using montmorillonite. morphological properties was determined. The results showed that The selective degradation of PLA in the PLA/PE blend by the stiffness of PLA improved after the addition of the fiber but montmorillonite in a toluene solution at 100 C forming the stress at break and strain at break decreased dramatically. Also it lactic acid oligomer (LA) with a small molecular weight. The PE was found that the stiffness of the composites is not affected with remained unchanged and was quantitatively recovered by the the injection number in spite of the molecular weight reduction reprecipitation method for material recycling. associated with the processing cycles, this behavior was attributed to the increasing of crystallinity in PLA phase after processing cycles In a similar procedure, chemical recycling of PLA/PBS blend was [8], whereas stress at break and strain at break decreased after the also carried out and compared by the two different procedures: injection cycles. The decreasing of stress at break and strain at break of the composites after the injection cycles was interpreted The direct separation of PLA and PBS by the solubility in by fiber damage during recycling, where the reduction in fiber toluene. length results in more strain concentrations and a higher risk of de- bonding. Also the reduction of molecular weight after the injection cycles could result in decreasing stress at break and strain at break of the material. The reduction of the molecular weights after the injection cycles was revealed by the rheological properties of the composites where it was found that after the injection cycles, the viscosity of the composites decreased sharply compared to that of the as-fabricated composite [9]. The same observation was reported by Hamad et al. [9], where the effect of processing cycles (extrusion and injection) on the properties of PLA/PS polymer blend was investigated. A simple blend containing 50% PLA and 50% PS was prepared using a single screw extruder and the granules of the blend were injected into dog bone-shaped samples. The samples were reprocessed again by grinding, extrusion, and injection. Solution viscosity, mechanical properties and rheological properties of the samples were determined. The results showed that intrinsic viscosity of the samples, which is directly related to the molecular weight, decreased after each processing cycle (Fig. 1) and it decreased steadily with increasing the processing number. The compounded blend (PLA50) had melt Fig. 1. Effect of processing number on the intrinsic viscosity of PLA/PS (50/50) blend viscosity of 3100 Pa.s, and after each processing cycle, the viscosity [9]. K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 2803

polyethylene (PE) and polypropylene (PP) using a triboelectrostatic technology. In this technology, negative and positive charges can be imparted to the particles of the two polymers in a mixture, and then they can be separated by passing through an external electric field. The kind of charges on the polymer depends on the ability of polymer to the electron loses or gains. The material with a higher affinity for electrons gains electrons and charges negatively, whereas the material with the lower affinity loses electrons and charges posi- tively. Fig. 4 shows the triboelectrostatic charging sequence of various polymers [13]. For the removal of PVC from two-component mixed plastics such as PVC/PET, PVC/PP, PVC/PE or PVC/PS, triboelectrostatic technology was used. Separation results showed a recovery of 96e99% with the pure extract content in excess of 90%. The mechanical recyclability of PVC sheets used in building floors applications was reported by Yarahmadi et al. [14]. The results has shown that PVC floorings as plastic waste can be mechanically recycled without upgrading, and without the addition of new plas- ticizer. Augier et al. [15] reported the effect of wood fiber fillers on the internal recycling of PVC based composites. For investigating the effect of the wood fibers content on the recyclability, the recyclability of pure PVC and wood fiber-reinforced PVC was compared through the effect of recycling on the mechanical properties of both Fig. 2. Effect of processing number on the stress at break and granules shape of PLA/PS PVC and the composites. The results showed that the addition of the (50/50) blend (Ex: extrusion cycle) [9]. wood fiber to PVC improves its recyclability where it was found that up to five processing cycles, the composite properties remained The sequential degradation of PLA/PBS blends using a lipase stable. However, after ten processing cycles and especially after fi rst to degrade PBS into cyclic oligomer, which was then re- twenty cycles, the flexural strength increased, whereas the other polymerized to produce a PBS. Next, PLA was degraded into mechanical properties remained almost constant. In general it was re-polymerizable LA oligomer (Fig. 3). to say that it is possible to recycle the composite waste five times, without adding raw materials, where no significant change It was found from the results of the two procedures that the appeared until five cycles. The same trend was reported by Petch- former procedure is more effective than the latter with respect to wattana et al. [16], where they studied the recycling of PVC/wood the recycling use of organic solvents. composites (woodeplastics composites (WPC)). The effect of reprocessing (up to seven times) on the mechanical and structure properties was investigated on a mixture of waste and virgin com- 2.2. Polyvinyl chloride (PVC) and its systems posites. The results showed that the molecular weights of PVC decreased due to the molecular chain scission induced by the shear The low cost and high performance of PVC products combined stress introduced to the material during reprocessing. Mechanical with the wide range of properties that can be obtained from different test results demonstrated that the composites could be reprocessed formulations has contributed to the widespread use of PVC in con- as WPC materials again without critically affecting its mechanical struction products. There has been a long time-lag between PVC performance, where the impact strength of the composite remained consumption and the amassing of PVC waste arising from the long rather constant and closes in the value to that of the composite from life of PVC products, which can be up to 50 years. It is obvious that all raw feed, and the flexural strength of the composite decreased the PVC that is being produced will become waste some day. The slightly with reprocessing times. European Association of Plastics Converters (EuPC) has estimated that the PVC waste for the periods between 2010 and 2020 will arise from the following sources. Various works have reported the recy- 2.3. Polyethylene’s (PE’s) and its systems cling of PVC and its systems since the beginning of the last decade. PE’s are one of the most widely used plastics characterized by a 2.2.1. Mechanical recycling density in the range 0.918e0.965 g/cm3 resulting in a range of Lee and Shin [12] separated PVC from different plastic mixtures toughness and flexibility. Their major application is in packaging including polystyrene (PS), polyethylene terephthalate (PET), film although their outstanding dielectric properties mean that

