# Recycling of Polystyrene/Polyethylene Micro/Nanolayered Films: Effect of Melt Rheological Properties Mismatch on Induced Morphological and Mechanical Performances

Hissein Bechibo, Mohamed Yousfi, Abderrahim Maazouz, Khalid Lamnawar

► To cite this version:

Hissein Bechibo, Mohamed Yousfi, Abderrahim Maazouz, Khalid Lamnawar. Recycling of Polystyrene/Polyethylene Micro/Nanolayered Films: Effect of Melt Rheological Properties Mismatch on Induced Morphological and Mechanical Performances. Polymer Engineering and Science, In press, 10.1002/pen.70137. hal-05273931

HAL Id: hal-05273931

https://hal.science/hal-05273931v1

Submitted on 23 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

RESEARCH ARTICLE

OPEN ACCESS

# Recycling of Polystyrene/Polyethylene Micro/Nanolayered Films: Effect of Melt Rheological Properties Mismatch on Induced Morphological and Mechanical Performances

Hissein Bechibo Adam¹ | Mohamed Yousfi¹ | Abderrahim Maazouz¹,² | Khalid Lamnawar¹

¹Université de Lyon, CNRS, UMR 5223, Ingénierie des Matériaux Polymères, Université Claude Bernard Lyon1, INSA Lyon, Université Jean Monnet, Villeurbanne, France | ²Hassan II Academy of Science and Technology, Rabat, Morocco

**Correspondence:** Khalid Lamnawar (khalid.lamnawar@insa-lyon.fr)

**Received:** 15 June 2025 |

**Revised:** 9 September 2025 |

**Accepted:** 10 September 2025

**Funding:** This work was supported by the INSA Lyon, PIA France 2030, and the French National Research Agency (ANR).

**Keywords:** forced assembly coextrusion | mechanical properties | morphology | multilayer films | recycling

## ABSTRACT

In the latest decade, intensive research work was dedicated to multi-micro/nanolayer systems as an alternative to classical multilayer systems, aiming to reduce the number of constituents. However, limited attention has been paid to their recyclability. The present study aimed to evaluate a novel eco-design approach for the valorization of polystyrene-based multilayer films as an alternative to conventional PS/PE multilayer systems. To facilitate their recyclability, films were prepared with a limited number of constituents, avoiding the use of tie layers. The forced assembly coextrusion technology was employed to manufacture multi-micro/nanolayer films composed of polystyrene and low-density polyethylene (PS/LDPE), as well as polystyrene and linear low-density polyethylene (PS/LLDPE), with a volumetric ratio of 90/10 mimicking the composition encountered in the packaging field. PS/PE multilayer systems were tailored based on their rheological properties (i.e., different viscosity and elasticity ratios between the polymer constituents) in order to control their morphology, even throughout the recycling process. Firstly, the effect of mechanical recycling steps (grinding and melt blending by twin-screw extrusion) on the evolution of the mechanical properties was investigated. Interestingly, it was found that the initial multi micro/nanolayered structure was transformed into a multi-micro/nanofibrillar or lamellar morphology, making it possible to obtain recycled films with a good compromise between rigidity and ductility. Furthermore, the Young's modulus was higher for 256L, 1024L, and 2048L prepared at DDR5 in comparison to 32-layer films with low confinement and interfacial area, whatever the recycling cycle. Overall, Young's modulus measurements suggest that the mechanical properties were preserved over multiple recycling cycles and, in some cases, even improved. The results demonstrated that eco-design through the recycling of multi-micro/nanolayer structures can represent a promising sustainable strategy to improve the recyclability of multilayer packaging films as produced. This approach reduces the number of components used in the multilayer system and supports the transition toward mono-materials in order to reduce environmental impact.

## 1 | Introduction

According to Plastics Europe's annual report, in 2022, 41% of plastic packaging managed by the European Union was recycled

[1, 2]. Further legislation aims to drastically reduce plastic waste and increase recycling rates. For example, a directive of the European Parliament sets a goal of a 60% recycling rate for plastic packaging waste by 2030. In France, there is an anti-waste

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Polymer Engineering & Science* published by Wiley Periodicals LLC on behalf of Society of Plastics Engineers.
## Summary

* PS/PE films (90/10 vol%) were successfully manufactured without tie layers.

* Films were recycled via melting in a twin-screw extruder.

* The number of micro/nanolayers controls the morphology of recycled films.

* Fibrillar or lamellar morphologies observed in recycled PS/LLDPE films.

* Mechanical properties (i.e., ductility) maintained or improved after recycling.

law published in 2021 setting targets for the reduction, reuse, and recycling of packaging in single-use plastic for the period up to 2030 [3].

Historically, the study of polymer systems composed of alternating polymer layers at the micrometric scale has made it possible to improve the physical properties of the resulting materials and to enhance the interactions and adhesion between constituents using tie layers as compatibilizers [4, 5]. As a result, many polymers with different mechanical and physico-chemical properties can be processed to obtain films with improved final properties.

Each year, more than 100 million tons of single-use multilayer thermoplastics are produced worldwide, which represents 25% of global plastic production. 60% of recycled conventional multilayer plastics eventually end up incinerated or landfilled, since reuse is not commercially viable [5, 6]. In terms of recycling of other non-multilayer thermoplastics, mechanical recycling is the most widespread. However, mechanical recycling is very limited for blends of multiple polymer materials. Chemical recycling is under development and is only industrialized to a limited degree, because it is currently not cost effective and can cause environmental problems related to the use of solvents and chemical agents. The main challenge when it comes to processing plastic waste thus consists in reconciling recyclability with retaining of useful properties.

