F. PEINADO et al.: RECYCLING OF THE STYRENE FRACTION FROM POST-CONSUMER WASTE
725–730
RECYCLING OF THE STYRENE FRACTION FROM
POST-CONSUMER WASTE
RECIKLIRANJE STIRENSKE FRAKCIJE V GOSPODINJSKIH
ODPADKIH
Francisco Peinado
1
, Miguel Aldas
1,2
, Juan López-Martínez
1*
,
María Dolores Samper
1
1
Institute of Materials Technology, Valencia Polytechnic University (Universitat, 03801 Alcoy-Alicante, Spain
2
Department of Food Science and Biotechnology, Faculty of Chemical and Agroindustrial Engineering, National Polytechnic School,
170517 Quito, Ecuador
Prejem rokopisa – received: 2020-03-17; sprejem za objavo – accepted for publication: 2020-05-02
doi:10.17222/mit.2020.043
This investigation studied the recovery of styrene materials with mechanical recycling of different natural domestic waste. The
aim of this work was to evaluate the possibility of obtaining a material capable of competing, in some respects, with the virgin
material. The two main problems of the recycling and recovery process were also addressed. These include the presence of
impurities from other plastic materials, such as polyvinyl chloride (PVC) or polypropylene (PP), and the degradation of the
recovered materials due to previous processing or the activities of external agents such as sunlight. In both cases, it was found
that the recovered material exhibits a decrease in its mechanical properties and it is also difficult to recover. The use of the
instrumental-analysis techniques, such as differential scanning calorimetry (DSC), is essential for predicting the quality of the
material recovered from the residue.
Keywords: recycling, styrene materials, polystyrene, impurities, waste
V ~lanku so avtorji {tudirali ponovno uporabo stirenskih materialov z mehanskim recikliranjem razli~nih naravnih doma~ih
odpadkov. Cilj tega dela je bil ovrednotiti mo`nosti pridobivanja materiala, ki bi bil enakovreden ~istemu stirenu za nekatere
vrste uporabe. Pri tem so analizirali dva glavna problema, ki nastopata med recikliranjem in pri ponovni uporabi. To so
predvsem ne~isto~e zaradi prisotnosti drugih plasti~nih materialov, kot sta na primer: polivinilklorid (PVC) in propilen (PP).
Drug problem pa je degradacija odpadnih materialov zaradi predhodnih procesov ali delovanja zunanjih vplivov, kot je son~na
svetloba. V vseh primerih so poleg poslab{anja mehanskih lastnosti naleteli tudi na te`ave pri procesu ponovne uporabe. Avtorji
ugotavljajo, da je pri tem klju~na uporaba modernih in{trumentalnih metod analize, med drugim diferencialna vrsti~na
kalorimetrija (DSC), za napoved kakovosti recikliranega materiala iz odpadkov.
Klju~ne besede: recikliranje, stirenski materiali, polistiren, ne~isto~e, odpadki
1 INTRODUCTION
At present, researchers are examining the appro-
priateness of the use of plastic materials due to the
impact of plastic waste on the environment. The accumu-
lated-plastic-waste generation between 1950–2015 was
estimated to be 381000 million tons.
1
In this period, the
industrial production of plastics grew very fast so that it
is estimated that only about 20 % of the generated plastic
waste was recycled or incinerated. Although this index
of recycling is expected to improve, the generation of
plastic waste and the problems linked to it will increase.
Due to their own characteristics and the variety of plastic
materials found in the municipal solid waste (MSW), the
polymeric materials found there are the most difficult to
treat and recover. The plastic waste represents 14 % of
the MSW even though it occupies about 40 % in volume
due to its low density. According to H. Ritchie, M.
Roser, with 302 million tons of plastic waste estimated
for 2015, the packaging industry is its largest generator,
as can be seen from the data in Table 1.
2
Table 1: Waste generation by the industrial sector estimated for 2015
Industry
Waste generation
(%)
Packaging 46.7 %
Textile 12.6 %
Consumer goods 12.3 %
Automotive and transport items 5.6 %
Building and construction 4.3 %
Electrics and electronics 4.3 %
Industrial machinery 0.3 %
Others 13.9 %
Developing countries are more problematic in this
respect than developed countries. Although developed
countries are the largest waste generators, they are also
more efficient at addressing these problems.
