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Plastic waste generation has become an important problem that critically affects marine and oceans environments. Fishing nets gear usually have a relatively short lifespan, and are abandoned, discarded and lost, what makes them one of the largest generators of ocean plastic waste. Recycled polyolefin resins from fishing nets (rFN), especially from polyethylene (PE), have poor properties due to the presence of contaminants and/or excessive degradation after its lifetime. These reasons limit the use of these recycled resins. This work aims to study the incorporation of recycled fishing nets PE-made to different grades of virgin PE, in order to evaluate the potential use of these rFN in the development of new products. The recovered fishing nets have been fully characterized to evaluate its properties after the collection and recycling process. Then, different PE virgin resins have been mechanically blended with the recovered fishing nets at different recycling contents to study its feasibility for fishing nets or packaging applications. Critical mechanical properties for these applications, as the elongation at break, impact strength or environmental stress cracking resistance have been deeply evaluated. Results show important limitations for the manufacture of fibers from recycled PE fishing nets due to the presence of inorganic particles from the marine environment, which restricts the use of rFN for its original application. However, it is proved that a proper selection of PE raw resins, to be used in the blending process, allows other possible applications, such as non-food contact bottles, which open up new ways for using the fishing nets recyclates, in line with the objectives pursued by the Circular Economy of Plastics.
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polymers
Article
Challenges and Opportunities for Recycled Polyethylene
Fishing Nets: Towards a Circular Economy
Rafael Juan 1,2 , Carlos Domínguez 1, 2, *, Nuria Robledo 1,2, Beatriz Paredes 1,2, Sara Galera 3
and Rafael A. García-Muñoz 1, 2, *


Citation: Juan, R.; Domínguez, C.;
Robledo, N.; Paredes, B.; Galera, S.;
García-Muñoz, R.A. Challenges and
Opportunities for Recycled
Polyethylene Fishing Nets: Towards a
Circular Economy. Polymers 2021,13,
3155. https://doi.org/10.3390/
polym13183155
Academic Editor: Abdel-Hamid
I. Mourad
Received: 29 August 2021
Accepted: 15 September 2021
Published: 17 September 2021
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Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1LATEP, Polymer Technology Laboratory, Rey Juan Carlos University, Tulipán St., Móstoles, 28933 Madrid,
Spain; rafael.juan@urjc.es (R.J.); nuria.robledo@urjc.es (N.R.); beatriz.paredes@urjc.es (B.P.)
2GIQA, Group of Environmental and Chemical Engineering, ESCET, Rey Juan Carlos University, Tulipán St.,
Móstoles, 28933 Madrid, Spain
3REPSOL, Repsol Technology Lab, Agustin de Betancourt St., Móstoles, 28935 Madrid, Spain;
sara.galera@repsol.com
*Correspondence: carlos.dominguez@urjc.es (C.D.); rafael.garcia@urjc.es (R.A.G.-M.)
Abstract:
Plastic waste generation has become an important problem that critically affects marine and
oceans environments. Fishing nets gear usually have a relatively short lifespan, and are abandoned,
discarded and lost, what makes them one of the largest generators of ocean plastic waste. Recycled
polyolefin resins from fishing nets (rFN), especially from polyethylene (PE), have poor properties
due to the presence of contaminants and/or excessive degradation after its lifetime. These reasons
limit the use of these recycled resins. This work aims to study the incorporation of recycled fishing
nets PE-made to different grades of virgin PE, in order to evaluate the potential use of these rFN
in the development of new products. The recovered fishing nets have been fully characterized to
evaluate its properties after the collection and recycling process. Then, different PE virgin resins
have been mechanically blended with the recovered fishing nets at different recycling contents to
study its feasibility for fishing nets or packaging applications. Critical mechanical properties for these
applications, as the elongation at break, impact strength or environmental stress cracking resistance
have been deeply evaluated. Results show important limitations for the manufacture of fibers from
recycled PE fishing nets due to the presence of inorganic particles from the marine environment,
which restricts the use of rFN for its original application. However, it is proved that a proper selection
of PE raw resins, to be used in the blending process, allows other possible applications, such as
non-food contact bottles, which open up new ways for using the fishing nets recyclates, in line with
the objectives pursued by the Circular Economy of Plastics.
Keywords:
recycled fishing nets (rFN); recycled polyethylene; high-density polyethylene (HDPE);
circular economy
1. Introduction
Plastics have become ubiquitous materials in our lives with a wide range of appli-
cations thanks to their excellent properties and low cost of production. Recent reports
estimate that around 50.7 million tonnes of plastics were demanded in Europe in 2019 [1].
These resins are used to manufacture a wide range of products, mainly in the packaging
(39.6%) and construction (20.4%) sectors. However, with the exponential growth of plas-
tic resins production, plastic pollution has become an important issue for countries and
ecosystems around the globe, and especially for marine environments [
2
4
]. According to
recent reports, is estimated that around 11 million metric tons of plastic debris ends into
the oceans every year, and is expected that without additional measures or actions, this
flow of plastics litter will nearly triple in 2040 (around 29 million metric tons per year) [
5
].
From this source of plastic waste, over 80% was originated from land, while the rest is
related to marine activities [
6
]. Measures have been currently adopted by governments
Polymers 2021,13, 3155. https://doi.org/10.3390/polym13183155 https://www.mdpi.com/journal/polymers
Polymers 2021,13, 3155 2 of 15
and industries to reduce plastic debris from land [
7
,
8
]. However, those related to marine
activities are still a challenge due to the difficulty to collect this plastic waste and the
mismanagement of discarded fishing gear.
In Europe, goods related to fishing activities (Abandoned, Lost or otherwise Discarded
Fishing Gear—ALDFG) are listed among the 10 most common debris found on coasts and
beaches (27% of total litter) [
9
]. Additionally, fishing goods have usually a short lifetime
that together with their loss at oceans and seas produces that around 640,000 tonnes of
fishing gear are discarded globally every year [
10
], although the underestimation about
the real metrics of abandoned fishing gear could hinder the real impact of these source of
contamination [
11
]. These debris lead not only to important effects on marine flora and
fauna due to plastic consumption and “ghost fishing” [
12
,
13
], but also cause and striking
economic and social harm [
14
]. To combat this concern and reduce the plastic pollution
at marine environments, important resources are being employed to develop new ways
to reduce, reuse and recycle these products [
15
], under the objective to achieve a Circular
Economy and the guidelines adopted in this sense by the European Union Council [16].