A novel innovative coextrusion technology has been developed called forced assembly coextrusion. This technique produces a structure made up of hundreds or even thousands of alternating layers, the thicknesses of which are controlled at the micron and nanometer scale. These very thin film thicknesses improve other characteristics such as lightness, optical properties, or gas barrier properties without using tie layers [7, 8]. Another advantage of the technology is reduced production costs thanks to the use of thin layers of technical polymers. However, the recycling of these multilayer films is tricky and currently underdeveloped [9].

Rigid packaging based on PS/PE multilayers (with a minimum of 85 vol% PS) represents a major component of plastic waste. It is widely used in the packaging industry (food and cosmetics, phone, glass, and automotive fields). Polystyrene provides rigidity, whereas polyethylene provides water vapor

barrier properties, sealability, and scratch-resistance. In general, polystyrene-based multilayer films are not easily recycled. In this context, mechanical recycling could be a reliable solution in order to give "another life" to this multilayer waste. Currently, typical tie-layer adhesives used in PS/PE co-extruded structures are based on ethylene vinyl acetate copolymer (EVA) [10]. EVA is compatible with polystyrene and ensures good adhesion between PS and polyethylene in the laminate. However, there are several disadvantages of using EVA-based tie-layer adhesives. One main drawback is that EVA-based polymers are known for poor temperature stability. EVA-based polymers degrade at high processing temperatures, contaminate the melt-extruded multilayer system, and may create gels in the laminates, making the recyclability of these materials extremely difficult [11].

The present study proposes and investigates a new approach to the eco-design of multilayer films based on immiscible polymers from polystyrene (PS-90 vol%) and polyethylene (PE-10 vol%) in order to facilitate and enhance their recyclability. Specifically, this new generation of films has been prepared without tie-layer adhesives.

The fabrication of multi micro/nanolayers based on the two materials was performed using a specific design of forced assembly coextrusion. The effect of their thermomechanical history during the recycling process (grinding, extrusion, granulation) on the final properties was explored. Subsequently, rheological, mechanical, thermal, and morphological properties of PS/LDPE and PS/LLDPE blends obtained from the recycling of PS/PE multilayer compositions with different viscosity ratios were investigated, and the effect of the number of recycling cycles on these properties was quantified.

# 2 | Experimental Section

## 2.1 | Raw Materials

The polystyrene used in this study was supplied by TRINSEO. It had an apparent density of 1.05 g cm⁻³, a weight average molecular weight of 450 kg mol⁻¹ and a glass transition temperature of 95°C. Two different polyethylene types were used. The first one was a metallocene linear low-density polyethylene (LLDPE), grade Marlex D139 purchased from Chevron Phillips Chemical (USA). It had an apparent density of 0.92 g cm⁻³, a weight average molecular weight of 93.8 kg mol⁻¹, a polydispersity index of 2.5, a melting temperature of 115°C, and a melt flow index of 1.0 g/10 min at 190°C and 2.16 kg. The second one was a low-density polyethylene (LDPE) supplied by ExxonMobil (USA), presenting an apparent density of 0.92 g cm⁻³, a weight average molecular weight of 239 kg mol⁻¹, a polydispersity index of 9, a melting temperature of 112°C, and a melt flow index of 0.33 g/10 min at 190°C and 2.16 kg.

## 2.2 | Sample Preparation—Processing of PS/LDPE Multilayers

Multilayer structures of micro/nanolayers were produced using a laboratory-built multilayer coextrusion setup, as
<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | Schematic diagram of the forced assembly layer coextrusion process used in this study.</figcaption></figure>

shown in Figure 1. From the feedblock, the initial two layers
flowed through a series of custom-made layer multiplication
dies which split the melt vertically and spread it horizontally
to its original width before stacking the layers, all the while
maintaining a constant melt thickness [12, 13]. The final num-
ber of layers was determined as a function of the number of
multiplication dies, which were placed in series between the
feedblock and the final film or sheet exit dies. During the co-
extrusion process, the melt temperature was set at 240°C for
the extruders, multipliers, and die. The chill roll temperature
was then adjusted to 60°C with a drawing velocity of 1.27 m/
min (denoted DDR1) and 6.35 m/min (denoted DDR5) for all
systems. The feed block configuration used for the systems
was a B/A configuration, where A and B corresponded to the
extruders. A set of n multipliers led to a film of 2ⁿ⁺¹, since
a two-layered feedblock was used for the PS/LDPE systems
giving samples with a 90/10 vol% composition. The number of
layers was either 32L, 256L, 1024L, or 2048L using 4, 7, 9, or
10 multiplication die elements, respectively.

**2.3 | Mechanical Recycling Process**

For clarity, the focus will be given to the recycled systems since
the interested reader can refer to our previous work [14] for virgin
multilayer properties. Once prepared, the multilayer films were
cryogenically ground using liquid nitrogen in the Pulverisette
11 grinder (Fritsch, Germany). This technique reduced the sam-
ples into flakes a few millimeters in length. In the second step,
these PS/LDPE flakes were extruded using a twin-screw micro-
compound (Haake Minilab II, Thermo Scientific, Germany)
at a temperature of 240°C and a screw speed of 54 rpm. The
extrudate was then either cooled and re-extruded again (after
granulation using a knife granulator [Pulverisette11, Fritsch,
Germany]) under the same conditions to study the effect of the
number of mechanical recycling cycles or directly transferred

in the molten state into a micro-injection machine (Minijet II,
Thermo Scientific, Germany) to prepare tensile test specimens
at barrel and mold temperatures of 240°C and 90°C, respec-
tively, under an injection pressure of 60 MPa, an injection time
of 5s, a holding pressure of 45 MPa, and a holding time of 10s.

