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[16] Mojtaba Salehi, Saeed Maleksaeedi, Mui Ling Sharon Nai, and Manoj Gupta. 2019. Towards additive manufacturing of magnesium alloys through integration of binderless 3D printing and rapid microwave sintering. *Addit. Manuf.* 29, (October 2019), 100790.
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# Effect of different auxetic cell design on the compression behavior of FDMed structures

NAPOLITANO Francesco¹,ª*, CARDENIO Ivano²,ᵇ, DEFINA Filippo³,ᶜ, MANCO Emanuele²,ᵈ, MANZO Alessandro³,ᵉ, PAPA Ilaria¹,²,>f and RUSSO Pietro¹,⁹

¹Institute of Polymers, Composites and Biomaterials, National Research Council, Via Campi Flegrei 34, 80078, Pozzuoli, Naples, Italy

²Department of Chemical, Materials and Industrial Production Engineering, University of Naples "Federico II", P.leTecchio 80, 80125, Naples, Italy

³3DnA srl, Consorzio "il Sole", Viale Impero, 80038 Pomigliano D'arco, Naples, Italy

ᵃfrancesco.napolitano@ipcb.cnr.it, ᵇcardenioivano@gmail.com, ᶜfilippo.defina@3dnasrl.it, ᵈemanuele.manco@unina.it, ᵉalessandro.manzo@3dnasrl.it, ᵃilaria.papa@unina.it, ᵇpietro.russo@ipcb.cnr.it

**Keywords:** Auxetic, Cellular Structures, Compression, Thermoplastic, FDM

**Abstract.** Among the different additive manufacturing technologies for polymeric materials, Fused Deposition Modelling has obtained great visibility and, thanks to the numerous studies on the process conditions, reached high quality of workpiece production. Nevertheless, the slicers are projected to replicate standard and easy to reproduce patterns, which don't take into account the final use of the workpiece produced. An optimization of the extruded material is represented by the design oriented to support specific loads, which are applied along specific directions. In this research work is proposed the use of auxetic geometry and two different cell types are proposed and compared with the related conventional workpieces, printed with the same volume density. The use of auxetic design allows for differentiation of the load-bearing and to obtain structures which don't lose their original shape and that can adsorb the compressive energy reaching the compaction which improves the final resistance.

**Introduction**

Sandwich structure composites consist of two rigid faces to resist external loads and an embedded cell core to increase the moment of inertia. Due to their high specific modulus and strength, they find applications in many fields, such as aerospace, automotive, marine and energy industries [1]. For this type of composite, the specific core architecture offers greater flexibility in tailoring their ultimate performance and, in this frame, particular interest is focused on auxetic structures [2,3]. Auxetic materials are characterized by a negative Poisson's ratio, the immediate effect of which is to allow a behaviour opposite to that found in classical structural materials; that is, when subjected to tensile stress they tend to increase their transverse dimension, while, if compressed, they tend to shrink transversely. [4,5]. An unexpected behaviour also occurs in bending where a material with auxetic behaviour tends to rise in the central area, conforming adequately to the curvature. The naturally auxetic materials are single crystals of arsenic [6] and cadmium [7], α-cristobalite [8], iron pyrites [9], and many cubic elemental metals [10]. Other examples regard biological materials, such as cat skin [11], salamander skin [12], cow teat skin [13], and load-bearing cancellous bone from human shins [14].

Studying the structures of these natural materials, in the late 1980s the researchers tried to replicate them to obtain auxetic structures from non-auxetic materials: an early example was an auxetic polyurethane foam [15,16].

