recycling of waste from polymer materials: an overview of the recent works
TRANSCRIPT
Accepted Manuscript
Recycling of waste from polymer materials: An overview of the recent works
Kotiba Hamad, Young Gun Ko, Mosab Kaseem, Fawaz Deri
PII: S0141-3910(13)00313-3
DOI: 10.1016/j.polymdegradstab.2013.09.025
Reference: PDST 7107
To appear in: Polymer Degradation and Stability
Received Date: 13 August 2013
Revised Date: 24 September 2013
Accepted Date: 25 September 2013
Please cite this article as: Hamad K, Ko YG, Kaseem M, Deri F, Recycling of waste from polymermaterials: An overview of the recent works, Polymer Degradation and Stability (2013), doi: 10.1016/j.polymdegradstab.2013.09.025.
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Recycling of waste from polymer materials: An overview of the recent
works
Kotiba Hamad*1,2. Young Gun Ko1. Mosab Kaseem2. Fawaz Deri2
1Plasticity Control and Mechanical Modeling Laboratory, School of Materials Science and
Engineering, Yeungnam University, Gyeongsan 712-749, South Korea
2Laboratory of Materials Rheology (LMR), Faculty of Science, Department of Chemistry,
University of Damascus, Damascus – Syria
*E-mail: [email protected]
Abstract
Polymer recycling is a way to reduce environmental problems caused by polymeric waste
accumulation generated from day-to-day applications of polymer materials such packaging
and construction. The recycling of polymeric waste helps to conserve natural resource
because the most of polymer materials are made from oil and gas. This paper reviews the
recent progress on recycling of polymeric waste form some traditional polymers and their
systems (blends and composites) such as polyethylene (PE), polypropylene (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 and processing properties of the recycled materials is highlighted in the present
paper.
Keywords: Polymer materials, Waste, Recycling, Properties
1. Introduction:
During last decades, the great population increase worldwide together with the need of people
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to adopt improved conditions of living led to a dramatically increase of the consumption of
polymers (mainly plastics). Materials appear interwoven with our consuming society where it
would be hard to imagine a modern society today without plastics which have found a myriad
of uses in fields as diverse as household appliances, packaging, construction, medicine,
electronics, and automotive and aerospace components.
A continued increase in the use of plastics has led to increase the amount of plastics ending
up in the waste stream, which motivated to more interest in the plastic recycling and reusing.
This review focuses on the reclamation and recycling of plastics. There are several options
for how this can be done: reuse, mechanical recycling, and chemical recycling:
� Reuse: the most common examples of reuse are with glass containers, where milk and
drinks bottles are returned to be cleaned and used again. Reuse is not widely practiced in
relation to plastic packaging - plastic products in general tend to be discarded after first
use. However, there are examples of reuse in the marketplace. For example, a number of
detergent manufacturers market refill sachets for bottled washing liquids and fabric
softeners. Consumers can refill and hence reuse their plastic bottles at home, but in all of
these cases the reusing of the plastic bottles and containers do not continue for long time
epically in the food applications.
� Mechanical recycling: also known as physical recycling. The plastic is ground down and
then reprocessed and compounded to produce a new component that may or may not be
the same as its original use [1].
� Chemical recycling: the polymer waste is turned back into its oil/hydrocarbon component
in the cases of polyolefin's and monomers in the case of polyesters and polyamides, which
can be used as raw materials for new polymer production and petrochemical industry, or
into the pure polymers using suitable chemical solvents [2].
A review of the works reported polymers recycling revealed that the mechanical recycling
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and chemical recycling are the most widely practiced of these methods and the most of the
studies discussed in the present paper are focused on the two methods. However, from
industrial point of view, the mechanical recycling is the most suitable because it’s low cost
and reliability. The aim of this paper is to discuss the most recent works reported the
mechanical and chemical recycling of some traditional polymers and their systems including
blends and composites.
2. Discussion:
2.1. Polylactic acid (PLA) and its systems:
PLA is one of the most important biodegradable polyesters derived from renewable sources
(mainly starch and sugar). Until the last decade, the main uses of PLA have been limited to
biomedical and pharmaceutical applications such as implant devices, tissue scaffolds, and
internal sutures, because of its high cost and low molecular weight. Since, the existence of
both hydroxyl and a carboxyl group in lactic acid enables it to be converted directly into
polyesters via a polycondensation reaction; a considerable interest has been paid to the
academic research associated with PLA polymer and its copolymers [3-5]. Although PLA is a
biodegradable material, which would significantly reduce environmental pollution associated
with its waste, the knowledge about the material recycling and changes in the properties of
PLA upon its multiple processing is a very important subject [6].