**2.4 | Characterizations of Multilayer Films and Their Recycled Systems**

**2.4.1 | Differential Scanning Calorimetry (DSC)**

A DSC Q10 from TA instrument, USA was used to perform mea-
surements from 20°C to 200°C with a scan rate of 10°C min⁻¹
under nitrogen environment.

**2.4.2 | Small-Amplitude Oscillatory Shear Measurements (SAOS)**

A stress-controlled DHR-2 (Discovery Hybrid Rheometer, TA Instruments), using a 25-mm parallel-plate geometry with a gap of 1 mm, was used for the neat polymers at temperatures ranging from 150°C to 250°C under a nitrogen atmosphere. Specimens were compression-molded for 5 min between two Teflon films at a pressure of 200 bars to reduce any residual stress and to obtain a smooth surface. The first experiment was a dynamic strain sweep with a maximum angular frequency of 628 rad/s, after which the strain value was set to 5% to ensure that the following measurements were in the linear viscoelastic domain. The following rheological tests were performed: (1) a dynamic time sweep test performed to verify the thermal stability of the neat polymers at different molten temperatures at a set angular frequency of 1 rad/s [15]; (2) a dynamic frequency sweep test with frequencies ranging from 628 to 0.01 rad/s at a fixed strain amplitude of 5% from 150°C to 250°C.
### 2.4.3 | Morphological Analyses

Morphologies of the PS/LDPE samples were characterized by scanning electron microscopy (SEM) using a Hitachi-S3000-N SEM instrument (Hitachi, Japan) with an accelerating voltage of 10 kV. Samples were cryofractured in liquid nitrogen and

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | The normalized $\eta^*(t)/\eta^*(t=0)$ as a function of time at 240°C with an angular frequency of 1 rad/s for the neat polymers.</figcaption></figure>

metalized for 120s with a thin gold/palladium layer of 10nm. Samples were cut either longitudinally, in the direction of the flow, or transversely, perpendicular to the flow direction.

### 2.4.4 | Tensile Measurements

Tensile tests were carried out on H2 dumbbell specimens using a Shimadzu AGS-X series tensile machine, according to the ISO 527-2 standard. A 5kN sensor was employed to measure the tensile force. Tests were done along the longitudinal axis of the injected specimen with a speed of 10 mm min⁻¹ at 23°C. An extensometer, attached to the sample at two points, was used to measure the deformation of the sample subjected to a tensile load.

## 3 | Results and Discussion

### 3.1 | Melt Rheological Study of the Selected Polymers

#### 3.1.1 | Thermal Stability of the Neat Polymers

Viscoelastic properties of polymeric materials are very sensitive to structural modifications that occur with temperature and time. Dynamic time sweep measurements were carried out to assess the thermal stability of neat polymers under conditions

<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | (a) Dynamic complex viscosity ($|\eta^*|$) versus angular frequency ($\omega$) for the studied neat polymers at a reference temperature of 240°C; (b) viscosity ratios for LDPE/PS and LLDPE/PS at 240°C; and (c) elasticity ratios for LDPE/PS and LLDPE/PS at 240°C.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | SEM images of LDPE/PS systems after varying rounds of recycling at DDR1.</figcaption></figure>

representative of the coextrusion process (high temperatures and prolonged durations associated with a large number of layers) [14]. In this context, Figure S1 depicts the evolution of the contact time during the coextrusion process as a function of the number of multiplying elements. Figure 2 shows the normalized complex viscosity $\eta^*(t)/\eta^*(t=0)$ as a function of time for the neat polymers at a reference temperature of 240°C with an angular frequency of 1 rad/s for 30 min. This time corresponds to the maximum residence time of the polymers when they pass through the multiplier elements during the coextrusion. The neat polymers were thermally stable for $t=30$ min, except for the LDPE, for which a significant increase in normalized viscosity was seen, especially for $t>18$ min. This behavior can be explained by crosslinking among the LDPE chains [16]. This result highlights the limit of recyclability of LDPE related to layer residence time.

### 3.1.2 | Rheological Characteristics

Figure 3a depicts the dynamic complex viscosity ($|\eta|^*$) modulus against the angular frequency ($\omega$) for the neat polymers at a reference temperature of 240°C. The evolution of the rheological behavior of LDPE, with an increase in complex viscosity at low frequencies, confirms the results obtained in the thermal stability section, where the viscosity of this polymer increases over time (see Figure 2). Unlike the other two polymers, LDPE did not reach the Newtonian region. According to the literature, the presence of long-chain branching (LCB) significantly broadens the frequency range over which pseudoplastic behavior is observed [17]. Consequently, the absence of a Newtonian plateau at low frequencies was expected, since LDPE exhibits a high content of LCB (2 LCB/1000C) compared with LLDPE (0.5 LCB/1000C) [14, 18].