<figure><img src="image_2450.png" /></figure>
# Effect of different auxetic cell design on the compression behavior of FDMed structures

NAPOLITANO Francesco¹,ª*, CARDENIO Ivano²,ᵇ, DEFINA Filippo³,ᶜ, MANCO Emanuele²,ᵈ, MANZO Alessandro³,ᵉ, PAPA Ilaria¹,²,>f and RUSSO Pietro¹,⁹

¹Institute of Polymers, Composites and Biomaterials, National Research Council, Via Campi Flegrei 34, 80078, Pozzuoli, Naples, Italy

²Department of Chemical, Materials and Industrial Production Engineering, University of Naples "Federico II", P.leTecchio 80, 80125, Naples, Italy

³3DnA srl, Consorzio "il Sole", Viale Impero, 80038 Pomigliano D'arco, Naples, Italy

ᵃfrancesco.napolitano@ipcb.cnr.it, ᵇcardenioivano@gmail.com, ᶜfilippo.defina@3dnasrl.it, ᵈemanuele.manco@unina.it, ᵉalessandro.manzo@3dnasrl.it, ᵃilaria.papa@unina.it, ᵇpietro.russo@ipcb.cnr.it

**Keywords:** Auxetic, Cellular Structures, Compression, Thermoplastic, FDM

**Abstract.** Among the different additive manufacturing technologies for polymeric materials, Fused Deposition Modelling has obtained great visibility and, thanks to the numerous studies on the process conditions, reached high quality of workpiece production. Nevertheless, the slicers are projected to replicate standard and easy to reproduce patterns, which don't take into account the final use of the workpiece produced. An optimization of the extruded material is represented by the design oriented to support specific loads, which are applied along specific directions. In this research work is proposed the use of auxetic geometry and two different cell types are proposed and compared with the related conventional workpieces, printed with the same volume density. The use of auxetic design allows for differentiation of the load-bearing and to obtain structures which don't lose their original shape and that can adsorb the compressive energy reaching the compaction which improves the final resistance.

**Introduction**

Sandwich structure composites consist of two rigid faces to resist external loads and an embedded cell core to increase the moment of inertia. Due to their high specific modulus and strength, they find applications in many fields, such as aerospace, automotive, marine and energy industries [1]. For this type of composite, the specific core architecture offers greater flexibility in tailoring their ultimate performance and, in this frame, particular interest is focused on auxetic structures [2,3]. Auxetic materials are characterized by a negative Poisson's ratio, the immediate effect of which is to allow a behaviour opposite to that found in classical structural materials; that is, when subjected to tensile stress they tend to increase their transverse dimension, while, if compressed, they tend to shrink transversely. [4,5]. An unexpected behaviour also occurs in bending where a material with auxetic behaviour tends to rise in the central area, conforming adequately to the curvature. The naturally auxetic materials are single crystals of arsenic [6] and cadmium [7], α-cristobalite [8], iron pyrites [9], and many cubic elemental metals [10]. Other examples regard biological materials, such as cat skin [11], salamander skin [12], cow teat skin [13], and load-bearing cancellous bone from human shins [14].

Studying the structures of these natural materials, in the late 1980s the researchers tried to replicate them to obtain auxetic structures from non-auxetic materials: an early example was an auxetic polyurethane foam [15,16].

<figure><img src="image_2443.png" /></figure>
The way to obtain an auxetic behaviour, from a conventional material, is to assemble a cellular structure in which each cell reproduces the load adsorption in the same manner as an auxetic material [17]. The elementary cells that constitute the basic structure of most auxetic materials can have different shapes of which it is possible, with trigonometric formulas, to calculate the rotation of the individual walls of each cell in relation to the load applied and therefore to have an idea of how to more accurately predict the overall mechanical behaviour of the structure. Over the years, some research studies were proposed with various geometries of the cells, aimed at achieving auxetic behaviour in cellular structures. The most simple to be designed, and the most adopted in the early stage of the research, is the hexagon concave [18–21], and its auxetic properties are demonstrated by finite elements analysis [22]. For these reasons, it was decided to design different cells to be tested in this experimental study.