2.1.1. Mechanical recycling:
Duigou et al. [7] studied the effect of recycling on mechanical behavior of biocompostable
flax/poly(L-lactide) composites which were used as an alternative to glass fiber-reinforced
petrochemical polymers. The composites were fabricated using a single screw extruder and
then molded using an injection machine. In order to investigate the recyclability of the
material, the fabricated composites were subjected to six injection cycles and the effect of the
number of the injection cycle on the rheological, mechanical and morphological properties
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was determined. The results showed that the stiffness of PLA improved after the addition of
the fiber but stress at break and strain at break decreased dramatically. Also it was found that
the stiffness of the composites is not affected with the injection number in spite of the
molecular weight reduction associated with the processing cycles, this behavior was
attributed to the increasing of crystallinity in PLA phase after processing cycles [8], whereas
stress at break and strain at break decreased after the injection cycles. The decreasing of
stress at break and strain at break of the composites after the injection cycles was interpreted
by fiber damage during recycling, where the reduction in fiber 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 viscosity
of 3100 Pa.s, and after each processing cycle, the viscosity of the blend decreased nearly by a
factor of 0.15-0.3 due to the reduction of the molecular weights with the processing cycles,
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these results are not consistent with earlier experimental observations reported for the
recycling of pure PLA [8] where it was found that the zero viscosity of pure PLA decreased
by a factor of 0.82 just after one processing cycle, and this difference was attributed to the
good thermal stability of PS compared to PLA.
In addition, the reduction of the molecular weight with the processing cycle resulted in
increasing the flow activation energy of the samples. The relationship between the flow
activation energy and molecular weight was reported by Collins and Metzger [10], where
they found that as the molecular weight of the polymer decreases, the influence of the
temperature on the melt viscosity (flow activation energy) increases. The mechanical results
showed that stress at break and strain at break of the sample decreased sharply after each
processing cycle (Fig. 2) due to the lower cohesion in the blend resulted from the molecular
weights decreasing after the processing cycles. The same behavior was noted from Young's
modulus measurements but in this case the reduction of the mechanical property (Young's
modulus) was not sharp after each processing cycle.
2.1.2. Chemical recycling:
Chemical recycling of PLA based polymer blends were reported by Tsuneizumi et al. [11] on
polylactic acid/polyethylene (PLA/PE) and polylactic acid/poly (butylene succinate)
(PLA/PBS) polymer blends. Two routes associated with the chemical recycling of PLA/PE
blend were performed:
� Direct separation of PLA and PE first by their different solubilities in toluene, followed by
the chemical recycling of PLA using montmorillonite.
� The selective degradation of PLA in the PLA/PE blend by montmorillonite in a toluene
solution at 100 °C forming the lactic acid oligomer (LA) with a small molecular weight.
The PE remained unchanged and was quantitatively recovered by the reprecipitation
method for material recycling.
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In a similar procedure, chemical recycling of PLA/PBS blend was also carried out and
compared by the two different procedures:
� The direct separation of PLA and PBS by the solubility in toluene.
� The sequential degradation of PLA/PBS blends using a lipase first to degrade PBS into
cyclic oligomer, which was then re-polymerized to produce a PBS. Next, PLA was
degraded into re-polymerizable LA oligomer (Fig. 3).
It was found from the results of the two procedures that the former procedure is more effective
than the latter with respect to the recycling use of organic solvents.
2.2. Polyvinyl chloride (PVC) and its systems:
The low cost and high performance of PVC products combined with the wide range of
properties that can be obtained from different formulations has contributed to the widespread
use of PVC in construction products. There has been a long time-lag between PVC
consumption and the amassing of PVC waste arising from the long life of PVC products,
which can be up to 50 years. It is obvious that all the PVC that is being produced will become
waste some day. The 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 recycling of PVC and its systems since the beginning of the
last decade.
2.2.1. Mechanical recycling:
Lee and Shin [12] separated PVC from different plastic mixtures including polystyrene (PS),
polyethylene terephthalate (PET), 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
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electrons gains electrons and charges negatively, whereas the material with the lower affinity
loses electrons and charges positively. 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 96-99% 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 plasticizer.
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 PVC and the composites. The results
showed that the addition of the wood fiber to PVC improves its recyclability where it was
found that up to five processing cycles, the composite properties remained stable. However,
after ten processing cycles and especially after twenty cycles, the flexural strength increased,
whereas the other mechanical properties remained almost constant. In general it was to say
that it is possible to recycle the composite waste five times, without adding raw materials,
where no significant change appeared until five cycles. The same trend was reported by
Petchwattana et al. [16], where they studied the recycling of PVC/wood composites (wood –
plastics 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 composites. The
results showed that the molecular weights of PVC decreased due to the molecular chain
scission induced by the shear stress introduced to the material during reprocessing.