Nevertheless, recycling of fishing goods is not an easy task, as they are made from
different plastics, mostly from polyamide (PA), polyethylene (PE), and polypropylene
(PP) [
17
]. In particular, high-quality polymers need to be used to manufacture these items
owing to strict requirements of most fishing gear, specifically the fishing nets. Among these
resins, recycling rates differs depending on their ease of recycling and availability. PA is
extensively used, mainly for manufacturing of gill nets due to its toughness and elasticity.
Discarded fishing nets from PA are currently both mechanical [
18
] and mainly chemically
recycled via depolymerization [
19
]. Chemical recycling allows transforming nylon nets
back into recycled materials, obtaining a yarn that could be used in the manufacture of
clothing or in the development of several new products [20].
On the other hand, fishing nets from PE and PP polyolefins have a lower value,
as both are usually used to produce trawl nets, which suffer from abrasive damage [
21
].
Another important concern that must considered by the recyclers is the habitual presence of
organic matter [
22
], which requires an exhaustive cleaning step that increase the economic
costs of the process. For this reason, recycled resins from these materials usually have a
lower performance and makes them difficult to compete with their virgin counterpart [
23
].
Most research have focused on the use of these recyclates as reinforcements fibers for
construction applications [
24
26
] since their use as a main material for the manufacture
of new products with the same application has been previously rejected due to their poor
mechanical performance [27].
Several companies are producing recycled PA, HDPE, and PP from fishing net materi-
als, via mechanical recycling [
28
,
29
]. Even if a sorting and separation step is essential to
improve the properties of the recycled resins obtained, mechanical recycling of fishing net
could still be a viable way of using these recycled plastics in the industry. To improve their
properties and extend their use as a recycled material, blending with raw resins at different
percentages could guarantee the requirements for the desired applications. Blending of
virgin and recycled materials was demonstrated to be a feasible strategy to reuse plastic
resins and help the development of a Circular Economy of Plastic [3032].
The objective of this work is to study the incorporation of recycled polyethylene
obtained from fishing nets to different grades of virgin PE (Figure 1). Although one of
the aims is to obtain a blending material that can be used again in the manufacture of
fishing nets, considering the properties of the recycled material, other potential applica-
tions related to bottle packaging were explored. The homogeneity and possible presence
of contaminants in the recycled material were determined through Fourier Transform
Infrared Spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), Thermogravimet-
ric Analysis (TGA) and Temperature Rising Elution Fractionation (TREF) measurements.
These techniques, together with Gel Permeation Chromatography (GPC) and rheological
measurements, were also used to study the chemical composition and homogeneity of all
blends prepared. Mechanical properties were established by tensile, flexural and impact
Polymers 2021,13, 3155 3 of 15
tests. The Environmental Stress Cracking Resistance (ESCR) of these blends was also
evaluated to overall determine the long-term performance of these materials and their
suitability for fishing nets and packaging applications.
Polymers 2021, 13, x FOR PEER REVIEW 3 of 16
measurements, were also used to study the chemical composition and homogeneity of all
blends prepared. Mechanical properties were established by tensile, flexural and impact
tests. The Environmental Stress Cracking Resistance (ESCR) of these blends was also eval-
uated to overall determine the long-term performance of these materials and their suita-
bility for fishing nets and packaging applications.
Figure 1. Flowchart of the experimental procedure.
2. Materials and Methods
2.1. Materials
Discarded HDPE fishing nets from Spanish harbors were sorted and separated from
other polymers such as PA and PP to reduce cross contamination. Once collected, they
were washed and cleaned, shredded, and extruded to obtain homogeneous pellets from
these recycled HDPE fishing nets (rFN). This recyclate was blended with five raw resins
used in different applications such as in fibers and packaging products. The blending pro-
cess has been carried out in a Collin ZK 50 counter-rotating twin-screw extruder, contain-
ing up to 75 wt.% of rFN, and the corresponding raw resin for the remaining portion. The
extrusion temperature goes from 185 to 240 °C with a screw speed of 60–80 rpm.
2.2. Molecular and Physical Characterization
To characterize the molecular structure of the resins, molecular weight (Mw), molec-
ular weight distribution (MWD), and short chain branching (SCB) distribution were de-
termined on a high temperature Gel Permeation Chromatography (GPC) GPC-IR6 (Poly-
mer Char). The set of columns and infrared detector equipped on the GPC-IR6 allows
distinguishing differences of 1 branch per 1000 atoms of carbon. Samples were dissolved
in 1,2,4-trichlorobenzene (TCB) stabilized with BHT (2,6-di-tert-butyl-4-methylphenol) at
a concentration of 0.75 mg/mL. The flow rate and temperature for the analysis was 1.0
mL/min and 160 °C, respectively.
TREF experiments were carried out on a commercial CRYSTAF-TREF instrument
model 300 (Polymer Char). Samples of 80 mg were dissolved in TCB at 160 °C. After 60
min. of gentle stirring, the solution was injected into the column and held at 130 °C for 45
min. Then, the temperature decreased from 130 to 35 °C at a cooling rate of 0.5 °C/min to
allow the polymer crystallization into the column. In the final step, the sample was eluted
at a constant flow rate of 1 mL/min while rising the temperature from 35 to 140 °C. The
Figure 1. Flowchart of the experimental procedure.
2. Materials and Methods
2.1. Materials
Discarded HDPE fishing nets from Spanish harbors were sorted and separated from
other polymers such as PA and PP to reduce cross contamination. Once collected, they were
washed and cleaned, shredded, and extruded to obtain homogeneous pellets from these
recycled HDPE fishing nets (rFN). This recyclate was blended with five raw resins used in
different applications such as in fibers and packaging products. The blending process has
been carried out in a Collin ZK 50 counter-rotating twin-screw extruder, containing up to
75 wt.% of rFN, and the corresponding raw resin for the remaining portion. The extrusion
temperature goes from 185 to 240 C with a screw speed of 60–80 rpm.