The study of auxetic structures is relatively recent [15], but the advantages of this cell architecture are now completely undeniable [23]. The potential and real applications of auxetic materials are rapidly increasing, but their diffusion sometimes seems hindered by the fact that this is a new research sector and the phenomena are not always very clear or in any case not always describable with mathematical formulas easy to use. Furthermore, production costs are very high, the creation of the cells is complex and it is therefore preferable to use traditional materials even if they have lower performance. Thanks to the customization given by 3D printing techniques, such as the Fused Deposition Modelling one (FDM), the design of auxetic cells is rapidly increasing in metallic and polymeric materials. This manufacturing approach allows for improved resistance under impact and compression strength [24].

3D printing offers a considerable number of advantages, the most significant of which is the ability to produce highly complex designs that would otherwise be impossible to create quickly, reducing waste of material and time for post-process machining while also offering many possibilities of 3D printing.

The contribution of the cellular structure could be improved through the choice of a properly reinforced polymer or polymeric blend which allows to performance of the expected response under a specific load. Furthermore, the cellular auxetic structures aid in managing the displacement of the singular cell element by driving the load transmission along the thicker and most resistant elements. If compared with a conventional 3D printed item, these new structures, having the same volume density, are characterized by an increased resistance given that the conventional FDM printing patterns are focused on the simple volume filling and not on the strategic filling.

In this frame, the work aims to design a reinforced polymer giving the best response under the expected load. An appropriate cell architecture will be chosen which ensures the best performances under compression load. The samples will be produced by the FDM process and, simultaneously, other samples will be produced by using traditional infill strategy to achieve the same volumetric density. All the structures will be tested under compression. The damaged samples were finally inspected.

## Materials and methods

### *Core design*

The in-plane behaviour of lattice composites is strongly affected by the core structure [25]. Here, two types of core topologies are considered. The main purpose of this work is to identify the cell deformation pattern and investigate how they affect compressive performances. Two types of cellular structures are fixed to have the same dimension as the unit cell. This approach provides a more straightforward observation of the divergent unit cell deformation pattern.

Gibson and Ashby proposed a numerical model to simplify the estimation of elemental cell behaviour and is based on the cell design in Figure 1 [26].
The way to obtain an auxetic behaviour, from a conventional material, is to assemble a cellular structure in which each cell reproduces the load adsorption in the same manner as an auxetic material [17]. The elementary cells that constitute the basic structure of most auxetic materials can have different shapes of which it is possible, with trigonometric formulas, to calculate the rotation of the individual walls of each cell in relation to the load applied and therefore to have an idea of how to more accurately predict the overall mechanical behaviour of the structure. Over the years, some research studies were proposed with various geometries of the cells, aimed at achieving auxetic behaviour in cellular structures. The most simple to be designed, and the most adopted in the early stage of the research, is the hexagon concave [18–21], and its auxetic properties are demonstrated by finite elements analysis [22]. For these reasons, it was decided to design different cells to be tested in this experimental study.

The study of auxetic structures is relatively recent [15], but the advantages of this cell architecture are now completely undeniable [23]. The potential and real applications of auxetic materials are rapidly increasing, but their diffusion sometimes seems hindered by the fact that this is a new research sector and the phenomena are not always very clear or in any case not always describable with mathematical formulas easy to use. Furthermore, production costs are very high, the creation of the cells is complex and it is therefore preferable to use traditional materials even if they have lower performance. Thanks to the customization given by 3D printing techniques, such as the Fused Deposition Modelling one (FDM), the design of auxetic cells is rapidly increasing in metallic and polymeric materials. This manufacturing approach allows for improved resistance under impact and compression strength [24].

3D printing offers a considerable number of advantages, the most significant of which is the ability to produce highly complex designs that would otherwise be impossible to create quickly, reducing waste of material and time for post-process machining while also offering many possibilities of 3D printing.

The contribution of the cellular structure could be improved through the choice of a properly reinforced polymer or polymeric blend which allows to performance of the expected response under a specific load. Furthermore, the cellular auxetic structures aid in managing the displacement of the singular cell element by driving the load transmission along the thicker and most resistant elements. If compared with a conventional 3D printed item, these new structures, having the same volume density, are characterized by an increased resistance given that the conventional FDM printing patterns are focused on the simple volume filling and not on the strategic filling.