Mechanical test results demonstrated that the composites could be reprocessed as WPC
materials again without critically affecting its mechanical performance, where the impact
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strength of the composite remained rather constant and closes in the value to that of the
composite from raw feed, and the flexural strength of the composite decreased slightly with
reprocessing times.
2.3. Polyethylene’s (PE’s) and its systems:
PE’s are one of the most widely used plastics characterized by a density in the range 0.918-
0.965 g/cm3 resulting in a range of toughness and flexibility. Their major application is in
packaging film although their outstanding dielectric properties mean that they are also widely
used as an electrical insulator. Other applications of PE’s including domestic ware, tubing,
squeeze bottles and cold water tanks are also well-known.
2.3.1. Mechanical recycling:
The effect of mechanical recycling on rheological and thermal properties of low density
polyethylene (LDPE) was reported by Jin et al. [17]. LDPE samples were subjected to
extensive extrusion cycles up to one hundred cycles. The results showed that the complex
viscosity of the samples at a low test frequency of 0.628 rad/s shown in Fig. a5 increased
with increasing the number of extrusion cycle, this observation was attributed to the
crosslinking reactions took place throughout LDPE chains during recycling process due to the
presence of the reactive carbon radicals [18, 19]. The same tend was observed in the melt
flow index (MFI) measurements where it was noted that MFI of the material decreased as the
number of extrusion cycle increased (Fig. b5). The results of this work revealed that the
processability of LDPE is only affected after the 40th extrusion cycle. Also the results
reported by Kartalis et al. [20] on the recycling of LDPE/medium density polyethylene
(MDPE) blend revealed that even after five successive extrusion cycles, the material shows
significant processing stability.
Vallim et al. [21] recycled high density polyethylene (HDPE) waste by blending with virgin
polyamide (PA6) using a twin-screw extruder. The characterization of the prepared blend
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revealed that the mechanical properties and thermal stability of the blend improve by the
using of PA6 which attributed to the decrease of size domains of the recycled HDPE.
2.3.2. Chemical recycling:
Puente et al. [22] studied the chemical recycling of LDPE using fluid catalytic cracking
(FCC) method at 500 °C in the presence of various commercial catalysts. The process was
performed in a solution of LDPE using toluene as a solvent. The results showed that the FCC
products were qualitatively similar in all the catalysts and the contribution is centered mainly
in the gasoline fraction, with high aromatic content, although the production of gases is also
important, with a high proportion of valuable light olefins C3–C4; isoparaffins C4–C5 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 kinetics of copyrolysis for 10 mass% solutions of wax/oil from
LDPE or PP with naphtha in the temperature range from 740 to 820 °C at residence times
from 0.09 to 0.54 s using industrial ethylene units were studied. The results showed that it is
possible to perform polyalkenes recycling via the copyrolysis of polyalkene oils and waxes
with conventional liquid steam cracking feedstocks on already existing industrial ethylene
units.
Thermal cracking method in the presence of phenol as a solvent was also used for the
chemical recycling process of HDPE in a work reported by Vicente et al. [24]; the effect of
phenol on the thermal cracking process was investigated. The results showed that the main
products in the cracking reaction were olefins which are very important for the petrochemical
industry and the presence of phenol as a solvent can promote the cracking reaction due to its
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role in favoring random scissions and chain reactions, as shown in Fig. 6, which can explain
the plastic conversions obtained as well as the yields and selectivity's of each hydrocarbon
product.
Achilias et al. [25] studied the chemical recycling of PE's (LDPE and HDPE) and PP
obtained from various applications including packaging film, bags, pipes, and food-retail
outlets using two techniques:
� Dissolution/reprecipitation method using different solvents and non-solvents.
� Catalytic pyrolysis using FCC method.
In the first technique the polymers were dissolved in xylene and reprecipitated using n-
hexane and the pure polymers were then dried. In the second one the polymers were thermal
cracked in the presence of FCC catalyst. The pure polymers obtained from the first technique
were mechanically tested and compared with the mechanical properties of the waste whereas
the products of the second technique were characterized using spectroscopes methods. The
first leads to high recovery of polymer with the disadvantage of using large amounts of
organic solvents. From the measurements of the tensile mechanical properties of samples
after dissolution/reprecipitation process, it was found that the pure polymers produced from
this process are almost identical to virgin polymers. Furthermore, pyrolysis was investigated
as a promising technique for thermochemical recycling of these polymers where a series of
alkanes and alkenes of different carbon number, which can be used in petrochemical industry,
could be recovered using pyrolysis method.