Fig. 3. Chemical recycling of PLA/PE and PLA/PBS blends. 2804 K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812

important, with a high proportion of valuable light olefins C3eC4; isoparaffins C4eC5 are significant as well. Hajekova and Bajus [23] studied the chemical recycling of LDPE and PP waste using two steps of the thermal cracking method, in the first one the polymer waste was decomposed individually in a batch reactor at 450 C and they converted to wax/oil products. In the second step the wax/oil products were dissolved in heavy naphtha to obtain steam cracking feedstock. The selectivity and Fig. 4. Triboelectrostatic charging sequence of various polymers. kinetics of copyrolysis for 10 mass% solutions of wax/oil from LDPE or PP with naphtha in the temperature range from 740 to 820 Cat residence times from 0.09 to 0.54 s using industrial ethylene units they are also widely used as an electrical insulator. Other applica- were studied. The results showed that it is possible to perform tions of PE’s including domestic ware, tubing, squeeze bottles and polyalkenes recycling via the copyrolysis of polyalkene oils and cold water tanks are also well-known. waxes with conventional liquid steam cracking feedstocks on already existing industrial ethylene units. 2.3.1. Mechanical recycling Thermal cracking method in the presence of phenol as a solvent The effect of mechanical recycling on rheological and thermal was also used for the chemical recycling process of HDPE in a work properties of low-density polyethylene (LDPE) was reported by Jin reported by Vicente et al. [24]; the effect of phenol on the thermal et al. [17]. LDPE samples were subjected to extensive extrusion cracking process was investigated. The results showed that the cycles up to one hundred cycles. The results showed that the main products in the cracking reaction were olefins which are very complex viscosity of the samples at a low-test frequency of important for the petrochemical industry and the presence of 0.628 rad/s shown in Fig. 5a increased with increasing the number phenol as a solvent can promote the cracking reaction due to its role of extrusion cycle, this observation was attributed to the cross- in favoring random scissions and chain reactions, as shown in Fig. 6, linking reactions took place throughout LDPE chains during recy- which can explain the plastic conversions obtained as well as the cling process due to the presence of the reactive carbon radicals yields and selectivity’s of each hydrocarbon product. [18,19]. The same tend was observed in the melt flow index (MFI) Achilias et al. [25] studied the chemical recycling of PE’s (LDPE measurements where it was noted that MFI of the material and HDPE) and PP obtained from various applications including decreased as the number of extrusion cycle increased (Fig. 5b). The packaging film, bags, pipes, and food-retail outlets using two results of this work revealed that the processability of LDPE is only techniques: affected after the 40th extrusion cycle. Also the results reported by Kartalis et al. [20] on the recycling of LDPE/medium density poly- Dissolution/reprecipitation method using different solvents ethylene (MDPE) blend revealed that even after five successive and non-solvents. extrusion cycles, the material shows significant processing stability. Catalytic pyrolysis using FCC method. Vallim et al. [21] recycled high-density polyethylene (HDPE) waste by blending with virgin polyamide (PA6) using a twin-screw In the first technique the polymers were dissolved in xylene and extruder. The characterization of the prepared blend revealed that reprecipitated using n-hexane and the pure polymers were then the mechanical properties and thermal stability of the blend dried. In the second one the polymers were thermal cracked in the improve by the using of PA6 which attributed to the decrease of size presence of FCC catalyst. The pure polymers obtained from the first domains of the recycled HDPE. technique were mechanically tested and compared with the mechanical properties of the waste whereas the products of the sec- 2.3.2. Chemical recycling ond technique were characterized using spectroscopes methods. Puente et al. [22] studied the chemical recycling of LDPE using The first leads to high recovery of polymer with the disadvantage of fluid catalytic cracking (FCC) method at 500 C in the presence of using large amounts of organic solvents. From the measurements of various commercial catalysts. The process was performed in a so- the tensile mechanical properties of samples after dissolution/ lution of LDPE using toluene as a solvent. The results showed that reprecipitation process, it was found that the pure polymers pro- the FCC products were qualitatively similar in all the catalysts and duced from this process are almost identical to virgin polymers. the contribution is centered mainly in the gasoline fraction, with Furthermore, pyrolysis was investigated as a promising technique high aromatic content, although the production of gases is also for thermochemical recycling of these polymers where a series of