In this frame, the work aims to design a reinforced polymer giving the best response under the expected load. An appropriate cell architecture will be chosen which ensures the best performances under compression load. The samples will be produced by the FDM process and, simultaneously, other samples will be produced by using traditional infill strategy to achieve the same volumetric density. All the structures will be tested under compression. The damaged samples were finally inspected.

# Materials and methods

## *Core design*

The in-plane behaviour of lattice composites is strongly affected by the core structure [25]. Here, two types of core topologies are considered. The main purpose of this work is to identify the cell deformation pattern and investigate how they affect compressive performances. Two types of cellular structures are fixed to have the same dimension as the unit cell. This approach provides a more straightforward observation of the divergent unit cell deformation pattern.

Gibson and Ashby proposed a numerical model to simplify the estimation of elemental cell behaviour and is based on the cell design in Figure 1 [26].
<figure><img src="image_2451.png" /><figcaption>Figure 1. Design and dimensions of the elemental cell.</figcaption></figure>

This cell could be loaded in direction 1, and in this case, the load is perpendicular to the vertical elements h which are also the most resistant. So the best loading direction is represented by direction 2, in this case, the load is parallel to the elements h which allows for improved cell resistance and is aided by the elements l which distribute the load on both sides. By considering the loading direction 2, the cell has two collapsing behaviour. The first one is the elastic buckling: in this case, the vertical elements h reach the instability before the fracture; the second one is the plastic collapse, in which case the rupture of the vertical elements causes the cell failure.

$$ (\sigma_{el})_2 = \frac{n^2 \pi^2}{24} E_s \frac{(t/l)^3}{(h/l)^2 \cos \theta} \quad (1) $$

$$ (\sigma_{pl})_2 = \sigma_{ys} \left(\frac{t}{l}\right)^2 \frac{1}{2 \cos^2 \theta} \quad (2) $$

The predominant failure mode is subjected to the maximum value which can reach the ratio between the thickness of the cell walls and the length of inclined elements. This ratio is:

$$ (t/l)_{critical} = \frac{12 (h/l)^2}{n^2 \pi^2 \cos \theta} \left(\frac{\sigma_{ys}}{E_s}\right) \quad (3) $$

Where

E_s : elastic modulus of the cell material

σ_ys : yield strength of the cell material

t : thickness of the cell elements h and l

n is a shape factor obtained from the ratio between vertical elements and inclined elements (Table 1).

<table><caption>Table 1. Shape factor n for different h/l ratios.</caption><thead><tr><td>h/l</td><td>n</td></tr></thead><tbody><tr><td>1</td><td>0.686</td></tr><tr><td>1.5</td><td>0.760</td></tr><tr><td>2</td><td>0.860</td></tr></tbody></table>

## Core design selection

The first step for the selection of the cells constituting the core of the structures under examination is to appropriately take the geometric relationships for the construction of the elementary cells necessary for the construction of the specimens of the experimental plan.

The values taken into consideration are reported in the table 2.
<figure><img src="image_2444.png" /><figcaption>Figure 1. Design and dimensions of the elemental cell.</figcaption></figure>

This cell could be loaded in direction 1, and in this case, the load is perpendicular to the vertical elements h which are also the most resistant. So the best loading direction is represented by direction 2, in this case, the load is parallel to the elements h which allows for improved cell resistance and is aided by the elements l which distribute the load on both sides. By considering the loading direction 2, the cell has two collapsing behaviour. The first one is the elastic buckling: in this case, the vertical elements h reach the instability before the fracture; the second one is the plastic collapse, in which case the rupture of the vertical elements causes the cell failure.