### 3.1.3 | Viscosity and Elasticity Ratios of the Neat Polymers

Melt viscosity ratio and/or elasticity ratio between the components is very important when evaluating the morphology of

<table><caption>**TABLE 1** | Size and polydispersity of the LDPE/PS as a function of the number of recycling steps at different DDR.</caption><thead><tr><th>LDPE/PS</th><th>Size (µm)</th><th>SD (µm)</th><th>DDR</th></tr></thead><tbody><tr><td>32L-R1</td><td>2.10</td><td>0.86</td><td>1</td></tr><tr><td>32L-R2</td><td>2.75</td><td>0.92</td><td>1</td></tr><tr><td>32L-R3</td><td>3.01</td><td>0.94</td><td>1</td></tr><tr><td>256L-R1</td><td>1.59</td><td>0.54</td><td>1</td></tr><tr><td>256L-R2</td><td>2.08</td><td>0.79</td><td>1</td></tr><tr><td>256L-R3</td><td>1.57</td><td>0.42</td><td>1</td></tr><tr><td>1024L-R1</td><td>0.82</td><td>0.17</td><td>1</td></tr><tr><td>1024L-R2</td><td>0.96</td><td>0.23</td><td>1</td></tr><tr><td>1024L-R3</td><td>0.99</td><td>0.25</td><td>1</td></tr><tr><td>2048L-R1</td><td>1.44</td><td>0.47</td><td>1</td></tr><tr><td>2048L-R2</td><td>1.40</td><td>0.20</td><td>1</td></tr><tr><td>2048L-R3</td><td>1.30</td><td>0.40</td><td>1</td></tr></tbody></table><table><thead><tr><th>LDPE/PS</th><th>Size (µm)</th><th>SD (µm)</th><th>DDR</th></tr></thead><tbody><tr><td>32L-R1</td><td>1.65</td><td>0.59</td><td>5</td></tr><tr><td>32L-R2</td><td>1.89</td><td>0.68</td><td>5</td></tr><tr><td>32L-R3</td><td>2.01</td><td>0.91</td><td>5</td></tr><tr><td>256L-R1</td><td>0.96</td><td>0.35</td><td>5</td></tr><tr><td>256L-R2</td><td>1.93</td><td>0.85</td><td>5</td></tr><tr><td>256L-R3</td><td>2.20</td><td>1.18</td><td>5</td></tr><tr><td>1024L-R1</td><td>1.14</td><td>0.20</td><td>5</td></tr><tr><td>1024L-R2</td><td>0.46</td><td>0.12</td><td>5</td></tr><tr><td>1024L-R3</td><td>0.49</td><td>0.17</td><td>5</td></tr><tr><td>2048L-R1</td><td>0.57</td><td>0.25</td><td>5</td></tr><tr><td>2048L-R2</td><td>0.44</td><td>0.11</td><td>5</td></tr><tr><td>2048L-R3</td><td>0.41</td><td>0.14</td><td>5</td></tr></tbody></table>
1024 L - DDR = 5

<figure><img src="image_7.png" /><figcaption>FIGURE 5 | SEM morphological observations of recycled LLDPE/PS systems (1024L) with a draw ratio of 5 at different recycling cycles.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIGURE 6 | SEM photographs of recycled LLDPE/PS systems (from 32L to 2048L) with a draw ratio of 5 at different cycles.</figcaption></figure>
<table><caption>**TABLE 2** | The effect of the stretching rate (DDR1 vs. DDR5) on the size of the dispersed LLDPE phase in the recycled PS/LLDPE systems.</caption><thead><tr><th>LLDPE/PS</th><th>Diameter (μm)</th><th>SD (μm)</th><th>DDR</th></tr></thead><tbody><tr><td>32L-R1</td><td>1.96</td><td>0.75</td><td>1</td></tr><tr><td>32L-R2</td><td>1.97</td><td>0.70</td><td>1</td></tr><tr><td>32L-R3</td><td>—</td><td>—</td><td>1</td></tr><tr><td>256L-R1</td><td>1.11</td><td>0.33</td><td>1</td></tr><tr><td>256L-R2</td><td>0.78</td><td>0.30</td><td>1</td></tr><tr><td>256L-R3</td><td>—</td><td>—</td><td>1</td></tr><tr><td>1024L-R1</td><td>0.97</td><td>0.20</td><td>1</td></tr><tr><td>1024L-R2</td><td>1.00</td><td>0.39</td><td>1</td></tr><tr><td>1024L-R3</td><td>0.59</td><td>0.20</td><td>1</td></tr><tr><td>2048L-R1</td><td>0.88</td><td>0.26</td><td>1</td></tr><tr><td>2048L-R2</td><td>0.98</td><td>0.39</td><td>1</td></tr><tr><td>2048L-R3</td><td>—</td><td>—</td><td>1</td></tr></tbody></table><table><thead><tr><th>LLDPE/PS</th><th>Diameter (μm)</th><th>SD (μm)</th><th>DDR</th></tr></thead><tbody><tr><td>32L-R1</td><td>1.79</td><td>0.70</td><td>5</td></tr><tr><td>32L-R2</td><td>1.56</td><td>0.69</td><td>5</td></tr><tr><td>32L-R3</td><td>—</td><td>—</td><td>5</td></tr><tr><td>256L-R1</td><td>1.14</td><td>0.24</td><td>5</td></tr><tr><td>256L-R2</td><td>0.90</td><td>0.27</td><td>5</td></tr><tr><td>256L-R3</td><td>—</td><td>—</td><td>5</td></tr><tr><td>1024L-R1</td><td>0.67</td><td>0.10</td><td>5</td></tr><tr><td>1024L-R2</td><td>0.81</td><td>0.35</td><td>5</td></tr><tr><td>1024L-R3</td><td>—</td><td>—</td><td>5</td></tr><tr><td>2048L-R1</td><td>0.75</td><td>0.09</td><td>5</td></tr><tr><td>2048L-R2</td><td>0.40</td><td>0.10</td><td>5</td></tr><tr><td>2048L-R3</td><td>—</td><td>—</td><td>5</td></tr></tbody></table>

the blends as well as layer stability. Figure 3b,c show the viscosity and elasticity ratios, designated as *k* and *k'*, respectively, for LDPE/PS and LLDPE/PS. The viscosity ratio of the LLDPE/PS pair was quite high and reached a maximum value of 5.5, whereas the viscosity ratio for the LDPE/PS system was close to 1. A viscosity ratio near 1 indicates that the interfaces between the layers are more stable and less likely to deform or mix. Moreover, this helps reduce coextrusion defects and is generally desirable for achieving uniform and stable multilayer structures.