2.2. Molecular and Physical Characterization
To characterize the molecular structure of the resins, molecular weight (Mw), molecu-
lar weight distribution (MWD), and short chain branching (SCB) distribution were deter-
mined on a high temperature Gel Permeation Chromatography (GPC) GPC-IR6 (Polymer
Char). The set of columns and infrared detector equipped on the GPC-IR6 allows dis-
tinguishing differences of 1 branch per 1000 atoms of carbon. Samples were dissolved
in 1,2,4-trichlorobenzene (TCB) stabilized with BHT (2,6-di-tert-butyl-4-methylphenol)
at a concentration of 0.75 mg/mL. The flow rate and temperature for the analysis was
1.0 mL/min and 160 C, respectively.
TREF experiments were carried out on a commercial CRYSTAF-TREF instrument
model 300 (Polymer Char). Samples of 80 mg were dissolved in TCB at 160
C. After
60 min.
of gentle stirring, the solution was injected into the column and held at 130
C for
45 min. Then, the temperature decreased from 130 to 35
C at a cooling rate of 0.5
C/min
to allow the polymer crystallization into the column. In the final step, the sample was
eluted at a constant flow rate of 1 mL/min while rising the temperature from 35 to 140
C.
The polymer concentration was measured by an infrared detector and the crystallization
curves were obtained from the first derivative.
Thermal measurements were developed at a heat rate of 10
C/min with a DSC
Mettler-Toledo 822e. Additionally, thermal stability of the recycled material was evaluated
Polymers 2021,13, 3155 4 of 15
using a TGA/DSC 1 thermogravimetric analyzer (Mettler-Toledo), from 20
C to 600
C at
a heating rate of 20 C/min.
FTIR spectra were collected in attenuated total reflection (ATR) mode using a FT-IR
Varian Excalibur 3100 spectrometer. The analysis was performed at room temperature and
ambient humidity between 4000 and 400 cm
1
, with 64 scans. The spectra were obtained
in transmittance mode.
Rheological measurements were performed in a TA Instruments (model DHR-2)
rheometer in the parallel plate mode. Sheets of around 2 mm of thickness have been previ-
ously compression molded and disks of suitable dimensions for rheological measurements
have been obtained. The oscillatory viscoelastic determinations were carried out over the
frequency range 0.01–100 rad
·
s
1
. Deformation was set at around 1%, which corresponds
to the linear viscoelastic region in all the pure polymers and blends, identified through
previous amplitude sweeps. The measurements were performed at 190 C.
Tensile and flexural tests were developed in a universal testing machine (MTS Alliance
RT/5) at 23
C and 50% relative humidity. Tensile tests were carried out using dumbbell
shaped specimens 1BA at a crosshead speed of 50 mm/min, according to UNE-EN ISO
527-2:2012. Flexural and Charpy impact tests were performed on rectangular bars samples
measuring 80 mm
×
10 mm
×
4 mm, extracted from a plate previously molded by com-
pression. Flexural tests were carried out with a three-point bending geometry according to
the ISO 178:2010 standard at a crosshead speed of 2 mm/min, while Charpy impact tests
were done following the UNE-EN ISO 179-1:2010.
2.3. Environmental Stress Crack Resistance (ESCR)
Environmental stress cracking resistance was evaluated according to ASTM D 1693-15,
under test condition B: Surface-active agent (Igepal CO-630) concentration of 10% volume in
water at 50
C. The tested materials have been previously obtained via
compression molding.
3. Results and Discussion
The potential use of recycled HDPE fishing nets for different applications were evalu-
ated through its blending with different raw resins. Table 1summarizes the main physico-
chemical properties of the materials used in this work and the main application of the raw
resins (fibers or packaging products).
Table 1. Physicochemical properties of raw resins and recycled fishing net (rFN).
Material Type Mw (kg/mol) Mw/Mn () SCB/1000C Tm (C)
PE1 Fibers 149 6.94 1.3 140.0
PE2 146 8.42 2.1 139.5
PE3
Packaging
209 29.7 0.3 132.4
PE4 348 38.7 0.3 132.5
PE5 274 22.6 1.0 130.3
rFN Recycled HDPE
Fishing Net 121 8.40 2.5 138.0
Blends of virgin and recycled materials have been prepared using a counter-rotating
twin-screw extruder to achieve a better homogenization and incorporating the recycled
material at different rates (up to 75% of recycled resin). Each system prepared has been
named according to the virgin resin and the recycled content (namely PEX-rFN-Y; where X
corresponds to raw resin 1, 2, 3, 4 or 5; and Y corresponds to wt.% of recycled fishing net).
3.1. Blends for Fishing Nets Gear Application
The first objective of this study is to evaluate the potential use of these recyclates for
the manufacture of fishing nets gear. Within this goal, two virgin resins have been chosen,
named PE1 and PE2, respectively. These raw materials have been selected as they are
Polymers 2021,13, 3155 5 of 15
typically used in the manufacture of fibers and nets, thus with similar properties to the
recycled resin.
All blends were characterized by GPC and DSC to assess that blending process was
successfully accomplished. Table 2summarizes the results obtained for both systems
(PE1—rFN and PE2—rFN). Both raw resins have a slightly higher Mw than rFN
(Table 1).
It is noteworthy that recycling process and degradation normally causes a drop in Mw
for recycled materials [
33
]. Moreover, discarded fishing gear are greatly exposed to envi-
ronmental conditions, being seriously affected by factors like ultraviolet light (UV) and
temperature, which favors the degradation process [
34
]. The difference between the virgin
PEs lies on their MWD and SCB distribution, being PE2 MWD slightly broader and with a
higher SCB content than that of the PE1 resin. During the blending process, a progressive
decrease in the Mw with the increasing of the rFN content is observed, for both systems. A
broadening of the MWD is also observed, probably caused by the blending and extrusion
process. Melting temperatures, for all blends prepared, were placed between the melting
point of raw and rFN materials (140.0 and 139.5
C for PE1 and PE2, and 138
C for the
recycled material). Moreover, the melting peak slightly shifted to lower temperatures with
the increasing content of rFN. All these results indicate a good miscibility among the virgin
and recycled materials, which confirms that the blending process was effective and a good
compatibility among all polyethylene resins was achieved.