$$ (\sigma_{el})_2 = \frac{n^2 \pi^2}{24} E_s \frac{(t/l)^3}{(h/l)^2 \cos \theta} \quad (1) $$

$$ (\sigma_{pl})_2 = \sigma_{ys} \left(\frac{t}{l}\right)^2 \frac{1}{2 \cos^2 \theta} \quad (2) $$

The predominant failure mode is subjected to the maximum value which can reach the ratio between the thickness of the cell walls and the length of inclined elements. This ratio is:

$$ (t/l)_{critical} = \frac{12 (h/l)^2}{n^2 \pi^2 \cos \theta} \left(\frac{\sigma_{ys}}{E_s}\right) \quad (3) $$

Where

$E_s$: elastic modulus of the cell material

$\sigma_{ys}$: yield strength of the cell material

$t$: thickness of the cell elements h and l

$n$ is a shape factor obtained from the ratio between vertical elements and inclined elements (Table 1).

<table><caption>Table 1. Shape factor $n$ for different $h/l$ ratios.</caption><thead><tr><td>h/l</td><td>n</td></tr></thead><tbody><tr><td>1</td><td>0.686</td></tr><tr><td>1.5</td><td>0.760</td></tr><tr><td>2</td><td>0.860</td></tr></tbody></table>

## Core design selection

The first step for the selection of the cells constituting the core of the structures under examination is to appropriately take the geometric relationships for the construction of the elementary cells necessary for the construction of the specimens of the experimental plan.

The values taken into consideration are reported in the table 2.
<table><caption>Table 2. Dimensions of the designed core topology</caption><thead><tr><td>Angle</td><td>t/l</td><td>h/l</td><td>t [mm]</td><td>l [mm]</td><td>h [mm]</td></tr></thead><tbody><tr><td>-30°</td><td>0.4000</td><td>1.8500</td><td>1</td><td>2.5</td><td>8.000</td></tr><tr><td>-45°</td><td>0.3100</td><td>2.0849</td><td>1</td><td>3.226</td><td>6.726</td></tr></tbody></table>

### Cell (-45°)

The elementary cell at -45° was designed starting from the geometric parameters generated by the numerical model and was dimensioned by defining the measurement of "l", of the first angle of 45° and of the thickness "t". The other measures were quoted based on the latter.

The dimensions were assigned to the average line defined as the construction line and then created an internal and external offset of 0.5 mm each respectively.

For compression sample construction, the sketch was subsequently extruded with a height of 30 mm. The constructed cell has a length of l=12.535mm, a width of L=3.0mm and a Volume factor of Vf=0.449 as shown in Figure 2 a), in Figure 2 b) is reported a three dimensional example of single cell extrusion.

<figure><img src="image_2452.png" /><figcaption>Figure 2. Model of Cell (-45°): a) CAD model; b) extruded cell.</figcaption></figure>

### Cell (-30°)

For comparison with the -45° cell, a free h parameter was chosen for the -30° cell. The construction phase instead followed the logic of the previous cell and an extrusion always for a height of 30mm in accordance with sample design for compression tests.

The constructed cell has a length of l=9.390 mm, a width of L=5.062 mm and a Volume factor of Vf=0.552 as shown in Figure 3 a), in Figure 3 b) is reported a three dimensional example of single cell extrusion.
<table><caption>Table 2. Dimensions of the designed core topology</caption><thead><tr><td>Angle</td><td>t/l</td><td>h/l</td><td>t [mm]</td><td>l [mm]</td><td>h [mm]</td></tr></thead><tbody><tr><td>-30°</td><td>0.4000</td><td>1.8500</td><td>1</td><td>2.5</td><td>8.000</td></tr><tr><td>-45°</td><td>0.3100</td><td>2.0849</td><td>1</td><td>3.226</td><td>6.726</td></tr></tbody></table>

### Cell (-45°)

The elementary cell at -45° was designed starting from the geometric parameters generated by the numerical model and was dimensioned by defining the measurement of "l", of the first angle of 45° and of the thickness "t". The other measures were quoted based on the latter.