## 3.2 | Morphological Properties

### 3.2.1 | Morphology Versus Recycling of Injection-Molded Parts

**3.2.1.1 | PS/LLDPE Systems.** SEM micrographs of the recycled LDPE/PS multilayer films at DDR1 are shown in Figure 4. In order not to weigh down the manuscript with too many figures,

we have chosen not to present the SEM results of the samples with DDR5 (see Appendix S1). Observations were made on specimens cut longitudinally (along the flow direction) and transversely (perpendicular to the direction of flow). LDPE, with white color, stands out from PS in gray. The structure observed in the transverse direction corresponds to deformed LDPE nodules dispersed in the PS matrix. The LDPE nodules identified in the cross-sections turned out to be fibrils [19]. Interestingly, we can see in the longitudinal section of the image the presence of fibrils throughout the layers in the direction of the flow.

The analysis of the morphological characteristics of the recycled multilayer films, as a function of recycling cycles and the number of layers, revealed, based on the longitudinal sections of the images that the LDPE layers elongated in the flow direction [20]. The recycled 32-layer and 256-layer films, where layer thickness was on the order of a few micrometers, showed a lack of adhesion as determined by the existence of voids at the interface and by the presence of cavities following the preparation by cryofracture, behavior that has been found in previous studies [21, 22]. In particular, the 256-layer film easily delaminated during the extrusion process, due to a lack of adhesion between the PS matrix and the LDPE phase, again as has been seen elsewhere [23]. These observations also revealed that for these two samples, the initial multilayered morphology was for the most part preserved. Morphological structures were more homogenous and the presence of voids was less visible for the 1024-layer and 2048-layer recycled films. When increasing the number of layers, the LDPE fibril diameters decreased after recycling.

ImageJ software was employed to quantify the size of the LDPE phase from cross-sectional images taken perpendicular to the flow direction. As shown in Table 1, with the exception of the LDPE/PS 2048L system, the size dispersity of the LDPE phase increases with the number of recycling cycles. The size dispersity is more favorable at DDR1 than at DDR5, except for the 256-layer film. Moreover, at DDR5, size dispersity increases with the number of cycles for the 32- and 256-layer films.

Figures 5 and 6 present SEM micrographs of recycled LLDPE/PS systems at different recycling cycles for DDR5; for LLDPE DDR1 is shown in the Appendix S1. The fibrillar morphology observed in these SEM images (Figures 5 and 6) was associated with the initial morphology of the multilayers [24]. In fact, the filaments diameters appeared to depend on the initial PE layer thickness, which was smaller in the 1024L and 2048L configurations. It can be assumed that the fibrillar morphology was generated both at the outlet of the micro-extruder and during the micro-injection step, the latter involving high elongation rates. Compared to LDPE/PS and despite a high viscosity ratio (> 4) of LLDPE/PS systems, the elasticity ratio was below unity, thereby promoting in situ fibrillation. Due to the higher viscosity of LLDPE relative to PS, the relaxation and breakup time of the LLDPE filaments were relatively long, which explains the temporal stability of the fibrillar morphology over three recycling cycles [25].

Furthermore, the stretching rate (DDR1 vs. DDR5) did not significantly affect the resulting morphology or size (Table 2). It should be emphasized that the morphological evolution of PS/PE blends depends strongly on the initial multi-layered film structure as well as the interfacial tension between PS and PE.
<figure><img src="image_9.png" /><figcaption>**FIGURE 7** | Evolution of Young's modulus for the recycled LDPE/PS multilayer films with 32, 256, 1024, and 2048 layers and a stretching ratio of either DDR1 or DDR5 after 1, 2, or 3 recycling cycles.</figcaption></figure>

According to Cole and Macosko [26], the interfacial tension in PS/PE systems is around 3 mN/m at 240°C, and the effect of the interfacial tension on the morphology increases with the number of interfaces (i.e., the surface-to-volume fraction). These authors suggested that, during the melt blending of multi-nanolayered PS/PE films, the interfaces and interphases are primarily affected due to interdiffusion phenomena between the layers.

### 3.2.2 | Mechanical Properties With Regard to Recycling Cycles and Numbers of Layer

Figure 7 shows the evolution of Young's modulus as a function of recycling cycles. Young's modulus was higher for 256L, 1024L, and 2048L prepared at DDR5 in comparison to 32-layer films, whatever the recycling cycle (see Table S1).