In another work reported by Achilias et al. [26], dissolution–reprecipitation technique was
investigated for recycling polymers from plastic packaging waste such as PE, PP, PET, and
PVC. The mechanical properties of the polymers were compared before and after recycling
process. The result showed that very good polymer recoveries could be obtained in almost all
waste samples examined, while lower values in some samples were attributed to the removal
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of additives present in the original waste products. Also it was found that the mechanical
properties of the polymers changed slightly after the recycling process.
In the work reported by Wei et al. [27], FCC method was also used for the chemical recycling
of plastics waste compounded from PP, LDPE and HDPE. The effect of two types of catalysts
(zeolitic and non-zeolitic) on the yield and the reaction selectivity was investigated and
compared. It was found that the zeolitic catalysts give higher yields comparing with non-
zeolitic catalysts in case of volatile hydrocarbons, various types of catalysts and their yield
and selectivity at 360 °C are shown in Table 1.
2.4. Polypropylene (PP) and its systems:
2.4.1. Mechanical recycling:
Aurrekoetxea et al. [28] determined the morphology and properties of PP samples subjected
to several injection cycles. The results showed that the melt viscosity of PP decreased after
processing which was attributed to the molecular weight decreasing of PP induced by
reprocessing. Also it was found that the recycled PP exhibited greater crystallization rate,
higher crystallinity and equilibrium melting temperature than those measured for virgin PP.
Young’s modulus and yield stress increased with the number of injection cycles due to the
higher crystallinity of PP after processing whereas decreasing of PP molecular weight
resulted in reduction of elongation at break and fracture toughness of the samples.
Phuong et al. [29] investigated the recyclability of polypropylene/organophilic modified
layered silicates nanocomposites using a twin screw extruder at different temperatures for ten
times throughout the changes in rheological and mechanical properties of the composites
after each processing cycle. The results showed that the MFI increased with increasing the
number of extrusion cycle; this behavior was reported in several works focused on the
recycling and stability of PP and was attributed to the thermal degradation of PP during the
extrusion [30, 31]. The mechanical results showed that tensile strength of the composites
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decreased with increasing the number of the extrusion cycle whereas the impact strength
reminded constant with increasing the extrusion cycle (Fig. 7). In another work [32], the
effect of recycling on the rigidity of PP/vegetal fiber composites prepared by extrusion
process was also investigated. The results showed that the rigidity of the composites nearly
reminded constant after the processing cycles due to the good stabilization of the fibers aspect
ratio after recycling.
The recyclability of other PP composites were also investigated by Bahlouli et al. [33], where
the effect of recycling on the properties of PP – based composites (ethylene propylene diene
monomer (EPDM)/PP and talc/PP) using extrusion process was examined. Rheological,
mechanical and structural properties of the composites were 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 mechanical properties of
the composites decreased with processing number. All noted changes in the composites
properties were attributed to the changes in the structural properties of the composites during
the processing cycle. The obtained results from this work were useful for optimizing the
recycling process and for a better use of the recycled materials in components design.
2.5. Polystyrene (PS) and its systems:
2.5.1. Mechanical recycling:
Brennan et al. [35] studied the effect of recycling on the properties of ABS, high impact
polystyrene (HIPS) and the blend of ABS waste and HIPS waste. Mechanical and thermal
properties of the virgin and recycled polymers (ABS and HIPS) were determined and
compared. The results showed that in the two cases (ABS and HIPS) the effect of recycling
on the tensile strength and tensile modulus of the blend, which has a small portion of one of
the two components (ABS or HIPS) were negligible (Figs. a8 and b8), but the strains at break
and impact strength of the blend reduced considerably comparing with virgin and pure
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components (ABS and HIPS) (Figs. c8 and d8). The previous results reveal that the blending
of small amount of ABS or HIPS could result in improving the tensile strength and modulus
of the blend after recycling. Elmaghor et al. [36] used poly(ethylene-co-vinyl acetate) and
poly(styrene-b-ethylene/butylenes-b-styrene) as compatibilizers for a ternary blend prepared
from waste polymers including PS, HDPE, and PVC using a single-screw extruder. For more
improving in the compatibility between the components of the prepared blend, the extrudates
were subjected to gamma radiation. The results showed that both the compatibilizers and
irradiation improved the mechanical properties of the blend where impact strength and
ductility of the blend was sharply enhanced and the improvement of tensile strength was
moderate.