The dimensions were assigned to the average line defined as the construction line and then created an internal and external offset of 0.5 mm each respectively.

For compression sample construction, the sketch was subsequently extruded with a height of 30 mm. The constructed cell has a length of l=12.535mm, a width of L=3.0mm and a Volume factor of Vf=0.449 as shown in Figure 2 a), in Figure 2 b) is reported a three dimensional example of single cell extrusion.

<figure><img src="image_2445.png" /><figcaption>Figure 2. Model of Cell (-45°): a) CAD model; b) extruded cell.</figcaption></figure>

### Cell (-30°)

For comparison with the -45° cell, a free h parameter was chosen for the -30° cell. The construction phase instead followed the logic of the previous cell and an extrusion always for a height of 30mm in accordance with sample design for compression tests.

The constructed cell has a length of l=9.390 mm, a width of L=5.062 mm and a Volume factor of Vf=0.552 as shown in Figure 3 a), in Figure 3 b) is reported a three dimensional example of single cell extrusion.
<figure><img src="image_2453.png" /><figcaption>Figure 3. Model of Cell (-30°): a) CAD model; b) extruded cell.</figcaption></figure>

### *Samples design*

The dimensions of the compression specimens were established according to the standard ASTM C365 [27].

In particular, three specimens for each design were produced: they have 30 mm x 30 mm x 30 mm dimensions. Four different sample designs were proposed, two auxetic and two traditional. Regarding the auxetic, the first one corresponds to the auxetic cell with $\theta = -30°$ and is named A-30, the second one corresponds to the auxetic cell with $\theta = -45°$ and is named A-45. The number of cell repetitions (n_r) to be carried out for each specimen was evaluated with the following formula:

$$nr = \frac{Ws/ls}{l/w} \qquad (4)$$

Where W and Ws are the cell and sample width, respectively, while l and ls are the cell and sample length.

To evaluate the effect of the auxetic structure on the mechanical performance, two traditional designs for comparison specimens were produced, they have the same dimensions and a filling infill equal to the volume factor, Vf, of the auxetic cells. For the infill strategy, the chosen pattern is +/- 45°, with different volume factor which is aimed to reproduce the same as auxetic. Fig. 5 a and b show respectively the infill strategy for the traditional which corresponds to auxetic 30° and 45°.

So, for the cell -30° an n_r=2.39 for both x and z directions were established and for the cell -45° an n_r=5.93 and n_r=3.19 were selected for x and z directions respectively (Figure 4).
<figure><img src="image_2446.png" /><figcaption>Figure 3. Model of Cell (-30°): a) CAD model; b) extruded cell.</figcaption></figure>

## Samples design

The dimensions of the compression specimens were established according to the standard ASTM C365 [27].

In particular, three specimens for each design were produced: they have 30 mm x 30 mm x 30 mm dimensions. Four different sample designs were proposed, two auxetic and two traditional. Regarding the auxetic, the first one corresponds to the auxetic cell with θ = -30° and is named A-30, the second one corresponds to the auxetic cell with θ = -45° and is named A-45. The number of cell repetitions (n_r) to be carried out for each specimen was evaluated with the following formula:

$$nr = \frac{Ws/ls}{l/w} \qquad (4)$$

Where W and Ws are the cell and sample width, respectively, while l and ls are the cell and sample length.

To evaluate the effect of the auxetic structure on the mechanical performance, two traditional designs for comparison specimens were produced, they have the same dimensions and a filling infill equal to the volume factor, Vf, of the auxetic cells. For the infill strategy, the chosen pattern is +/- 45°, with different volume factor which is aimed to reproduce the same as auxetic. Fig. 5 a and b show respectively the infill strategy for the traditional which corresponds to auxetic 30° and 45°.