Fig. 5. Complex viscosity and MFI of LDPE after extrusion cycles [17]. K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 2805

Fig. 6. Role of phenol in the thermal cracking of HDPE [24]. alkanes and alkenes of different carbon number, which can be used exhibited greater crystallization rate, higher crystallinity and in petrochemical industry, could be recovered using pyrolysis equilibrium melting temperature than those measured for virgin method. PP. Young’s modulus and yield stress increased with the number of In another work reported by Achilias et al. [26], dissolutione injection cycles due to the higher crystallinity of PP after processing reprecipitation technique was investigated for recycling polymers whereas decreasing of PP molecular weight resulted in reduction of from plastic packaging waste such as PE, PP, PET, and PVC. The elongation at break and fracture toughness of the samples. mechanical properties of the polymers were compared before and Phuong et al. [29] investigated the recyclability of poly- after recycling process. The result showed that very good polymer propylene/organophilic modified layered silicates nanocomposites recoveries could be obtained in almost all waste samples examined, using a twin screw extruder at different temperatures for ten times while lower values in some samples were attributed to the removal throughout the changes in rheological and mechanical properties of additives present in the original waste products. Also it was of the composites after each processing cycle. The results showed found that the mechanical properties of the polymers changed that the MFI increased with increasing the number of extrusion slightly after the recycling process. cycle; this behavior was reported in several works focused on the In the work reported by Wei et al. [27], FCC method was also recycling and stability of PP and was attributed to the thermal used for the chemical recycling of plastics waste compounded from degradation of PP during the extrusion [30,31]. The mechanical PP, LDPE and HDPE. The effect of two types of catalysts (zeolitic and results showed that tensile strength of the composites decreased non-zeolitic) on the yield and the reaction selectivity was investi- with increasing the number of the extrusion cycle whereas the gated and compared. It was found that the zeolitic catalysts give impact strength reminded constant with increasing the extrusion higher yields comparing with non-zeolitic catalysts in case of vol- cycle (Fig. 7). In another work [32], the effect of recycling on the atile hydrocarbons, various types of catalysts and their yield and rigidity of PP/vegetal fiber composites prepared by extrusion pro- selectivity at 360 C are shown in Table 1. cess was also investigated. The results showed that the rigidity of the composites nearly reminded constant after the processing cy- 2.4. Polypropylene (PP) and its systems cles due to the good stabilization of the fibers aspect ratio after recycling. 2.4.1. Mechanical recycling The recyclability of other PP composites was also investigated by Aurrekoetxea et al. [28] determined the morphology and Bahlouli et al. [33], where the effect of recycling on the properties of properties of PP samples subjected to several injection cycles. The PP-based composites (ethylene propylene diene monomer (EPDM)/ results showed that the melt viscosity of PP decreased after pro- PP and talc/PP) using extrusion process was examined. Rheological, cessing which was attributed to the molecular weight decreasing of mechanical and structural properties of the composites were PP induced by reprocessing. Also it was found that the recycled PP determined and compared after each extrusion cycle. The results showed that the melt viscosity of the composites decreases with processing number in the same way of pure PP [34], also the me- Table 1 chanical properties of the composites decreased with processing Summary of the main products of LDPE/HDPE blend degradation at reaction temperature of 360 C over various catalysts. number. All noted changes in the composites properties were attributed to the changes in the structural properties of the com- Degradation results Catalyst type posites during the processing cycle. The obtained results from this USY ZSM-5 MOR ASA MCM-41 work were useful for optimizing the recycling process and for a (zeolitic (zeolitic (zeolitic (non-zeolitic (non-zeolitic better use of the recycled materials in components design. catalysts) catalysts) catalysts) catalysts) catalysts)