The effect of the reprocessing cycles on properties and structure of PS nanocomposites
containing 5 wt. % organophilic clay, which is commercialized under the trade name Cloisite
15 A, was investigated by Remili et al. [37]. Rheological, mechanical, and structural
properties of the composites were determined after each processing cycle and compared to
those of pure PS. The results presented in Fig. 9 showed that the composite (PS/Cloisite 15
A) has better recyclability and reprocessability compared to pure PS where the decreasing of
the melt viscosity and mechanical performance was more pronounced in pure PS comparing
with that in the composite. This behavior was attributed to the increase in the molecular
weights of the composites after 8 processing cycles due the occurrence of some crosslinking
[38], whereas the molecular weights of pure PS decreased by ~49 % after 8 cycles.
2.5.2. Chemical recycling:
The chemical recycling of PS waste was reported firstly in the work of Lee et al. [39] by
using of clinoptilolites as catalysts, they found that clinoptilolites possess a good catalytic
activity for the degradation of PS with very high selectivity to aromatic liquids. Arandes et al.
[40] studied the thermal cracking of PS and polystyrene-butadiene (PSB) dissolved in light
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cycle oil (LCO) in the presence of mesoporous silica as a catalyst. The cracking of the
PS/LCO blend produced high yields of styrene, whereas the cracking of the PSB/LCO blend
resulted in a stream of products with petrochemical interest.
The using of FCC method for the chemical recycling of PS waste was reported by Williams et
al. [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 commercial waste products as the feedstock. It was
found that the pyrolysis oil fraction could be re-polymerized again to produce virgin PS.
However, aromatic compounds included in this fraction may act as chain transfer agents,
resulting in alterations in the shape of the reaction rate curve and lowering significantly the
average molecular weight and the glass transition temperature of the PS prepared form the
pyrolysis process.
Dissolution–reprecipitation process for chemical recycling of PS waste was reported recently
[43, 44] and the effect of the temperature and oils from natural sources as solvents on
stability of PS chains during the process was investigated. The results showed that the
solubility of PS in the solvents increased as the temperature of dissolving increased and these
solvents have slightly effect on the PS chains so it was to say that it is possible to use these
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solvents for PS waste recycling at high temperature (up to 60 ºC) without sharp decreasing in
the molecular weight of the recycled polymer as well as the possibility of reusing the solvents
again.
2.6. Acrylonitrile–butadiene–styrene copolymer (ABS) and its systems:
2.6.1. Mechanical recycling:
Boronat et al. [45] studied the effect of reprocessing cycle conditions (temperature and shear
rate) on the properties of ABS. Two grade of ABS were injected and tested (high viscosity
grade and low viscosity grade). It was found that each of the two grades shows a different
behavior upon reprocessing where the low viscosity grade showed a reduction of viscosity
with increasing the number of processing cycles, which was attributed to the degradation of
this polymer, whereas the high viscosity grade, conversely, showed an increase of melt
viscosity as the number of processing cycles increased.
Perez et al. [46] studied the effect of reprocessing on mechanical, thermal and rheological
properties of ABS. The results showed that neither melt viscosity nor tensile strength were
affected by the number of processing cycles, but the impact strength decreased slightly, so it
was to say that ABS has good mechanical recyclability and for improving impact strength
after recycling, toughness agents are needed. These results are in consistent with those
obtained by Karahaliou and Tarantili [47] where the stability of ABS subjected to five
extrusion cycle was investigated. The mechanical and rheological properties showed that
ABS has good stability during the processing cycles.
In another work [48], ABS waste was used as an additive to virgin ABS and the effect of
waste content on mechanical properties of the blend was evaluated. The finding of this work
revealed that there is no obvious effect of ABS waste content on the tensile strength,
elongation at yield, flexural strength, flexural modulus, and impact strength. However,
hardness, melt flow index, and glass transition temperature of blend increased with increasing
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the waste content in the blend. The effect of ABS waste content on the properties of a blend
consisted of virgin and recycled ABS was also investigated by Scaffaro et al. [49]. The effect
of reprocessing cycles on the physical properties of the prepared blends was also examined.
The results showed that the effect of recycled ABS content on the viscosity of the blend was
not significant. However, at all blend compositions the viscosity of the blends decreases with
the number of the processing cycle (Fig. a10). In the case of mechanical properties it was
found that the tensile strength, tensile modulus, elongation at break and impact strength of the
blends decreased with the number of the processing cycle and decreased slightly with
increasing the recycled ABS content in the blend (Fig. b10).
Yeh et al. [50] used ABS waste for preparing ABS/wood composites and compared with
counterpart composites prepared by virgin ABS. The composites with 50% wood and a
coupling agent were prepared using a twin-screw extruder and characterized in term of
mechanical properties. It was found that while the impact strength and ductility of the virgin
and recycled polymers were significantly different, the composite properties differed only
slightly from each other. Recently Bai et al. [51] suited the effect of reprocessing on the
mechanical properties of ABS/CaCO3 composites. It was found that at a low content of
CaCO3 (less than 10 %), the impact strength of the composites decreased with the number of
processing cycle which was attributed to the thermal degradation of the rubber phase in ABS,
whereas at a high content of CaCO3 (higher than 15 %) impact strength of the composites
increased with the number of processing cycle.