So, for the cell -30° an n_r=2.39 for both x and z directions were established and for the cell -45° an n_r=5.93 and n_r=3.19 were selected for x and z directions respectively (Figure 4).
<figure><img src="image_2454.png" /><figcaption>Figure 4. Compression samples: a) A-45; b) A-30°.</figcaption></figure>

<figure><img src="image_2455.png" /><figcaption>Figure 5. Infill strategy of the traditional compression samples.</figcaption></figure>

The material used is a commercial PLA black by Flashforge and the samples were produced by Flashforge 3D printer at 3DNa srl in Naples with a build volume equal to 300 × 270 × 200 mm was used. The extruders move along the x and y axes, while the print bed moves along the z axis. The process starts by uploading the STL file into the related software FlashPrint. It is possible to set process parameters such as materials, deposition strategy; printing speed and temperature are set as optimal according to the material (Table 3).

<table><caption>Table 3. Printer parameters selected.</caption><tr><td>Extruder Temperature [°C]</td><td>220</td></tr><tr><td>Plate Temperature [°C]</td><td>55</td></tr><tr><td>Layer height [mm]</td><td>0.2</td></tr><tr><td>Print speed [mm/s]</td><td>300</td></tr></table>

## Sample characterization

Compression tests were performed using an Instron mod. 4505, equipped with a 100kN load cell and having a crosshead speed 2 mm/min.
<figure><img src="image_2447.png" /><figcaption>Figure 4. Compression samples: a) A-45; b) A-30°.</figcaption></figure>

<figure><img src="image_2448.png" /><figcaption>Figure 5. Infill strategy of the traditional compression samples.</figcaption></figure>

The material used is a commercial PLA black by Flashforge and the samples were produced by Flashforge 3D printer at 3DNa srl in Naples with a build volume equal to 300 × 270 × 200 mm was used. The extruders move along the x and y axes, while the print bed moves along the z axis. The process starts by uploading the STL file into the related software FlashPrint. It is possible to set process parameters such as materials, deposition strategy; printing speed and temperature are set as optimal according to the material (Table 3).

<table><caption>Table 3. Printer parameters selected.</caption><tr><td>Extruder Temperature [°C]</td><td>220</td></tr><tr><td>Plate Temperature [°C]</td><td>55</td></tr><tr><td>Layer height [mm]</td><td>0.2</td></tr><tr><td>Print speed [mm/s]</td><td>300</td></tr></table>

## Sample characterization

Compression tests were performed using an Instron mod. 4505, equipped with a 100kN load cell and having a crosshead speed 2 mm/min.
## Results and discussion

The following Table 4 shows the average and the standard deviation for the traditional specimens. Table 5 summarizes the average and standard deviation values for the auxetic specimens. Figure 6 shows the four representative curves of the compression test, each referring to one different sample design. The process of compression of the serrations of auxetic 3D structures shows a nature related to the characteristics of the destruction of the structures. Once the critical deformation is reached, the brittle material fractures and initially splits into several large parts and the stress drops to almost zero. Going to increase the deformation of the fragments obtained, a packing phenomenon is obtained, the stress increases slightly, which leads to the further fracture of large parts and the formation of smaller ones, then the process is repeated. Such a cracking mechanism leads to the formation of serrations on the load curve. The envelope of this curve also indicates the presence of a pronounced plateau. The area under this curve characterizes the adsorbed strain energy. This phenomenon was just recorded in [28].

<table><caption>Table 4. Average and the standard deviation for the traditional specimens.</caption><thead><tr><td>Sample</td><td>Max stress [MPa]</td><td>Young's modulus [MPa]</td></tr></thead><tbody><tr><td>T -30</td><td>24.95 ± 0.44</td><td>860.77 ± 2.47</td></tr><tr><td>T -45</td><td>19.95 ± 1.68</td><td>742.36 ± 2.67</td></tr></tbody></table>