3,4
On the
contrary, developing countries have less control over the
waste generated.
5
This can be confirmed with the data
indicating mismanaged plastic waste, shown in Fig-
ure 1.
2
Regardless of the region, pollution is a problem
Materiali in tehnologije / Materials and technology 54 (2020) 5, 725–730 725
UDK 67.017:658.567.3:620.28 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)725(2020)
*Corresponding author's e-mail:
jlopezm@mcm.upv.es (Juan López-Martínez)

that transcends borders and nations as seen in the case of
the microplastic pollution of the sea.
6
Therefore, the problem of plastic waste can be
addressed with the recycling of materials,
7
incineration
of waste
8
and use of biodegradable materials.
9
Among
these possibilities, in this work, we addressed the im-
provement of the recycling technology. Within different
types of plastics, the recovery of polystyrene materials
constitutes an important line of research as they are the
most consumed polymeric materials after polyolefins
and PVC, covering approximately 7.5 % of the total con-
sumption. In the group of polystyrene materials, it is
possible to find both common-use materials, such us
polystyrene (PS), styrene-butadiene (SB), and technical
plastics, such as styrene-acrylonitrile (SAN), acrylo-
nitrile styrene acrylate (ASA), acrylonitrile butadiene
styrene (ABS).
Moreover, the recovery of household polystyrene
waste was analysed in some studies. Polystyrene is a
material present in packaging as well as in many every-
day objects such as toys, hangers, office supplies and
others. Among plastic materials, polystyrene represents
about6%o ft h ew aste generated, exceeded only by
polyolefins and polyethylene terephthalate (PET), as can
be seen in Table 2.
1
It ranks seventh in the list.
Table 2: Generation of waste by type of plastic material as estimated
in 2015
Plastic material Percentage
Low-density polyethylene, LDPE 20.0 %
Polypropylene, PP 19.3 %
Polypropylene fibre reinforced polymer,
PP-fibre
14.7 %
High-density polyethylene, HDPE 14.0 %
Polyethylene terephthalate, PET 11.2 %
Polystyrene, PS 6.0 %
Polyurethane, PU 5.6 %
Polyvinyl chloride, PVC 5.3 %
Others 3.9 %
Regarding the possibilities of plastic-waste recovery
and revalorization, PS presents several problems.
10
Due
to the variety of its uses, it is difficult to visually separate
PS from a waste stream. In this respect, it is very
different from PE and PET whose packaging residues are
easy to identify. Moreover, during a water-residue sepa-
ration, when floating, its separation is difficult due to the
density (1.05 g/cm
3
).
11
When the separation of waste residues is easy,
excellent-quality recycled materials are obtained, for
example, in the case of the waste from polystyrene
foams.
12,13
Nevertheless, when some impurities are
found, the separation process becomes difficult. The
existence of aromatic rings causes PS to have a low
compatibility even with the polymers derived from
styrene, such as acrylonitrile-butadiene styrene (ABS),
where the use of compatibilizers for their mixtures is
recommended.
13
To assess this behaviour, the aim of this work was to
study the influences of5%ofimpurities on the mecha-
nical properties of recycled polystyrene. As impurities,
we selected the materials that can be found in packaging
(PS, PET and PVC),
14
materials from electronic waste
(polyoxymethylene (POM), ABS and polycarbonate
(PC))
15
and biodegradable materials mainly found in the
packaging industry (poly(lactic acid) (PLA), thermoplas-
tic starch (TPS) and Poly(3-hydroxybutyrate) (PHB)).
16
2 EXPERIMENTAL PART
2.1 Materials
Recycled polystyrene (PS) from food packaging and
recycled polypropylene (PP) with reference PP1B were
obtained from scrap. Neat low-density polyethylene
(LDPE) Resistex 1614 FG and neat high-density poly-
ethylene (HDPE) Alcudia 4910 were provided as pellets.