Table 2. GPC and DSC results for PE1—rFN and PE2—rFN blend systems.
Blend System % rFN Mw (kg/mol) Mw/Mn () Tm (C)
PE1—rFN
0 149 6.94 140.0
10 147 7.17 140.2
15 144 7.19 138.8
25 139 7.56 137.9
50 135 8.42 137.4
100 121 8.40 138.0
PE2—rFN
0 146 8.42 139.5
10 141 8.50 138.9
15 141 8.93 138.5
25 138 8.68 137.9
50 132 9.00 138.1
100 121 8.40 138.0
After addressing the blend compatibility, the influence of the incorporation of rFN to
virgin resins have been studied through the characterization of mechanical properties. Ten-
sile stress-strain curves, flexural modulus and impact strength resistance were determined
for both blend systems at room temperature (Figure 2a–d, respectively). At lower recycled
contents (10 and 15 wt.%), tensile strength (Figure 2a) and elongation at break
(Figure 2b)
are little affected, with values close to those obtained for both raw resins. However, for
higher recycled contents these properties are more affected compared to those of both
virgin materials, with a greater dispersion of the measurements. rFN shows lower values
and close to half of those observed for both raw resins. These facts could be explained due
to the decrease in Mw with the increasing rFN content. Flexural modulus is less affected
by the presence of the recycled net, with values between 1200 and 1300 MPa for all blends
(Figure 2c). Finally, impact strength resistance is greatly affected by the incorporation of
the recycled material, especially for the system “PE1—rFN”, which have a higher initial
impact resistance than the system “PE2—rFN” that is closer to the rFN resin (Figure 2d).
Polymers 2021,13, 3155 6 of 15
Polymers 2021, 13, x FOR PEER REVIEW 6 of 16
less affected by the presence of the recycled net, with values between 1200 and 1300 MPa
for all blends (Figure 2c). Finally, impact strength resistance is greatly affected by the in-
corporation of the recycled material, especially for the system “PE1—rFN”, which have a
higher initial impact resistance than the system “PE2—rFN” that is closer to the rFN resin
(Figure 2d).
Figure 2. (a) Tensile strength, (b) Elongation at break, (c) Flexural modulus and (d) Impact strength resistance results for
PE1—rFN and PE2—rFN systems.
Moreover, cross contamination with other polymers or inorganic impurities could
also be the cause of the premature rupture of the specimens during tensile tests since this
is a common phenomenon for recycled polyolefins [35] that may also affect its long-term
performance [36]. To confirm if the presence of unwanted substances could be the reason
of the results obtained, further tests were performed for the rFN resin. Figure 3 shows the
FTIR spectrum obtained for the rFN material. First, a doublet corresponding to asymmet-
ric (2914 cm1) and symmetric (2848 cm1) stretching of CH2 are observed. Two more dou-
blets are seen at 1470 and 725 cm1, which correspond to bending and rocking defor-
mations, respectively. All these peaks confirm the presence of PE [37]. No other signals
are obtained in the spectrum, which could indicate that the separation from other fishing
nets was effective.
Figure 2.
(
a
) Tensile strength, (
b
) Elongation at break, (
c
) Flexural modulus and (
d
) Impact strength resistance results for
PE1—rFN and PE2—rFN systems.
Moreover, cross contamination with other polymers or inorganic impurities could
also be the cause of the premature rupture of the specimens during tensile tests since
this is a common phenomenon for recycled polyolefins [
35
] that may also affect its long-
term performance [
36
]. To confirm if the presence of unwanted substances could be the
reason of the results obtained, further tests were performed for the rFN resin. Figure 3
shows the FTIR spectrum obtained for the rFN material. First, a doublet corresponding to
asymmetric (2914 cm
1
) and symmetric (2848 cm
1
) stretching of CH
2
are observed. Two
more doublets are seen at 1470 and 725 cm
1
, which correspond to bending and rocking
deformations, respectively. All these peaks confirm the presence of PE [
37
]. No other
signals are obtained in the spectrum, which could indicate that the separation from other
fishing nets was effective.
To further verify that cross contamination with other polymers did not take place
during the recycling process, DSC and TREF analyses were performed (Figure 4). The
DSC thermogram reveals a single melting peak at 138
C, which corresponds to PE. No
other melting peaks are observed, neither at 160
C (PP) nor at 220
C (PA-6) [
38
], which
confirms the homogeneity of the recyclate and the successful previous separation process.
However, DSC analysis is not really confident for the determination of PP contents under
2 wt.%,
and TREF analyses were performed. TREF allows fractionating different polymer
chains as functions of their crystallinity to understand the microstructure and the phase
behavior of these materials. This technique, commonly used to determine the chemical
composition distribution of different polymer blends, has recently proven to be more
Polymers 2021,13, 3155 7 of 15
accurate for detection and quantification of PP impurities at low quantities than standard
methodologies [
39
]. The thermograms reveals a main region between 90 and 105
C
composed by a very intense peak, which corresponds to the crystalline fraction of the
HDPE chains (100
C). The absence of other peaks, especially in the range of 120
C, which
corresponds to PP region, confirms that no or negligible cross contamination occurred
during the recycling process [40].
Polymers 2021, 13, x FOR PEER REVIEW 7 of 16
Figure 3. FTIR Spectrum of the recycled fishing net (rFN).
To further verify that cross contamination with other polymers did not take place
during the recycling process, DSC and TREF analyses were performed (Figure 4). The DSC
thermogram reveals a single melting peak at 138 °C, which corresponds to PE. No other
melting peaks are observed, neither at 160 °C (PP) nor at 220 °C (PA-6) [38], which con-
firms the homogeneity of the recyclate and the successful previous separation process.