For the 32-layer films, the recycling cycles had no impact, or only a very limited impact, on Young's modulus. By contrast, the 1024-layer films stretched at DDR1 exhibited a significant increase in Young's modulus between Cycles 1 and 2, as well as between Cycles 1 and 3. The same films, when stretched at DDR5, showed a significant difference in Young's modulus only between Cycles 1 and 3 (see Table S3). For the 2048-layer films, Young's modulus increases with the number of recycling cycles, except in the case of Cycle 2 compared to Cycle 3 for DDR5, where the differences are not statistically significant (p value > 0.05) (see Table S4). The standard deviations recorded for the 1024-layer samples processed at DDR5 during the second and third recycling cycles appear high. However, these values should be interpreted with caution. The differences observed with respect to stretching speeds are not statistically significant (see

Table S2), probably due to the magnitude of the standard deviations. Overall, Young's modulus measurements suggest that the mechanical properties were preserved over multiple recycling cycles and, in some cases, even improved. For reference, the Young's modulus for the neat PS and LDPE were 3 and 0.23 GPa, respectively, whereas the LDPE/PS blends prepared from virgin LDPE and PS granules exhibited a modulus of 2.5 GPa.

Figure 8 highlights the evolution of the mechanical properties of the recycled materials—namely Young's modulus, elongation at break, and yield strength—as functions of the number of layers (32L, 256L, 1024L, 2048L), recycling cycles (1, 2, or 3), and stretching rates (DDR1 and DDR5). The data for Young's modulus are the same as those presented in Figure 7; however, the purpose of this separate presentation is specifically to emphasize the effect of DDR on the layers (Figure 7). For configurations with a low number of layers (32L and 256L), Young's modulus remains stable across the recycling cycles, regardless of the stretching rate. In contrast, configurations with a high number of layers (1024L and 2048L) exhibited a significant increase in Young's modulus during the recycling cycles, indicating an increased rigidity. Moreover, these high-number of layer configurations consistently showed higher Young's modulus values compared to those with fewer layers, highlighting the positive impact of a high number of layers on material stiffness [14]. The evolution of the elongation at break was nonlinear and varied depending on the configurations and stretching rates, reflecting a complex mechanical response to applied stresses.

To avoid overloading this section with histograms, the data related to the LLDPE/PS blend have been compiled in Table S5.
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | Evolution of the Young's modulus, elongation at break, and yield strength of recycled LDPE/PS multilayer films as a function of the number of layers at different DDR.</figcaption></figure>

Overall, the Young's modulus measured at DDR5 is higher than that obtained at DDR1, regardless of the number of layers or the recycling cycle. However, at DDR5, the modulus tends to decrease as the number of cycles increases, probably due to the progressive degradation of the linear structure of LLDPE under repeated thermal treatments. With regard to elongation at break, the values obtained at DDR5 generally exceed those at DDR1 for films with 32–1024 layers during the first two recycling cycles. In contrast, for the 2048-layer films, DDR1 yields higher values. After the third cycle, elongations at break measured at DDR5 nevertheless remain superior, irrespective of the number of layers. Overall, elongation at break tends to improve with the increase in the number of recycling cycles. Finally, the yield strength remains systematically higher at DDR5 than at DDR1, regardless of the number of layers or recycling cycle. Moreover, it appears to be very little, if at all, affected by the increase in recycling cycles.

The mechanical properties of recycled LDPE/PS and LLDPE/PS blends with 1024 layers are shown in Figure 9. The LLDPE/PS system displayed a progressive decrease in Young's modulus with increasing recycling cycles, in contrast to the LDPE/PS blend, which exhibited an improvement in mechanical properties (see Table S3 for LDPE/PS and Table S6 for LLDPE/PS). However, the elongation at break and yield strength for the LLDPE/PS system remained more or less stable throughout the recycling cycles while being significantly higher than those observed for the LDPE/PS blend. The *p* values are all less than 0.01; therefore, the difference is highly significant (see Table S7). The LDPE/PS blend demonstrated a brittle mechanical behavior, characterized by a limited elongation at break, averaging around 7%, whereas the LLDPE/PS blend displayed a more ductile mechanical behavior, with an average elongation at break of approximately 18%.
<figure><img src="image_11.png" /><figcaption>**FIGURE 9** | Evolution of Young's modulus, the elongation at break, and the yield strength of recycled 1024L LDPE/PS and LLDPE-PS multilayer films at a DDR5 stretching ratio.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**FIGURE 10** | Comparison of Young's modulus, the elongation at break, and the yield strength of virgin LDPE/PS and LLDPE/PS systems.</figcaption></figure>
Figure 10 illustrates a comparison between blends obtained solely by twin-screw extrusion, without using a layer-multiplying unit. The mechanical properties of these blends were examined, and the results show that the LLDPE-PS blend exhibits a higher elongation at break, regardless of the number of recycling cycles, compared to the LDPE-PS blend (see Table S8). This performance could be explained by the linear structure of the LLDPE phase, which provides the material with good ductility as well as increased stability under stress. In contrast, the LDPE phase, due to its branched structure, tends to stiffen, resulting in certain brittleness and lower tensile strength.

The Young's modulus of the LLDPE-PS blend decreases with the increasing number of recycling cycles. This change may be attributed to the scission of the macromolecular chains of the LLDPE phase induced by repeated thermomechanical processing. Conversely, the modulus of the LDPE-PS blend shows a slight improvement between the first and third cycles, probably due to a crosslinking phenomenon in the LDPE phase. Overall, the LLDPE-PS blend exhibits a higher Young's modulus than the LDPE-PS blend during the first two recycling cycles. However, by the third cycle, the crosslinking of LDPE gives the LDPE-PS blend a slightly higher Young's modulus than the LLDPE-PS blend.