Recycled polycarbonate (PC), recycled acrylonitrile
butadiene styrene (ABS) and polyoxymethylene (POM)
came from electrical- and electronic-equipment waste
(WEEE). These materials were kindly supplied by
Acteco S.L. (Ibi, Spain).
Polyethylene-terephthalate (PET) type Bripet 1000
for bottle manufacturing, with an intrinsic-viscosity
index of 0.80 dL/g, was obtained from Novatpet
(Huesca, Spain). Recycled polyvinyl chloride (PVC)
from credit-card waste was supplied by Crearplast, S.L.
(Ibi, Spain). This PVC has a k-value of 62 (the kwert
value is the viscosity measurement correlated with the
molecular weight obtained in line with ISO 1628-2).
With regard to the biodegradable materials,
poly(lactic acid) (PLA), commercial grade 6201D
IngeoTM Biopolymer, was kindly supplied by Nature-
Works LLC (Minnetonka, USA). This grade contains
2 % of D-lactic acid. Thermoplastic starch (TPS), com-
mercial grade Mater-Bi® NF 866, was kindly supplied
from Novamont SPA (Novara, Italy). Poly(3-hydro-
xybutyrate) (PHB), commercial grade P226 with an
F. PEINADO et al.: RECYCLING OF THE STYRENE FRACTION FROM POST-CONSUMER WASTE
726 Materiali in tehnologije / Materials and technology 54 (2020) 5, 725–730
Figure 1: Mismanaged plastic waste by region for 2015

average molecular weight of 426 KDa, was supplied by
Biomer (Krailling, Germany). These materials were used
in the form of pellets.
The main technical characteristics of the materials
are presented in Table 3.
Table 3: Main properties of the used materials
Material
Melt-flow index (MFI)
(g/10 min)
Density
(g/cm
3
)
PS 9 (200 °C/5 kg) 1.05
PP 20 (230 °C/2.16 kg) 0.905
LDPE 1.4 (190 °C/2.16 kg) 0.916
HDPE 0.90 (190 °C/2.16 kg) 0.949
PC 6.1 (230 °C/5 kg) 1.2
ABS 20.5 (230 °C/5 kg) 1.05
POM 9 (190 °C/2.16 kg) 1.41
PLA 22.5 (210 °C/5 kg) 1.24
TPS 3.5 (150 °C/5 kg) 1.27
PHB 10 (180 °C/5 kg) 1.25
2.2 Sample preparation
All blends were formulated, containing 95 % of PS
and5%ofanother material considered as an impurity.
The blending process of PS with biodegradable polymers
was carried out using a screw extruder from Dupra S.L.
(Castalla, Spain). The extrusion conditions included a
rotor speed of 30 min
–1
and temperature of 180–200 °C.
After that, samples for mechanical characterization were
prepared with injection moulding on a Babyplast 6/6
from Cronoplanst S.L. (Abrera, Spain). The injection
samples were normalized specimens for the tensile test
with dimensions following ISO-527.
2.3 Sample characterization
The mechanical characterization was carried out
using tensile and hardness tests. Tensile properties were
assessed using a universal machine ELIB 30 from S.A.E
Ibertest (Madrid, Spain) under the ISO-527 standard
conditions. All the tests were carried out at room tempe-
rature at a speed of 10 mm/min, with a load cell of 5 KN.
Five test pieces of each samples were characterized.
The Charpy impact resistance was measured using a
Metrotec SA machine (San Sebastian, Spain), under the
ISO-179 standard, with a 6-J pendulum. Five specimens
were tested.
Fourier transformed infrared spectroscopy (FTIR)
measurements were carried out using a Perkin-Elmer
Spectrum BX infrared spectrometer from Perkin-Elmer
Spain S.L. (Madrid, Spain) in a range of 4000–650 cm
–1
,
with a resolution of 4 cm
–1
and 32 scans.
Differential scanning calorimetry (DSC) was
conducted with a Mettler Toledo DSC 821 equipment
from Mettler Toledo (Schwerzenbach, Switzerland)
using samples of 4–6 mg. The heating and cooling prog-
rams were performed at 20 °C/min in a nitrogen atmos-
phere (60 mL/min), with the first heating from 25 °C to
200 °C, then a cooling cycle from 200 °C to 50 °C and
the second heating from 50 °C to 200 °C.