However, DSC analysis is not really confident for the determination of PP contents under
2 wt.%, and TREF analyses were performed. TREF allows fractionating different polymer
chains as functions of their crystallinity to understand the microstructure and the phase
behavior of these materials. This technique, commonly used to determine the chemical
composition distribution of different polymer blends, has recently proven to be more ac-
curate for detection and quantification of PP impurities at low quantities than standard
methodologies [39]. The thermograms reveals a main region between 90 and 105 °C com-
posed by a very intense peak, which corresponds to the crystalline fraction of the HDPE
chains (100 °C). The absence of other peaks, especially in the range of 120 °C, which cor-
responds to PP region, confirms that no or negligible cross contamination occurred during
the recycling process [40].
Figure 3. FTIR Spectrum of the recycled fishing net (rFN).
Polymers 2021, 13, x FOR PEER REVIEW 8 of 16
Figure 4. (a) DSC melting thermogram and (b) TREF curves for the recycled fishing net (rFN).
Finally, the thermal stability of rFN was evaluated through thermogravimetric anal-
ysis (Figure 5). The results show a residue around 6 wt.%, which could be assigned to the
presence of CaCO3 (fillers). However, this residual high value in the sample also could
indicate the presence of other inorganic contaminants (mainly silicates) present in marine
environments. The presence of these impurities, even after the recycling process for the
obtention of rFN, reveals the difficulty of washing and cleaning these kinds of recycled
materials.
Figure 5. TGA curve for the recycled fishing net.
The presence of undesirable particles in the recycled samples may explain the results
obtained during the mechanical characterization of both systems (PE1—rFN and PE2—
rFN), as key properties such as elongation at break or impact strength resistance are
greatly affected by the presence of contaminants. This fact clearly endangers its possible
use in the manufacture of fishing nets gear since the presence of impurities could act as a
stress riser and fracture initiator that may break the fibers during the manufacture and
Figure 4. (a) DSC melting thermogram and (b) TREF curves for the recycled fishing net (rFN).
Polymers 2021,13, 3155 8 of 15
Finally, the thermal stability of rFN was evaluated through thermogravimetric anal-
ysis (Figure 5). The results show a residue around 6 wt.%, which could be assigned to
the presence of CaCO
3
(fillers). However, this residual high value in the sample also
could indicate the presence of other inorganic contaminants (mainly silicates) present in
marine environments. The presence of these impurities, even after the recycling process
for the obtention of rFN, reveals the difficulty of washing and cleaning these kinds of
recycled materials.
Polymers 2021, 13, x FOR PEER REVIEW 8 of 16
Figure 4. (a) DSC melting thermogram and (b) TREF curves for the recycled fishing net (rFN).
Finally, the thermal stability of rFN was evaluated through thermogravimetric anal-
ysis (Figure 5). The results show a residue around 6 wt.%, which could be assigned to the
presence of CaCO3 (fillers). However, this residual high value in the sample also could
indicate the presence of other inorganic contaminants (mainly silicates) present in marine
environments. The presence of these impurities, even after the recycling process for the
obtention of rFN, reveals the difficulty of washing and cleaning these kinds of recycled
materials.
Figure 5. TGA curve for the recycled fishing net.
The presence of undesirable particles in the recycled samples may explain the results
obtained during the mechanical characterization of both systems (PE1—rFN and PE2—
rFN), as key properties such as elongation at break or impact strength resistance are
greatly affected by the presence of contaminants. This fact clearly endangers its possible
use in the manufacture of fishing nets gear since the presence of impurities could act as a
stress riser and fracture initiator that may break the fibers during the manufacture and
Figure 5. TGA curve for the recycled fishing net.
The presence of undesirable particles in the recycled samples may explain the results
obtained during the mechanical characterization of both systems (PE1—rFN and PE2—
rFN), as key properties such as elongation at break or impact strength resistance are greatly
affected by the presence of contaminants. This fact clearly endangers its possible use in
the manufacture of fishing nets gear since the presence of impurities could act as a stress
riser and fracture initiator that may break the fibers during the manufacture and failing in
the process of obtaining monofilaments and nets. As a result, rFNs do not seem suitable
for the original application and strengthening us to search for other possible uses for this
recycled material.
3.2. Blends for Packaging Application
The results attained for PE1—rFN and PE2—rFN ascertained its limited use in the
manufacture of fishing nets, mainly due to the contamination present in the recycled resin.
However, the overall properties obtained for the recycled materials could be useful for
other applications. As previously mentioned, packaging industries are one of the biggest
consumers of PE resins. The European regulations and objectives regarding Circular
Economy of Plastics are expecting to incorporate recycled resins in packaging sector to
reduce the raw plastic consumption in the next few years [
41
]. Within this aim, we have
evaluated the recycled fishing nets as a possible recyclate source for the manufacture of
non-food contact bottles, such as those used to contain cleaning or cosmetic products.
Three high-molecular weight blow molding raw HDPE resins have been chosen
(named PE3, PE4 and PE5), in order to reinforce the expected loss of properties after
blending with the recyclate. In this sense, the use of other additives or compatibilizers
was avoided, as its use could increase the economic cost of the process hindering their
recyclability [
42
]. Thermal and molecular characterization were carried out for all blending
systems (PE3-4-5—rFN). Results are summarized in Table 3. All virgin resins have higher
Mw and broader distributions than the rFN material. As the content of rFN increases
Polymers 2021,13, 3155 9 of 15
the Mw of the blend decreases, and the distribution is becoming narrower. Both aspects
suggest a good compatibility between both virgin and recycled resins, which is supported
by the DSC curves that show a single melting peak for all systems (Table 3). Regarding
the melting temperature, all blend systems appear to shift to higher temperatures with the
increasing content of rFN, respecting the virgin PEs, which also implies good miscibility
among the components.
Table 3. GPC and DSC results for PE3—rFN, PE4—rFN and PE5—rFN blend systems.