Yield (wt% feed) Gaseous 87.5 93.1 90.2 85.6 87.3 2.5. Polystyrene (PS) and its systems Liquid 3.7 3.3 4.3 4.7 5.6 Residue 8.8 3.6 5.5 9.7 7.1 2.5.1. Mechanical recycling Involatile 4.7 2.4 2.9 7.4 5.2 Brennan et al. [35] studied the effect of recycling on the prop- residue erties of ABS, high impact polystyrene (HIPS) and the blend of ABS Coke 4.1 1.2 2.6 2.3 1.9 waste and HIPS waste. Mechanical and thermal properties of the 2806 K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812

The using of FCC method for the chemical recycling of PS waste was reported by Williams and Bagri [41]. Two catalysts were used; zeolite ZSM-5 and Y-zeolite and the effect of the temperature on the yield of the process were studied. The results showed that the main product from the uncatalysed process of polystyrene was an oil consisting mostly of styrene and other aromatic hydrocarbons. The gas produced for the process was found to consist of methane, ethane, ethene, propane, propene, butane and butene. In the presence of either catalyst an increase in the yield of gas and a decrease in the amount of oil produced was reported, but there was significant formation of carbonaceous coke on the catalyst. Increasing the temperature in the case of Y-zeolite catalyst and also the amount of the catalyst in the catalyst bed led to a decrease in the yield of the oil and increase in the yield of the gas. Achilias et al. [42] reported catalytic and non-catalytic pyrolysis of PS waste in a fixed bed reactor using either model polymer or Fig. 7. Effect of the extrusion cycle on the mechanical properties and MFI of poly- commercial waste products as the feedstock. It was found that the propylene/organophilic modified layered silicates nanocomposites [29]. pyrolysis oil fraction could be re-polymerized again to produce virgin PS. However, aromatic compounds included in this fraction virgin and recycled polymers (ABS and HIPS) were determined and may act as chain transfer agents, resulting in alterations in the compared. The results showed that in the two cases (ABS and HIPS) shape of the reaction rate curve and lowering significantly the the effect of recycling on the tensile strength and tensile modulus of average molecular weight and the glass transition temperature of the blend, which has a small portion of one of the two components the PS prepared form the pyrolysis process. (ABS or HIPS) were negligible (Fig. 8a and b), but the strains at break Dissolutionereprecipitation process for chemical recycling of PS and impact strength of the blend reduced considerably comparing waste was reported recently [43,44] and the effect of the temper- with virgin and pure components (ABS and HIPS) (Fig. 8c and d). ature and oils from natural sources as solvents on stability of PS The previous results reveal that the blending of small amount of chains during the process was investigated. The results showed ABS or HIPS could result in improving the tensile strength and that the solubility of PS in the solvents increased as the temperature modulus of the blend after recycling. Elmaghor et al. [36] used of dissolving increased and these solvents have slightly effect on poly(ethylene-co-vinyl acetate) and poly(styrene-b-ethylene/bu- the PS chains so it was to say that it is possible to use these solvents tylenes-b-styrene) as compatibilizers for a ternary blend prepared for PS waste recycling at high temperature (up to 60 C) without from waste polymers including PS, HDPE, and PVC using a single- sharp decreasing in the molecular weight of the recycled polymer screw extruder. For more improving in the compatibility between as well as the possibility of reusing the solvents again. the components of the prepared blend, the extrudates were subjected to gamma radiation. The results showed that both the 2.6. Acrylonitrileebutadieneestyrene copolymer (ABS) and its compatibilizers and irradiation improved the mechanical proper- systems ties of the blend where impact strength and ductility of the blend were sharply enhanced and the improvement of tensile