2.6.2. Chemical recycling:
Szabo et al. [52] recycled ABS and ABS/polymethylmethacrylate (PMMA) blend waste using
thermal decomposition method. The results of this work showed that the decomposition
temperature of the blend is less than that of pure ABS, but the additional products derived
from ABS detain the direct feedstock recycling of the MMA monomer. In another work [53],
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the thermal degradation of denitrogenated ABS samples (DABS) prepared by sequential
hydrolysis of ABS using polyethyleneglycol (PEG)/NaOH was reported and it was found that
effective denitrogenation of ABS before pyrolysis is beneficial to produce clean oil during
pyrolysis.
2.7. Polycarbonate (PC) and its systems:
2.7.1. Mechanical recycling:
Elmaghor et al. [54] used virgin ABS grafted maleic (ABS-g-MA) anhydride for modifying
PC waste. Blends of ABS-g-MA/PC were prepared using a twin-screw extruder and
characterized. It was found that the mechanical properties of the waste could be enhanced by
the addition of ABS-g-MA which was explained based on the reaction took place between
MA functional group in ABS-g-MA and end hydroxyl groups of PC resulting in bridges
formation between various phases in the blend and enhancement of the mechanical
performance of the material.
In another work [55], blends of PC waste and polyethylene terephthalate (PET) waste were
prepared in different ratio using a twin-screw extruder at 270 °C. The obtained granules of
the prepared blends were molded using two different methods, compression molding and
injection molding; and characterized in term of thermal, mechanical and rheological
properties (Fig. 11). It was found that the blend (PC waste/PET waste) showed good
mechanical properties which attributed to the interfacial reaction between the blend
components (transesterification between PET waste and PC waste occurs during blending in
the molten state resulting in a good compatibility in the blend. The compatibility between the
blend components was detected using the Tg values where it was found from differential
scanning calorimetry (DSC) measurements that the Tg associated with the PET phase in all
blends is higher than that in the case of the individual component (PET) whereas it was
higher in PC after blending (Fig. 12). In another work, Maria and Sanchez [56] studied the
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recyclability of PC/butylene terephthalate (PBT) blend through evaluation the influence of
recycling on the mechanical properties of the blend. The result showed that both tensile
strength and tensile modulus were slightly affected with the recycling process so it was to say
that the PC/PBT blend has good mechanical recyclability.
2.7.2. Chemical recycling:
The chemical recycling of PC waste was reported by Hidaka et al. [57], the finding of this
work showed that the main products of the chemical recycling of PC waste are bisphenol A
(BPA) and carbohydrate carbonates. Hata et al. [58] reported the obtaining of 1,3-dimethyl-2-
imidazolidinone (DMI) and BPA from the chemical recycling of PC waste through the
treatment of PC waste with N,N′-dimethyl-1,2-diaminoethane (DMDAE) in the presence of
dioxane as a solvent.
Recently a simple chemical recycling method of PC waste was suggested by Tsintzou et al.
[59], they studied the chemical recycling of PC waste with water in microwave reactor in the
presence of NaOH under controlled conditions of temperature and pressure, the results
showed that PC degradation higher than 80% can be obtained at 160 °C after a microwave
irradiation time of either 40 min or 10 min using either a 5 or 10% (w/v) NaOH solution.
2.8. Polyethylene terephthalate (PET) and its system:
2.8.1. Mechanical recycling:
Navarro et al. [60] prepared a blend of virgin HDPE and PET waste using the extrusion
process in efforts to improve the performance of PET waste. The prepared blend, in different
ratio, was injected to obtain test samples for mechanical characterization. Thermal and
rheological properties of the prepared blends were also evaluated. It was found that the
presence of HDPE in the blend reduces the melt viscosity of the blend indicating good flow
ability compared to PET waste (Fig. 13). However, incompatibility between HDPE and PET
in the blend was detected at a content of HDPE higher than 5% resulting in poor mechanical
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properties compared 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 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 and ABS was fabricated and for reducing the interfacial tensions
between PET and ABS in the blend, SiO2 was incorporated. The results of this study showed
that the mechanical properties of composites improve with increasing the SiO2 content.