Blend System % rFN Mw (kg/mol) Mw/Mn () Tm (C)
PE3—rFN
0 205 22.1 132.4
10 202 20.6 133.9
15 201 20.1 133.5
25 193 18.9 135.9
50 175 15.1 136.7
75 147 13.2 137.1
100 121 8.40 138.0
PE4—rFN
0 348 38.7 132.5
10 307 34.7 133.5
15 307 33.2 134.1
25 286 31.7 134.8
50 255 27.1 135.7
75 192 18.4 137.3
100 121 8.40 138.0
PE5—rFN
0 277 21.7 130.3
10 251 19.5 131.2
15 250 19.3 132.3
25 240 19.7 133.9
50 205 15.2 135.1
75 162 12.1 137.4
100 121 8.40 138.0
For using in non-food contact bottles, the materials need to keep the stiffness and
a high impact strength and environmental stress cracking resistances. For these reasons,
tensile, flexural and impact test were performed for all blends. Additionally, the resistance
to environmental stress cracking (ESCR) was measured to evaluate the influence of the
recycled material into this critical parameter for packaging applications. The values
obtained in all tests were compared with the standard values for common raw HDPEs
used for packaging applications. Reference values indicated in Figure 6, as dot lines, were
obtained as an average of different blow molding HDPE commercial bottles.
Figure 6a,b depicts a comparative of yield strength and elongation at break values
obtained for all prepared systems. For yield strength and elongation at break, reference val-
ues are 26 MPa and 700%, respectively. Both properties are influenced by the incorporation
of recycled resin into the blend, decreasing as the rFN content is higher, which results in
the reduction of the Mw of the material. Nevertheless, for PE4 and PE5 systems contents
up to 50% of recycled material makes the values remaining above or around the threshold.
The higher Mw of both virgin resins allows incorporating more recycled material into
the blend.
Regarding flexural modulus, the reference value must be closer to 1150 MPa to guaran-
tee the minimum stiffness required for the bottles. As it can be seen in Figure 6c, all virgin
and recycled resins have similar values. PE4 and PE5 have slightly lower rigidity, but with
the increasing content of rFN the flexural modulus slightly increases. All blends with a
50 wt.%
content of recycled material have values above the 1150 MPa, thus the stiffness is
not a restriction for the use of this recyclate for packaging application.
Polymers 2021,13, 3155 10 of 15
Polymers 2021, 13, x FOR PEER REVIEW 10 of 16
For using in non-food contact bottles, the materials need to keep the stiffness and a
high impact strength and environmental stress cracking resistances. For these reasons,
tensile, flexural and impact test were performed for all blends. Additionally, the resistance
to environmental stress cracking (ESCR) was measured to evaluate the influence of the
recycled material into this critical parameter for packaging applications. The values ob-
tained in all tests were compared with the standard values for common raw HDPEs used
for packaging applications. Reference values indicated in Figure 6, as dot lines, were ob-
tained as an average of different blow molding HDPE commercial bottles.
Figure 6a,b depicts a comparative of yield strength and elongation at break values
obtained for all prepared systems. For yield strength and elongation at break, reference
values are 26 MPa and 700%, respectively. Both properties are influenced by the incorpo-
ration of recycled resin into the blend, decreasing as the rFN content is higher, which re-
sults in the reduction of the Mw of the material. Nevertheless, for PE4 and PE5 systems
contents up to 50% of recycled material makes the values remaining above or around the
threshold. The higher Mw of both virgin resins allows incorporating more recycled mate-
rial into the blend.
Figure 6. (a) Yield strength, (b) Elongation at break, (c) Flexural modulus and (d) Impact strength resistance results for
PE3—PE4 and PE5—rFN blend systems.
Regarding flexural modulus, the reference value must be closer to 1150 MPa to guar-
antee the minimum stiffness required for the bottles. As it can be seen in Figure 6c, all
virgin and recycled resins have similar values. PE4 and PE5 have slightly lower rigidity,
but with the increasing content of rFN the flexural modulus slightly increases. All blends
Figure 6.
(
a
) Yield strength, (
b
) Elongation at break, (
c
) Flexural modulus and (
d
) Impact strength resistance results for
PE3—PE4 and PE5—rFN blend systems.
Figure 6d also shows the impact resistance for the different blend systems analyzed.
The impact strength is the most sensitive mechanical property to the recycled content and
for the different blends is critically influenced by the low impact resistance of the rFN resin,
around 5 kJ/m
2
. Minimum required impact resistance needs to be above 9 kJ/m
2
, value not
reached for any blends of system PE3—rFN. The Mw of the raw resin critically mediates on
the impact strength resistance of the systems, which rapidly decrease with the incorporation
of rFN. However, results are improved when higher Mw resins such as PE4 and PE5, are
used. Both systems allow reaching a recycled content of up to 50 wt.%, maintaining the
minimum specifications. The higher Mw of PE4 and PE5 raw resins increases the overall
impact resistance of the material, as a higher degree of entanglement and tie molecules
density difficult the rupture of PE bonds and the fracture process, allowing the material
to absorb more energy. The impact resistance falls dramatically with the increment of
the recycled resin, as the rFN has almost half the Mw of the virgin resins. Additionally,
degradation and inorganic impurities make the rFN more brittle, which lower the impact
of the material after the blending process.
Finally, the environmental stress cracking resistance (ESCR) has been evaluated. This
long-term mechanical property is crucial for the application considered, as in the presence
of stress cracking agents, such as detergents or alcohols, PE suffers breakage after an interval
of time [
43
,
44
]. Results for rFN show a low resistance to ESCR (~10 h). This recyclate has a
low Mw, a property that greatly influences the environmental stress cracking resistance
of PE materials. Moreover, the presence of inorganic impurities, previously detected,
together with a certain level of degradation caused during the recycling process [
45
,
46
]
also determines the lower ESCR performance of the rFN. On the other hand, raw resins
Polymers 2021,13, 3155 11 of 15
PE3, PE4 and PE5 have overall higher ESCR values due to its high Mw, which helps to
increase the tie molecular density [4749].