strength 2.6.1. Mechanical recycling was moderate. Boronat et al. [45] studied the effect of reprocessing cycle con- The effect of the reprocessing cycles on properties and structure ditions (temperature and shear rate) on the properties of ABS. Two of PS nanocomposites containing 5 wt. % organophilic clay, which is grades of ABS were injected and tested (high viscosity grade and commercialized under the trade name Cloisite 15 A, was investi- low viscosity grade). It was found that each of the two grades shows gated by Remili et al. [37]. Rheological, mechanical, and structural a different behavior upon reprocessing where the low viscosity properties of the composites were determined after each process- grade showed a reduction of viscosity with increasing the number ing cycle and compared to those of pure PS. The results presented in of processing cycles, which was attributed to the degradation of Fig. 9 showed that the composite (PS/Cloisite 15 A) has better this polymer, whereas the high viscosity grade, conversely, showed recyclability and reprocessability compared to pure PS where the an increase of melt viscosity as the number of processing cycles decreasing of the melt viscosity and mechanical performance was increased. more pronounced in pure PS comparing with that in the composite. Perez et al. [46] studied the effect of reprocessing on mechani- This behavior was attributed to the increase in the molecular cal, thermal and rheological properties of ABS. The results showed weights of the composites after 8 processing cycles due the that neither melt viscosity nor tensile strength was affected by the occurrence of some crosslinking [38], whereas the molecular number of processing cycles, but the impact strength decreased weights of pure PS decreased by w49% after 8 cycles. slightly, so it was to say that ABS has good mechanical recyclability and for improving impact strength after recycling, toughness 2.5.2. Chemical recycling agents are needed. These results are in consistent with those ob- The chemical recycling of PS waste was reported firstly in the tained by Karahaliou and Tarantili [47] where the stability of ABS work of Lee et al. [39] by using of clinoptilolites as catalysts, they subjected to five extrusion cycle was investigated. The mechanical found that clinoptilolites possess a good catalytic activity for the and rheological properties showed that ABS has good stability degradation of PS with very high selectivity to aromatic liquids. during the processing cycles. Arandes et al. [40] studied the thermal cracking of PS and In another work [48], ABS waste was used as an additive to polystyrene-butadiene (PSB) dissolved in light cycle oil (LCO) in the virgin ABS and the effect of waste content on mechanical properties presence of mesoporous silica as a catalyst. The cracking of the PS/ of the blend was evaluated. The finding of this work revealed that LCO blend produced high yields of styrene, whereas the cracking of there is no obvious effect of ABS waste content on the tensile the PSB/LCO blend resulted in a stream of products with petro- strength, elongation at yield, flexural strength, flexural modulus, chemical interest. and impact strength. However, hardness, melt flow index, and glass K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 2807

Fig. 8. Mechanical properties recycled ABS and recycled HIPS blend [35]. transition temperature of blend increased with increasing the examined. The results showed that the effect of recycled ABS con- waste content in the blend. The effect of ABS waste content on the tent on the viscosity of the blend was not signiﬁcant. However, at all properties of a blend consisted of virgin and recycled ABS was also blend compositions the viscosity of the blends decreases with the investigated by Scaffaro et al. [49]. The effect of reprocessing cycles number of the processing cycle (Fig. 10a). In the case of mechanical on the physical properties of the prepared blends was also properties it was found that the tensile strength, tensile modulus,