Very recently Mantia et al. [62] modify PET waste by the addition of small amounts of virgin
PLA using melt mixing technology. The effect of the small amounts of PLA on the
mechanical and rheological properties of the prepared blend was investigated. The results
showed that the viscosity of the blend is less than that of PET, but the blend possessed higher
thermal sensitivity compared to PET waste (Fig. 14). The mechanical results revealed that the
small amount of PLA do not affect on the tensile properties, where the mechanical properties
of the blend were similar to those of PET
2.8.2. Chemical recycling:
For chemical recycling of PET waste, the microwave irradiation in the presence of ethylene
glycol (EG) and zinc acetate as catalysts was used [63]. The yield of the main product (bis(2-
hydroxyethyl) terephthalate (BHET)) was nearly same as that obtained by conventional
electric heating. However, the time taken for completion of reaction was reduced drastically
from 8 h to 35 min leading to substantial saving in energy. Shukla et al. [64] used ethylene
glycol and sodium sulfate for the chemical recycling of PET waste. The BHET obtained as a
main product of the reaction was used to hydrophobic disperse dyes for synthetic textiles.
Fonseca et al. [65, 66] investigated the effect of various metal salts (zinc acetate, sodium
carbonate, sodium bicarbonate, sodium sulphate and potassium sulphate) 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
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it could control the yield of the reaction where the main product of the reaction was BHET
and the highest yields of the reaction were obtained with zinc acetate and sodium carbonate at
196 °C.
Achilias et al. [67] depolymerized PET waste in the presence of diethylene glycol
(glycolysis) under microwave irradiation. The reaction was carried out in a sealed microwave
reactor in which the pressure and temperature could be controlled. The depolymerization
products were characterized using FTIR. The results were compared to that obtained without
using microwave irradiation. It was found that the complete degradation of PET can be done
at temperatures higher than 180 °C. Also the results showed that in the normal condition
(without irradiation), the reaction needs more than 4 h to complete the degradation of PET,
which confirms the importance of the microwave power technique and the substantial energy
saving achieved.
Also in another work done by Achilias et al. [68] PET waste were depolymerized in the
presence of ethanolamine (aminolytic) under microwave irradiation to enhance the waste
degradation. The main product of the reaction was bis(2-hydroxyethyl) terephthalamide
(BHETA). The results showed that the complete degradation of PET waste was done at
temperatures higher than 260 °C. By comparing with the previous work reported the using of
diethylene glycol [67], it could be said that the presence of diethylene glycol caused in
complete degradation of PET at lower temperature compared to the presence of ethanolamine.
2.9. Polyamides (PA’s) and their systems:
2.9.1. Mechanical recycling:
Su et al. [69] studied the effect of the reprocessing on the mechanical and rheological
properties of PA6. PA6 sample was processed sixteen times and its properties were compared
with those of the virgin PA6. The results showed a reduction in the molecular weight and an
increase in the molecular weight distribution as a consequence of a decrement in melt
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viscosity of PA6, but no chemical changes were noted in the FTIR spectra of PA6 structure
during the recycling process. The mechanical test showed that the tensile strength increased
after each processing cycle while the impact strength decreased. Bernasconi et al. [70]
blended waste of short glass fiber reinforced PA66 with virgin materials and studied 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 increasing the content of the waste.
Goitisolo et al. [71] studied the effect of reprocessing on the properties of PA6
nanocomposites using injection molding for five times. The properties of the nanocomposite
were determined after each processing cycle. Also, no chemical changes in the FTIR spectra
of PA6 were observed in the composite during the processing cycles as same as in the case of
pure PA6 recycling [69, 72], but the viscosity of the composite decreased sharply with the
number of processing cycle which was attributed to the decrease of the molecular weight. In
addition, the results of this work showed that strain at break of the composite decreased with
the number of processing cycle, which indicated less ductility of the material.
Hassan et al. [73] studied the mechanical, thermal and morphological properties of irradiated
PA6/66 waste and rubber waste powder blends. The properties of this blend were compared
to that of the non-irradiated blend. The results showed that the incorporation of rubber waste
into recycled PA led to decrease the mechanical performance because of the too weak
interfacial adhesion between the two components in the blend. However, the irradiation
process resulted in improve the compatibility of the prepared blend. In another work by
Hassan et al. [74], the effect of carbon black (CB) content on the properties of irradiated
blends containing PA6/66 waste and rubber waste powder was reported. The results showed
that the tensile strength, elongation at break and Young's modulus of the blend increase with
increasing CB content. Also El-Nemr et al. [75] studied the effect of acrylonitrile butadiene
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rubber (NBR) content on the properties of irradiated blends of PA6/66 waste and rubber
waste. The results showed that the mechanical properties improve by the addition of NBR.
Dorigato and Fambri [76] used recycled short fiber of PA66 in the reinforcing process of the
commercial grade of PA12, and studied the effect of PA66 content on the thermal and
mechanical properties of PA66/PA12 blend. It was found from this study that Tg of PA12
increases with increasing the fiber content. Mechanical properties and the morphology of the
material indicated good interfacial adhesion between the components whereas the thermal
stability of PA12 decreased slightly as the content of PA66 increased (Fig. 16).