As shown in Figure 7, the same trend is observed for all blend systems. As the content
of recycled resin increases the ESCR of each blend exponentially decreases, even at low
contents of recyclate. When PE3 is used as a raw material, only 25 wt.% of rFN could be
incorporated into the blend in order to fulfill the resistance for packaging applications,
fixed at a threshold value of 70 h from commercial HDPEs used as reference. The loss of
ESCR properties, promoted by the incorporation of rFN in the blends, can be compensated
and commensurate with raw resins with higher Mw, such as PE4 or PE5 resins. Thus,
higher ESCR values are achieved when PE5 is chosen as raw resin. This could be explained
because of the higher SCB content of PE5 (Table 1), which in the case of the blends is
mainly located in the high-molecular weight region, and therefore increasing the ESCR
performance of the material [
47
]. In this way, it is possible to incorporate up to 50% of
recycled resin in the PE5—rFN system, guaranteeing the standard value of the commercial
reference materials (70 h).
Figure 7. ESCR F50 values (h) vs. recycled content for all blends.
Finally, for PE5—rFN system, which shows the best results in terms of mechanical and
ESCR properties, a molecular model has been introduced to confirm the miscibility of the
blend components from a morphological point of view. The double reptation model [
50
]
for miscible blends, as expressed in Equation (1), is frequently applied to explain the
viscoelastic behavior of polyolefin melts [
51
,
52
]. Indeed, rheology has become a potent tool
for inferring morphological state.
G(t)="
i
φiG1/c
i(t)#2
(1)
Figure 8represents the viscoelastic function tan
δ
, which is the ratio between loss and
storage shear moduli (tan
δ
=G”/G
0
). As it can be observed, this additive model perfectly
predicts the experimental response up to 50% of recycled content, which confirms the
good miscibility among the PE5 and rFN, despite the differences in molecular weight of
both resins. For PE5—rFN—75, a slight deviation of the model is observed, which can be
attributable to the presence of impurities, which are more noticeable when the rFN is the
main component in the blends.
Polymers 2021,13, 3155 12 of 15
Polymers 2021, 13, x FOR PEER REVIEW 13 of 16
Figure 8. Tan δ versus the angular frequency for PE5—rFN system at 190 °C. Solid lines represent
the results of the additive model for miscible blends.
Blends that incorporate virgin PEs with higher Mw enhance the mechanical perfor-
mance of the recycled PE, even with the presence of slight contents of impurities or con-
taminants, which seems to be a good alternative for this kind of recycled HDPE from fish-
ing nets. These results could be further improved with better handling and management
of the discarded fishing nets, which would enhance the properties of the recycled material
and helping to its better recyclability, even for different applications than its original use,
and thus closing the loop on discarded PE fishing nets.
4. Conclusions
Plastic pollution is becoming an increasing problem in our world, with the marine
environments especially affected by plastic debris. Discarded fishing nets, particularly
those made of polyolefins, are subjected to harsh conditions that affect its properties, low-
ering its value as recycled materials. To evaluate possible uses for this recycling stream,
rFN has been blended with different raw resins. First, blends with virgin HDPE were ex-
plored to be used again for the manufacture of fishing nets. This study produced poor
results, despite the good compatibility exhibited between virgin and recycled resins. Me-
chanical properties showed a high scatter in the results, especially due to the inorganic
contaminants detected in the rFN, which made more difficult the obtention of fibers with
proper quality to satisfy the demanding requirements for fishing nets. This result dramat-
ically decreases its use on the original application.
As other possible applications for the rFN, the use of this material in the manufacture
of non-food contact bottles has been explored. To strength the properties of the recyclate,
rFN was blended with three different high-molecular blow molding virgin resins, to en-
sure the minimum requirements for the packaging application. Results show that while
some properties are less influenced by the recycled content, other key properties such as
impact strength or environmental stress cracking resistances rapidly decrease. However,
when using raw resins with remarkably higher Mw than rFN, as PE4 or PE5 resins, the
incorporation of up to 50 wt.% of recycled material that meets the specifications was
achieved. In this sense, it has been proven that is critical to know the properties of the
recyclate to identify its possible uses. Additionally, a proper selection of virgin materials
helps to compensate the loss of properties of the recycled materials and open up the way
to new applications different from the original. The approach herein developed can help
Figure 8.
Tan
δ
versus the angular frequency for PE5—rFN system at 190
C. Solid lines represent
the results of the additive model for miscible blends.
Blends that incorporate virgin PEs with higher Mw enhance the mechanical per-
formance of the recycled PE, even with the presence of slight contents of impurities or
contaminants, which seems to be a good alternative for this kind of recycled HDPE from
fishing nets. These results could be further improved with better handling and manage-
ment of the discarded fishing nets, which would enhance the properties of the recycled
material and helping to its better recyclability, even for different applications than its
original use, and thus closing the loop on discarded PE fishing nets.
4. Conclusions
Plastic pollution is becoming an increasing problem in our world, with the marine
environments especially affected by plastic debris. Discarded fishing nets, particularly
those made of polyolefins, are subjected to harsh conditions that affect its properties,
lowering its value as recycled materials. To evaluate possible uses for this recycling stream,
rFN has been blended with different raw resins. First, blends with virgin HDPE were
explored to be used again for the manufacture of fishing nets. This study produced
poor results, despite the good compatibility exhibited between virgin and recycled resins.
Mechanical properties showed a high scatter in the results, especially due to the inorganic
contaminants detected in the rFN, which made more difficult the obtention of fibers
with proper quality to satisfy the demanding requirements for fishing nets. This result
dramatically decreases its use on the original application.
As other possible applications for the rFN, the use of this material in the manufacture
of non-food contact bottles has been explored. To strength the properties of the recyclate,
rFN was blended with three different high-molecular blow molding virgin resins, to
ensure the minimum requirements for the packaging application. Results show that while
some properties are less influenced by the recycled content, other key properties such as
impact strength or environmental stress cracking resistances rapidly decrease. However,
when using raw resins with remarkably higher Mw than rFN, as PE4 or PE5 resins, the
incorporation of up to 50 wt.% of recycled material that meets the specifications was
achieved. In this sense, it has been proven that is critical to know the properties of the
recyclate to identify its possible uses. Additionally, a proper selection of virgin materials
helps to compensate the loss of properties of the recycled materials and open up the way
to new applications different from the original. The approach herein developed can help
Polymers 2021,13, 3155 13 of 15
in the design of new products, as crucial step for the growth of a resilient and sustainable
industry of plastics, in line with the objectives pursued by the Circular Economy.