Similarly, the LLDPE-PS blend shows a higher yield strength than the LDPE-PS blend during the first two recycling cycles. Nevertheless, in the third cycle, the yield strength of the LDPE-PS blend surpasses that of the LLDPE-PS blend, likely due to the crosslinking of the LDPE phase. The work of Felgel-Farnholz et al. [16] supports these analyses and highlights the two main reaction mechanisms of polyethylene: chain scission and crosslinking. Due to its linear structure, LLDPE is primarily affected by chain scission, leading to a decrease in its molecular weight. In contrast, the long-chain branched structure of LDPE promotes crosslinking, resulting in an increase in its molecular weight.

## 4 | Conclusions

The context of this work has been to explore an eco-design approach through the recycling of multi-micro-nano-layer materials. Two configurations, that is, an LDPE/PS and an LLDPE/PS system, were developed and then recycled multiple times using a thermomechanical process. The morphological and mechanical properties of the materials were analyzed after each recycling cycle.

In the first step, we investigated the rheological behavior of virgin LDPE and LLDPE. LLDPE exhibited thermal stability throughout the 30-min test, in contrast to LDPE, which showed an increase in viscosity after 18 min, probably due to the crosslinking of macromolecular chains. The absence of a Newtonian plateau in the case of LDPE is most likely explained by its high content of LCB. Furthermore, the LLDPE/PS blend displayed a higher viscosity than the LDPE/PS system at high frequencies, whereas the latter distinguished itself by higher elasticity compared to its LLDPE/PS counterpart.

Next, we examined the morphological behavior of the LDPE/PS and LLDPE/PS blends. LDPE/PS films showed more defects than the LLDPE/PS films, including the presence of localized areas with interfacial distortions, such as layer rupture and droplet formation. By increasing the number of layers in the LDPE/PS system, we found that the thickness of the LDPE domains decreased, making the morphological structures more homogeneous. Furthermore, the presence of voids became less visible in recycled films with 1024 and 2048 layers. In conclusion, the LLDPE/PS system generally exhibited a finer morphology and improved properties compared to the LDPE/PS system, likely owing to the more linear molecular structure, which facilitated a more homogeneous phase dispersion.

Finally, we studied the influence of recycling cycles and stretching speeds on the mechanical behavior of the LDPE/PS blend. For this system, increasing the stretching speeds improved Young's modulus in most cases, except for the 32-layer films. Recycling cycles preserve the mechanical properties, and in some cases, even enhance them.

The LDPE/PS blend adopted a brittle mechanical behavior, reflected by a lower elongation at break, whereas the LLDPE/PS blend was more ductile, with a higher elongation at break.

The present work has thus demonstrated that the design of multi-micro/nano-layer films represents a promising solution to industrial challenges related to the valorization of recycled materials, without the need for compatibilizers. However, it is important to note that this study was conducted at the laboratory scale on coextruded films. The transfer of this micro/nano-layer technology to industrial-scale film production processes, as well as a deeper understanding of the nanostructural mechanisms involved, will require further studies in the future.

## Author Contributions

**Hissein Bechibo Adam:** conceptualization, methodology, data curation, formal analysis, writing – review and editing, writing – original draft, software, visualization. **Mohamed Yousfi:** conceptualization, methodology, supervision, data curation, visualization, validation, writing – review and editing, writing – original draft. **Abderrahim Maazouz:** conceptualization, methodology, supervision, resources, project administration, visualization, validation, funding acquisition, writing – original draft, writing – review and editing. **Khalid Lamnawar:** conceptualization, methodology, supervision, funding acquisition, visualization, writing – review and editing, writing – original draft, project administration, resources, validation.

## Acknowledgments

The authors thank the “Bonus Qualité Recherche” of INSA Lyon for its financial support of project DE2M. The authors thank INSA Lyon for its support through BQR “DEVMAN-2022” grants. The authors are also grateful also for the financial support from PIA France 2030 and the French National Research Agency (ANR) through the PEPR program Plastics 22-PERE-0002. The authors also express their thanks to the French Ministry of Superior Education and Research for the doctoral study grant.
## Data Availability Statement

The data presented in this study are available on request from the corresponding author.

## References

1. M. Bredács, C. Barretta, J. Geier, et al., "Progress Towards Composition-Based Inline Sorting and Recycling of Multilayer Polymer Packaging," *Sustainable Materials and Technologies* 44 (2025): e01385.

2. C. T. de Mello Soares, M. Ek, E. Östmark, M. Gällstedt, and S. Karlsson, "Recycling of Multi-Material Multilayer Plastic Packaging: Current Trends and Future Scenarios," *Resources, Conservation and Recycling* 176 (2022): 105905.

3. S. F. Wamba, M. Fotso, E. Mosconi, and J. Chai, "Assessing the Potential of Plastic Waste Management in the Circular Economy: A Longitudinal Case Study in an Emerging Economy," *Annals of Operations Research* 1 (2023): 1-23.

4. J. L. Self, A. J. Zervoudakis, X. Peng, W. R. Lenart, C. W. Macosko, and C. J. Ellison, "Linear, Graft, and Beyond: Multiblock Copolymers as Next-Generation Compatibilizers," *JACS Au* 2, no. 2 (2022): 310-321.

5. M. C. Mulakkal, A. C. Castillo, A. C. Taylor, et al., "Advancing Mechanical Recycling of Multilayer Plastics Through Finite Element Modelling and Environmental Policy," *Resources, Conservation and Recycling* 166 (2021): 105371.