3. Conclusion:
Form the previous discussion about the recycling process of polymeric waste it could be
concluded that the mechanical recycling is the most preferred and used recycling method
comparing with the chemical recycling method in which the waste are subject to complicated
chemical treatments.
Based on mechanical recycling results, the incorporation of minor amounts of virgin
polymers with waste from same or other polymers in the presence of suitable compatibilizers
can result in good properties compared to those of the waste, for example, the blending of
PLA with PET waste resulted in a good thermal stability of the fabricated blend which was
almost similar to that of PET waste. Also, in the case of PC waste the addition of ABS
together with compatibilizers led to improve the mechanical properties compared to those of
the waste.
As a final conclusion, the using of the blending technique in the presence of suitable
compatibilizers for the mechanical recycling of waste form polymer materials should be
given more interest in the future.
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Table Captions:
Table. 1 Summary of the main products of LDPE/HDPE blend degradation at reaction
temperature of 360 ºC over various catalysts
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Figure captions:
Fig. 1 Effect of processing number on the intrinsic viscosity of PLA/PS (50/50) blend [9]
Fig. 2 Effect of processing number on the stress at break and granules shape of PLA/PS
(50/50) blend (Ex: extrusion cycle) [9]
Fig. 3 Chemical recycling of PLA/PE and PLA/PBS blends
Fig. 4 Triboelectrostatic charging sequence of various polymers
Fig. 5 Complex viscosity and MFI of LDPE after extrusion cycles [17]
Fig. 6 Role of phenol in the thermal cracking of HDPE [24]
Fig. 7 Effect of the extrusion cycle on the mechanical properties and MFI of polypropylene/
organophilic modified layered silicates nanocomposites [29]
Fig. 8 Mechanical properties recycled ABS and recycled HIPS blend [35]
Fig. 9 Effect of recycling on the melt viscosity and mechanical properties of PS and
PS/Cloisite 15 A nanocomposite [37]
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]
Fig. 11 Recycling process of PC/PET blend
Fig. 12 Effect of the reaction time on Tg values of PC/PET blend components [55]
Fig. 13 Effect of HDPE content on the viscosity of the recycled PET [60]
Fig. 14 Effect of PLA content on the thermal stability of the recycled PET [62]
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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]
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Table. 1 Summary of the main products of LDPE/HDPE blend degradation at reaction temperature of 360 ºC over various catalysts
Degradation results
Catalyst type
USY
(zeolitic c
atalysts)
ZSM-5
(zeolitic catalyst
s)
MOR
(zeolitic catalyst
s)
ASA
(non-zeolitic cat
alysts)
MCM-41
(non-zeolitic cat
alysts)
Yield (wt% feed)
Gaseous 87.5 93.1 90.2 85.6 87.3
Liquid 3.7 3.3 4.3 4.7 5.6
Residue 8.8 3.6 5.5 9.7 7.1
Involatile residue 4.7 2.4 2.9 7.4 5.2
Coke 4.1 1.2 2.6 2.3 1.9
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Fig. 1 Effect of processing number on the intrinsic viscosity of PLA/PS (50/50) blend [9]
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Fig. 2 Effect of processing number on the stress at break and granules shape of PLA/PS (50/50) blend (Ex: extrusion cycle) [9]
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Fig. 3 Chemical recycling of PLA/PE and PLA/PBS blends
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Fig. 4 Triboelectrostatic charging sequence of various polymers
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Fig. 5 Complex viscosity and MFI of LDPE after extrusion cycles [17]
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Fig. 6 Role of phenol in the thermal cracking of HDPE [24]
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Fig. 7 Effect of the extrusion cycle on the mechanical properties and MFI of polypropylene/organophilic modified layered silicates nanocomposites [29]
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Fig. 8 Mechanical properties recycled ABS and recycled HIPS blend [35]
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Fig. 9 Effect of recycling on the melt viscosity and mechanical properties of PS and PS/Cloisite 15 A nanocomposite [37]
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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]
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Fig. 11 Recycling process of PC/PET blend
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Fig. 12 Effect of the reaction time on Tg values of PC/PET blend components [55]
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Fig. 13 Effect of HDPE content on the viscosity of the recycled PET [60]
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Fig. 14 Effect of PLA content on the thermal stability of the recycled PET [62]
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Fig.15 Reaction and analytical procedure for the glycolysis of PET wastes [65]
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Fig. 16 Effect of recycled PA66 content on the thermal stability of virgin PA12 [76]