Author Contributions:
R.J.: Investigation, Writing—original draft. C.D.: Conceptualization, Writing—
review & editing, Funding acquisition. N.R.: Investigation, Writing—review & editing. B.P.: Writing—
review & editing. S.G.: Writing—review & editing. R.A.G.-M.: Supervision, Conceptualization,
Writing—review & editing, Funding acquisition. All authors have read and agreed to the published
version of the manuscript.
Funding:
This research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
We acknowledge the Taller de Inyección de la Industria del Plástico (TIIP) from
Zaragoza University for their support with blends preparation. We also especially acknowledge
the technical staff of the Polymer Technology Laboratory (LATEP) for the great technical work
and support.
Conflicts of Interest: The authors declare no conflict of interest.
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... Recycling has been one of the ways of managing plastic waste. Thus, the scientific literature has very recent reports as lithium-ion battery recycling (dos Santos et al., 2021;Yıldızbaşı et al., 2022), structural pavement rehabilitation with recycled materials (Freire et al., 2022), circularity of phosphorus of disposable baby nappy waste (Chowdhury & Wijayasundara, 2021), recycling plastic in construction (Arora et al., 2021;Balletto et al., 2021;Norouzi et al., 2021), recycled polyethylene fishing nets (Juan et al., 2021), plastic recycling and remolding circular economy using blockchain (Khadke et al., 2021), artificial intelligence-based solution for sorting COVID-19-related medical waste streams (Kumar et al., 2021), description of non-household end-use plastics (Kleinhans et al., 2021), post-consumer plastic packaging waste flow (Pimentel Pincelli et al., 2021), and recycling and utilization of polymers for road construction projects (Anwar et al., 2021). Then other efforts were developed as government experiences of plastic waste (Lacko et al., 2021;Wu et al., 2021), 3D print of plastics (Zhu et al., 2021), review of the European Union's strategy for plastics (Matthews et al., 2021), lean green production for reducing variable costs of plastic (Diaz et al., 2022), chemicals advances of plastic (Aurisano et al., 2021), using worms for plastic agriculture (Gan et al., 2021), regulation of plastic (Syberg et al., 2021), review of bioplastics (Di Bartolo et al., 2021), algae biopolymer (Devadas et al., 2021), prevention of marine plastic pollution (Fadeeva & Van Berkel, 2021). ...
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We will begin with an introduction to the circular economy, an abstract concept that can be applied to any industry or business. This chapter will address the principles based on theoretical and empirical models that support the decision of universities to influence and improve the circular economy policies to ensure a sustainable impact in the long term, including the pillars of sustainable development. Likewise, we will look at the role of universities as drivers of education towards ecological economics that minimizes the production of residues and waste that puts effectiveness and efficiency at risk. Finally, we will examine the possible limitations and conflicts of concepts in the circular economy.
... Recycling has been one of the ways of managing plastic waste. Thus, the scientific literature has very recent reports as lithium-ion battery recycling (dos Santos et al., 2021;Yıldızbaşı et al., 2022), structural pavement rehabilitation with recycled materials (Freire et al., 2022), circularity of phosphorus of disposable baby nappy waste (Chowdhury & Wijayasundara, 2021), recycling plastic in construction (Arora et al., 2021;Balletto et al., 2021;Norouzi et al., 2021), recycled polyethylene fishing nets (Juan et al., 2021), plastic recycling and remolding circular economy using blockchain (Khadke et al., 2021), artificial intelligence-based solution for sorting COVID-19-related medical waste streams (Kumar et al., 2021), description of non-household end-use plastics (Kleinhans et al., 2021), post-consumer plastic packaging waste flow (Pimentel Pincelli et al., 2021), and recycling and utilization of polymers for road construction projects (Anwar et al., 2021). Then other efforts were developed as government experiences of plastic waste (Lacko et al., 2021;Wu et al., 2021), 3D print of plastics (Zhu et al., 2021), review of the European Union's strategy for plastics (Matthews et al., 2021), lean green production for reducing variable costs of plastic (Diaz et al., 2022), chemicals advances of plastic (Aurisano et al., 2021), using worms for plastic agriculture (Gan et al., 2021), regulation of plastic (Syberg et al., 2021), review of bioplastics (Di Bartolo et al., 2021), algae biopolymer (Devadas et al., 2021), prevention of marine plastic pollution (Fadeeva & Van Berkel, 2021). ...
Chapter
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link 50 days' free access: https://authors.elsevier.com/a/1boSO3QCo9Yn0F High-density polyethylene (HDPE) is one of the most used and demanded plastic, not only for packaging, but also for construction and within this application especially for non-pressure and pressure pipes, which makes this material the most abundant in the municipal waste stream. On the basis of the Circular Economy and the sustainable life that promotes, it is important to explore new applications for recycled HDPE (rHDPE) to increase the polymer recycled uptake. However, recycled HDPE is not currently being used in pressure pipes, mainly due to the high structural and loading requirements that must be met. The present study evaluates the potential use of post-consumer rHDPE from different origins in the manufacture of polyethylene pressure pipes. Different rHDPE sources are blended in different ratios with raw HDPE with PE100 grade quality. Blends are fully characterized to determine their feasibility to be used for pipe applications. Properties such as tensile strength at yield, elongation at break and flexural modulus for all blends yield values above the minimum required for PE100 grades. Furthermore, two important mechanical properties of polyethylene pipes, Slow Crack Growth (SCG) and Rapid Crack Propagation (RCP) resistances, are deeply evaluated. Remarkably, a dual correlation of SCG and RCP with the content of recycled PE in blends was established, allowing to develop predictive capabilities that guarantee the requirements and specifications for pressure pipe applications. Finally, through the evaluation of different waste streams, it can be concluded that handling, sorting, separation and selection of polyethylene’s waste is critical to achieve the required pipe specifications, and to increase the percentage of post-consumer rHDPE into the final product. This investigation is in line with the sustainability objective and the commitment to boost the circular economy by replacing part of the conventional HDPE raw material with recycled HDPE to increase close-loop recycling on PE for pipe application, and the basis for the recycling of rHDPE from pipe at its end-life, after 50 years in service.