Fig. 9. Effect of recycling on the melt viscosity and mechanical properties of PS and PS/Cloisite 15 A nanocomposite [37]. 2808 K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 elongation at break and impact strength of the blends decreased term of thermal, mechanical and rheological properties (Fig. 11). It with the number of the processing cycle and decreased slightly was found that the blend (PC waste/PET waste) showed good me- with increasing the recycled ABS content in the blend (Fig. 10b). chanical properties which attributed to the interfacial reaction Yeh et al. [50] used ABS waste for preparing ABS/wood com- between the blend components (transesterification between PET posites and compared with counterpart composites prepared by waste and PC waste occurs during blending in the molten state virgin ABS. The composites with 50% wood and a coupling agent resulting in a good compatibility in the blend). The compatibility were prepared using a twin-screw extruder and characterized in between the blend components was detected using the Tg values term of mechanical properties. It was found that while the impact where it was found from differential scanning calorimetry (DSC) strength and ductility of the virgin and recycled polymers were measurements that the Tg associated with the PET phase in all significantly different, the composite properties differed only blends is higher than that in the case of the individual component slightly from each other. Recently Bai et al. [51] suited the effect of (PET) whereas it was higher in PC after blending (Fig. 12). In another reprocessing on the mechanical properties of ABS/CaCO3 compos- work, Maria and Sanchez [56] studied the recyclability of PC/ ites. It was found that at a low content of CaCO3 (less than 10%), the butylene terephthalate (PBT) blend through evaluation the influ- impact strength of the composites decreased with the number of ence of recycling on the mechanical properties of the blend. The processing cycle which was attributed to the thermal degradation result showed that both tensile strength and tensile modulus were of the rubber phase in ABS, whereas at a high content of CaCO3 slightly affected with the recycling process so it was to say that the (higher than 15%) impact strength of the composites increased with PC/PBT blend has good mechanical recyclability. the number of processing cycle. 2.7.2. Chemical recycling 2.6.2. Chemical recycling The chemical recycling of PC waste was reported by Hidaka et al. Szabo et al. [52] recycled ABS and ABS/polymethylmethacrylate [57], the finding of this work showed that the main products of the (PMMA) blend waste using thermal decomposition method. The chemical recycling of PC waste are bisphenol A (BPA) and carbo- results of this work showed that the decomposition temperature of hydrate carbonates. Hata et al. [58] reported the obtaining of 1,3- the blend is less than that of pure ABS, but the additional products dimethyl-2-imidazolidinone (DMI) and BPA from the chemical derived from ABS detain the direct feedstock recycling of the MMA recycling of PC waste through the treatment of PC waste with N,N0- monomer. In another work [53], the thermal degradation of deni- dimethyl-1,2-diaminoethane (DMDAE) in the presence of dioxane trogenated ABS samples (DABS) prepared by sequential hydrolysis as a solvent. of ABS using polyethyleneglycol (PEG)/NaOH was reported and it Recently a simple chemical recycling method of PC waste was was found that effective denitrogenation of ABS before pyrolysis is suggested by Tsintzou et al. [59], they studied the chemical recy- beneficial to produce clean oil during pyrolysis. cling of PC waste with water in microwave reactor in the presence of NaOH under controlled conditions of temperature and pressure, 2.7. Polycarbonate (PC) and its systems the results showed that PC degradation higher than 80% can be obtained at 160 C after a microwave irradiation time of either 2.7.1. Mechanical recycling 40 min or 10 min using either a 5 or 10% (w/v) NaOH solution. Elmaghor et al. [54] used virgin ABS grafted maleic (ABS-g-MA) anhydride for modifying PC waste. Blends of ABS-g-MA/PC were 2.8. Polyethylene terephthalate (PET) and its system prepared using a twin-screw extruder and characterized. It was found that the mechanical properties of the waste could be 2.8.1. Mechanical recycling enhanced by the addition of ABS-g-MA which was explained based Navarro et al. [60] prepared a blend of virgin HDPE and PET on the reaction took place between MA functional group in ABS-g- waste using the extrusion process in efforts to improve the per- MA and end hydroxyl groups of PC resulting in bridges formation formance of PET waste. The prepared blend, in different ratio, was between various phases in the blend and enhancement of the injected to obtain test samples for mechanical characterization. mechanical performance of the material. Thermal and rheological properties of the prepared blends were In another work [55], blends of PC waste and polyethylene also evaluated. It was found that the presence of HDPE in the blend terephthalate (PET) waste were prepared in different ratio using a reduces the melt viscosity of the blend indicating good flow ability twin-screw extruder at 270 C. The obtained granules of the pre- compared to PET waste (Fig. 13). However, incompatibility between pared blends were molded using two different methods, HDPE and PET in the blend was detected at a content of HDPE compression molding and injection molding; and characterized in higher than 5% resulting in poor mechanical properties compared