3 RESULTS AND DISCUSSION
Regarding the mechanical behaviour reported in
Figure 2, the influence of the impurities on the PS
properties changes according to the material used. For
example, when mixed with POM, the reduction of the
tensile strength reaches 20 %. This means that the tensile
strength of the blend with impurities is 80 % of the
tensile strength of the pure PS. On the other hand, some
materials, such as LDPE or HDPE, cause a 5-% de-
crease. Technical materials, like PET or PC, seem to
have a larger influence on the properties of the recycled
PS. This is due not only to the chemical incompatibility
but also to the high temperatures required for processing
technical plastics.
The Charpy impact energy allows us to easily verify
the influence of different impurities on the properties of
the recycled materials. In Figure 2, it is possible to
observe that the decrease in the toughness (linked to the
impact energy) is acceptable when using polyolefins
(HDPE, LDPE). Nevertheless, when using technical
plastics (PC, PET, POM), the decrease in the toughness
becomes important. In the case of POM, its easy degra-
dation behaviour becomes a disadvantage in a PS recov-
ery process. With regard to the presence of a biodegrad-
able material, PLA does not have a high influence, but
TPS and PHB exert an important influence on PS.
Therefore, in a PS-recycling process, it is important to
control the grade of impurities of different materials. In
addition, it is important to establish the tolerance limits
depending on the final properties desired.
With regard to certain impurities, it is necessary to
confirm their presence in the recycled PS. In this case,
F. PEINADO et al.: RECYCLING OF THE STYRENE FRACTION FROM POST-CONSUMER WASTE
Materiali in tehnologije / Materials and technology 54 (2020) 5, 725–730 727
Figure 2: Effects of 5-% impurities on the mechanical properties of
polystyrene

infrared spectroscopy is a very useful control tool. The
FTIR spectra of PS are very characteristic, allowing us to
identify the presence of impurities, as can be seen in Fig-
ure 3.
In the case of polyolefins, due to the difficulties of
comparing the FTIR spectra of the contaminated sam-
ples, differential scanning calorimetry (DSC) is a more
efficient technique for detecting the presence of impuri-
ties. In Figure 4, we can see DSC curves of the PS and
PS with impurities. Due to the amorphous microstructure
of PS, fusion enthalpies of the polyolefins are clearly
detected. Moreover, depending on the temperature of the
peak, it is possible to infer the type of polyolefin (PP,
HDPE, LDPE) contaminating the recycled PS.
On the other hand, the contaminations of PS due to
PVC are easily detectable. Its presence is quantifiable
with thermogravimetry (TGA). This is due to the jump
detected at about 200 °C caused by the hydrochloric
acid. In addition, depending on the mass loss detected
with TGA, it is possible to calculate the percentage of
PVC in a sample. The jump in a neat PVC represents
64 % of the mass loss.
17
In the cases of PS and PVC blends, both materials
have some compatibility. Nevertheless, the presence of
PVC decreases the toughness of the PS. There is an easy
way to predict the compatibility of the two polymeric
materials, i.e., a comparison of the solubility parameters
( ). These parameters are reported in reference,
18
but it
can also be estimated through the method proposed by P.
A. Small. Each group of organic compounds has a value
linked to the molar attraction. The sum of the contri-
butions of all the components, divided by the molecular
mass and multiplied by the density gives a theoretical
value of this parameter.
19
Based on this method, the
solubility values and the molar attraction of the mole-
cular groups calculated for the PS-PVC blends are
shown in Table 4.
Table 4: Solubility values calculated as Small’s molar-attraction
constants
Group
F
*
(cal
1/2
c.c.
1/2
)
Group
F
*
(cal
1/2
c.c.
1/2
)
-CH3
214 -C
6
H
5
735
-CH2
- 133 -O- 70
-CH< 28 -H 80–100
>C< –93 -OH 83
>C=O 275 -Cl 270
Polymer Structure  (cal) (cal
1/2
c.c.