6. H. B. Adam, M. Yousfi, A. Maazouz, and K. Lamnawar, "Recycling of Multilayer Polymeric Barrier Films: An Overview of Recent Pioneering Works and Main Challenges," *Macromolecular Materials and Engineering* 310 (2025): 2400414.

7. G. Cabrera, I. Touil, E. Masghouni, A. Maazouz, and K. Lamnawar, "Multi-Micro/Nanolayer Films Based on Polyolefins: New Approaches From Eco-Design to Recycling," *Polymers* 13, no. 3 (2021): 413.

8. N. Antón, Á. González-Fernández, and A. Villarino, "Reliability and Mechanical Properties of Materials Recycled From Multilayer Flexible Packages," *Materials* 13, no. 18 (2020): 3992.

9. K. Hamad, M. Kaseem, and F. Deri, "Recycling of Waste From Polymer Materials: An Overview of the Recent Works," *Polymer Degradation and Stability* 98, no. 12 (2013): 2801-2812.

10. M. A.-A. AlMa'adeed and I. Krupa, "Polyolefin Compounds and Materials," in *Springer Series on Polymer and Composite Materials* (Springer, 2016), 271-284.

11. M. Patel, S. Pitts, P. Beavis, et al., "Thermal Stability of Poly(Ethylene-Co-Vinyl Acetate) Based Materials," *Polymer Testing* 32, no. 4 (2013): 785-793.

12. M. Ponting, A. Hiltner, and E. Baer, "Polymer Nanostructures by Forced Assembly: Process, Structure, and Properties," in *Macromolecular Symposia*, vol. 294 (Wiley Online Library, 2010), 19-32.

13. J. Cheng, X. You, Z. Cao, D. Wu, C. Liu, and H. Pu, "Effective Control of Laser-Induced Carbonization Using Low-Density Polyethylene/Polystyrene Multilayered Structure via Nanolayer Coextrusion," *Macromolecular Materials and Engineering* 304, no. 5 (2019): 1800726.

14. J. Li, I. Touil, G. Sudre, et al., "Fabrication of Architectured Multilayers With Mismatched Rheological Behaviors: Layer Stability, Structure, and Confinement Dictate Polyethylene-Based Film Properties," *Industrial & Engineering Chemistry Research* 63, no. 4 (2024): 1953-1964.

15. T. Bernal-Lara, R. Liu, A. Hiltner, and E. Baer, "Structure and Thermal Stability of Polyethylene Nanolayers," *Polymer* 46, no. 9 (2005): 3043-3055.

16. A. Felgel-Farnholz, A. Schweighuber, C. W. Klampfl, and J. Fischer, "Comparative Study on the Degradation of HDPE, LLDPE and LDPE During Multiple Extrusions," *Polymer Degradation and Stability* 216 (2023): 110486.

17. D. Yan, W.-J. Wang, and S. Zhu, "Effect of Long Chain Branching on Rheological Properties of Metallocene Polyethylene," *Polymer* 40, no. 7 (1999): 1737-1744.

18. J. Li, I. Touil, C. F. de Alba, et al., "Structure-Rheology Properties of Polyethylenes With Varying Macromolecular Architectures," *Journal of Polymer Research* 30, no. 12 (2023): 454.

19. M. Yousfi, T. Dadouche, D. Chomat, et al., "Development of Nano-fibrillar Morphologies in Poly(L-Lactide)/Poly(Amide) Blends: Role of the Matrix Elasticity and Identification of the Critical Shear Rate for the Nodular/Fibrillar Transition," *RSC Advances* 8, no. 39 (2018): 22023-22041.

20. T. Dadouche, M. Yousfi, C. Samuel, M. F. Lacrampe, and J. Souletin, "(Nano) Fibrillar Morphology Development in Biobased Poly(Butylene Succinate-Co-Adipate)/Poly(Amide-11) Blown Films," *Polymer Engineering & Science* 61, no. 5 (2021): 1324-1337.

21. W. Schrenk and T. Alfrey, Jr., "Coextruded Multilayer Polymer Films and Sheets," in *Polymer Blends* (Academic Press, 1978), 129-165.

22. O. Kwon and D. Zumbrunnen, "Progressive Morphology Development to Produce Multilayer Films and Interpenetrating Blends by Chaotic Mixing," *Journal of Applied Polymer Science* 82, no. 7 (2001): 1569-1579.

23. S. Luo, J. Xue, Y. Xiong, J. Shen, and S. Guo, "Light-Scattering Properties of Linear Low Density Polyethylene/Polystyrene Films Fabricated Through Layer-Multiplying Technology," *Journal of Applied Polymer Science* 133, no. 33 (2016): 43826.

24. M. Yousfi, C. Samuel, J. Souletin, and M.-F. Lacrampe, "Rheological Considerations in Processing Self-Reinforced Thermoplastic Polymer Nanocomposites: A Review," *Polymers* 14, no. 3 (2022): 637.

25. S. Scholtyssék, R. Adhikari, V. Seydewitz, G. H. Michler, E. Baer, and A. Hiltner, "Evaluation of Morphology and Deformation Micromechanisms in Multilayered PP/PS Films: An Electron Microscopy Study," in *Macromolecular Symposia*, vol. 294 (Wiley Online Library, 2010), 33-44.

26. P. J. Cole and C. W. Macosko, "Polymer-Polymer Adhesion in Melt-Processed Layered Structures," *Journal of Plastic Film & Sheeting* 16, no. 3 (2000): 213-222.

## Supporting Information

Additional supporting information can be found online in the Supporting Information section. **Appendix S1**: pen70137-sup-0001-Appendix.docx.