Fig. 10. Effect of the recycled ABS content and number of extrusion cycle (R1, R2 and R3) on (a) MFI and (b) tensile strength of virgin ABS [49]. K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 2809

Fig. 11. Recycling process of PC/PET blend. to PET. The results of this work also indicated that it will be possible to modify PET waste using a small amount of virgin HDPE (less than Fig. 13. Effect of HDPE content on the viscosity of the recycled PET [60]. 5%.) The using of PET waste in the preparation of polymer systems was also reported by Shi et al. [61], in this work a blend of PET waste main product of the reaction was BHET and the highest yields of the and ABS was fabricated and for reducing the interfacial tensions reaction were obtained with zinc acetate and sodium carbonate at between PET and ABS in the blend, SiO2 was incorporated. The 196 C. results of this study showed that the mechanical properties of Achilias et al. [67] depolymerized PET waste in the presence of composites improve with increasing the SiO2 content. diethylene glycol (glycolysis) under microwave irradiation. The Very recently Mantia et al. [62] modify PET waste by the addi- reaction was carried out in a sealed microwave reactor in which the tion of small amounts of virgin PLA using melt mixing technology. pressure and temperature could be controlled. The depolymeriza- The effect of the small amounts of PLA on the mechanical and tion products were characterized using FTIR. The results were rheological properties of the prepared blend was investigated. The compared to that obtained without using microwave irradiation. It results showed that the viscosity of the blend is less than that of was found that the complete degradation of PET can be done at PET, but the blend possessed higher thermal sensitivity compared temperatures higher than 180 C. Also the results showed that in to PET waste (Fig. 14). The mechanical results revealed that the the normal condition (without irradiation), the reaction needs small amount of PLA do not affect on the tensile properties, where more than 4 h to complete the degradation of PET, which conﬁrms the mechanical properties of the blend were similar to those of PET. the importance of the microwave power technique and the substantial energy saving achieved. 2.8.2. Chemical recycling Also in another work done by Achilias et al. [68] PET waste was For chemical recycling of PET waste, the microwave irradiation depolymerized in the presence of ethanolamine (aminolytic) under in the presence of ethylene glycol (EG) and zinc acetate as catalysts microwave irradiation to enhance the waste degradation. The main was used [63]. The yield of the main product (bis(2-hydroxyethyl) product of the reaction was bis(2-hydroxyethyl) terephthalamide terephthalate (BHET)) was nearly same as that obtained by con- (BHETA). The results showed that the complete degradation of PET ventional electric heating. However, the time taken for completion waste was done at temperatures higher than 260 C. By comparing of reaction was reduced drastically from 8 h to 35 min leading to with the previous work reported the using of diethylene glycol [67], substantial saving in energy. Shukla et al. [64] used ethylene glycol it could be said that the presence of diethylene glycol caused in and sodium sulfate for the chemical recycling of PET waste. The complete degradation of PET at lower temperature compared to the BHET obtained as a main product of the reaction was used to hy- presence of ethanolamine. drophobic disperse dyes for synthetic textiles. Fonseca et al. [65,66] investigated the effect of various metal salts (zinc acetate, sodium carbonate, sodium bicarbonate, sodium sulfate and potassium sulfate) as depolymerization catalysts in the presence ethylene glycol at different temperature for PET recycling as shown in Fig. 15. The type of the salt used in the reaction has no effect on the kind of the product but it could control the yield of the reaction where the

Fig. 12. Effect of the reaction time on Tg values of PC/PET blend components [55]. Fig. 14. Effect of PLA content on the thermal stability of the recycled PET [62]. 2810 K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812

Fig. 15. Reaction and analytical procedure for the glycolysis of PET wastes [65].