1/2
)
Polystyrene (C8H8)
n 8.7
PVC (C2
H
3
Cl)
n
9.7
DEHP (C6
H
4
(COOC
8
H
17
)
2
) 9.5
A PS residue that has PVC impurities will not only
cause problems in the recycling process but also generate
incompatibility phenomena leading to relatively low
mechanical properties of the resulting material. There-
fore, a direct recovery of these materials is not feasible.
To solve this problem, a more efficient prior separation
must be done, although the use of compatibilizers may
also be a solution. The compatibilizers may be reactive
products or products that contribute to the mixing of the
components, such as the use of plasticizers, which can be
natural.
19,20
In this work, bis(2-ethylhexyl) benzene-1,2-
dicarboxylate, known as di(2-ethylhexyl) phthalate
(DEHP) was selected as the plasticizer. This product has
a high compatibility with PVC
21
and polar groups due to
ester and a non-polar group due to olefinic chains. These
F. PEINADO et al.: RECYCLING OF THE STYRENE FRACTION FROM POST-CONSUMER WASTE
728 Materiali in tehnologije / Materials and technology 54 (2020) 5, 725–730
Figure 3: Comparative FTIR spectra of PVC, recycled PS without
impurities and recycled PS with different percentages of PVC
impurities
Figure 4: DSC curves of recycled PS without impurities and PS with 2- and 6-% PP

characteristics make it an excellent compatibilizing
agent, with a solubility parameter close to those of both
polymers (PS and PVC). The molecular structure of
DEHP can be seen in Figure 5. Thus, the incorporation
of DEHP improves the recycling process, acting in two
ways, as a lubricant and as a compatibilizer of the two
materials.
Figure 6 shows the effect of DEHP on the Charpy
impact resistance when PS is blended with different
amounts of PVC as the impurity. Without the compati-
bilizer, the materials show a clear incompatibility, as can
be seen in the evolution of the resilience values. This
behaviour improves with the incorporation of 30 phr
(parts per hundred) of DEHP per 70 phr of PVC.
23
However, DEHP should be restricted in its used since an
excess would result in a loss of PVC properties due to
migration processes.
For a PVC:DEHP ratio of 70:30, the results show an
improvement in both the final properties and the com-
patibility of the mixture. Therefore, the incorporation of
plasticizers improves both the recovery process and the
compatibility of the mixture. For this reason, the
recovering process can be interesting from an industrial
point of view. The resulting material will have accept-
able macroscopic and microscopic properties.
4 CONCLUSIONS
The recovery and recycling of styrene is very inte-
resting since this material is responsible for6%ofthe
post-consumer waste generated. However, its recovery is
almost zero for some types of waste, such as packaging
products. The main problem for the recovery of
polystyrene (PS) is the presence of impurities in the
plastic waste. Regarding the impurities, it should be
noted that if they are polyolefins, PS is not greatly
affected, accepting5%ofimpurities. This amount can
be easily controlled with differential scanning calori-
metry. On the contrary, impurities from technical
materials, such as PET, PC or POM, make the PS
recycling and recovery difficult, even in low percentages.
Its detection can be easily done with infrared spec-
troscopy. If this kind of contamination is detected, a
more efficient separation of the residues must be per-
formed. With respect to the new biodegradable materials,
whose use is increasing, the behaviour is variable. PS
allows the presence of PLA, but it changes the
mechanical properties when having other contaminants,
such as PHB. Finally, although there is a chemical
compatibility between PS and PVC, the presence of PVC
directly affects the PS recycling process. The use of
compatibilizers allows a technically and economically
profitable recovery.
Acknowledgment
This work was supported by the Spanish State
Research Agency, within period 2018–2021 and project
MAT2017-84909-C2-2-R (Processing and optimization
of advanced materials derived from protein structures
and lignocellulosic components).
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F. PEINADO et al.: RECYCLING OF THE STYRENE FRACTION FROM POST-CONSUMER WASTE
730 Materiali in tehnologije / Materials and technology 54 (2020) 5, 725–730