Fig. 16. Effect of recycled PA66 content on the thermal stability of virgin PA12 [76].

2.9. Polyamides (PA’s) and their systems and Young’s modulus of the blend increase with increasing CB content. Also El-Nemr et al. [75] studied the effect of acrylonitrile 2.9.1. Mechanical recycling butadiene rubber (NBR) content on the properties of irradiated Su et al. [69] studied the effect of the reprocessing on the me- blends of PA6/66 waste and rubber waste. The results showed that chanical and rheological properties of PA6. PA6 sample was pro- the mechanical properties improve by the addition of NBR. cessed sixteen times and its properties were compared with those Dorigato and Fambri [76] used recycled short fiber of PA66 in the of the virgin PA6. The results showed a reduction in the molecular reinforcing process of the commercial grade of PA12, and studied weight and an increase in the molecular weight distribution as a the effect of PA66 content on the thermal and mechanical proper- consequence of a decrement in melt viscosity of PA6, but no ties of PA66/PA12 blend. It was found from this study that Tg of chemical changes were noted in the FTIR spectra of PA6 structure PA12 increases with increasing the fiber content. Mechanical during the recycling process. The mechanical test showed that the properties and the morphology of the material indicated good tensile strength increased after each processing cycle while the interfacial adhesion between the components whereas the thermal impact strength decreased. Bernasconi et al. [70] blended waste of stability of PA12 decreased slightly as the content of PA66 increased short glass fiber reinforced PA66 with virgin materials and studied (Fig. 16). the effect of the waste content on the tensile strength of the blend. Tensile test results showed a decrease of both elastic modulus and tensile strength, whereas the strain at break increased, for 3. Conclusion increasing the content of the waste. Goitisolo et al. [71] studied the effect of reprocessing on the Form the previous discussion about the recycling process of properties of PA6 nanocomposites using injection molding for five polymeric waste it could be concluded that the mechanical recy- times. The properties of the nanocomposite were determined after cling is the most preferred and used recycling method comparing each processing cycle. Also, no chemical changes in the FTIR spectra with the chemical recycling method in which the waste are subject of PA6 were observed in the composite during the processing cycles to complicated chemical treatments. as same as in the case of pure PA6 recycling [69,72], but the vis- Based on mechanical recycling results, the incorporation of cosity of the composite decreased sharply with the number of minor amounts of virgin polymers with waste from same or other processing cycle which was attributed to the decrease of the mo- polymers in the presence of suitable compatibilizers can result in lecular weight. In addition, the results of this work showed that good properties compared to those of the waste, for example, the strain at break of the composite decreased with the number of blending of PLA with PET waste resulted in a good thermal stability processing cycle, which indicated less ductility of the material. of the fabricated blend which was almost similar to that of PET Hassan et al. [73] studied the mechanical, thermal and waste. Also, in the case of PC waste the addition of ABS together morphological properties of irradiated PA6/66 waste and rubber with compatibilizers led to improve the mechanical properties waste powder blends. The properties of this blend were compared compared to those of the waste. to that of the non-irradiated blend. The results showed that the As a final conclusion, the using of the blending technique in the incorporation of rubber waste into recycled PA led to decrease the presence of suitable compatibilizers for the mechanical recycling of mechanical performance because of the too weak interfacial waste form polymer materials should be given more interest in the adhesion between the two components in the blend. However, the future. irradiation process resulted in improve the compatibility of the prepared blend. In another work by Hassan et al. [74], the effect of References carbon black (CB) content on the properties of irradiated blends containing PA6/66 waste and rubber waste powder was reported. [1] Cui J, Forssberg E. Mechanical recycling of waste electric and electronic The results showed that the tensile strength, elongation at break equipment: a review. J Hazard Mater B 2003;99:243e63. K. Hamad et al. / Polymer Degradation and Stability 98 (2013) 2801e2812 2811

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