<table><caption>Table 5. Average and the standard deviation values for the auxetic specimens.</caption><thead><tr><td>Sample</td><td>Max stress 1 [MPa]</td><td>Young's modulus 1 [MPa]</td><td>Max stress 2 [MPa]</td><td>Young's modulus 2 [MPa]</td><td>Max stress 3 [MPa]</td><td>Young's modulus 3 [MPa]</td></tr></thead><tbody><tr><td>A -30</td><td>14.98 ± 0.36</td><td>624.89 ± 11.23</td><td>12.96 ± 1.75</td><td>383.71 ± 85.06</td><td>14.95 ± 1.29</td><td>273.44 ± 14.99</td></tr><tr><td>A -45</td><td>7.36 ± 0.44</td><td>259.93 ± 18.99</td><td>7.20 ± 1.01</td><td>127.37 ± 31.44</td><td>7.51 ± 2.16</td><td>160.44 ± 50.12</td></tr></tbody></table>

<figure><img src="image_2456.png" /><figcaption>Figure 6. Curves of the compression test comparison.</figcaption></figure>

Table 6 reports the results regarding the elastic springback and the shape ratio of the tested samples. The elastic springback is evaluated by considering the ratio between the final height of the sample, under the applied compression load, and the height of the sample after the load release.
## Results and discussion

The following Table 4 shows the average and the standard deviation for the traditional specimens. Table 5 summarizes the average and standard deviation values for the auxetic specimens. Figure 6 shows the four representative curves of the compression test, each referring to one different sample design. The process of compression of the serrations of auxetic 3D structures shows a nature related to the characteristics of the destruction of the structures. Once the critical deformation is reached, the brittle material fractures and initially splits into several large parts and the stress drops to almost zero. Going to increase the deformation of the fragments obtained, a packing phenomenon is obtained, the stress increases slightly, which leads to the further fracture of large parts and the formation of smaller ones, then the process is repeated. Such a cracking mechanism leads to the formation of serrations on the load curve. The envelope of this curve also indicates the presence of a pronounced plateau. The area under this curve characterizes the adsorbed strain energy. This phenomenon was just recorded in [28].

<table><caption>Table 4. Average and the standard deviation for the traditional specimens.</caption><thead><tr><td>Sample</td><td>Max stress [MPa]</td><td>Young's modulus [MPa]</td></tr></thead><tbody><tr><td>T -30</td><td>24.95 ± 0.44</td><td>860.77 ± 2.47</td></tr><tr><td>T -45</td><td>19.95 ± 1.68</td><td>742.36 ± 2.67</td></tr></tbody></table>

<table><caption>Table 5. Average and the standard deviation values for the auxetic specimens.</caption><thead><tr><td>Sample</td><td>Max stress 1 [MPa]</td><td>Young's modulus 1 [MPa]</td><td>Max stress 2 [MPa]</td><td>Young's modulus 2 [MPa]</td><td>Max stress 3 [MPa]</td><td>Young's modulus 3 [MPa]</td></tr></thead><tbody><tr><td>A -30</td><td>14.98 ± 0.36</td><td>624.89 ± 11.23</td><td>12.96 ± 1.75</td><td>383.71 ± 85.06</td><td>14.95 ± 1.29</td><td>273.44 ± 14.99</td></tr><tr><td>A -45</td><td>7.36 ± 0.44</td><td>259.93 ± 18.99</td><td>7.20 ± 1.01</td><td>127.37 ± 31.44</td><td>7.51 ± 2.16</td><td>160.44 ± 50.12</td></tr></tbody></table>

<figure><img src="image_2449.png" /><figcaption>Figure 6. Curves of the compression test comparison.</figcaption></figure>

Table 6 reports the results regarding the elastic springback and the shape ratio of the tested samples. The elastic springback is evaluated by considering the ratio between the final height of the sample, under the applied compression load, and the height of the sample after the load release.
The shape ratio is evaluated by considering the ratio between the final in-plane dimensions of the sample and the initial dimensions. It is reported a significant reduction in the shape ratio between traditional samples and auxetic ones.

<table><caption>Table 6. Elastic spring back and the shape ratio of the tested samples.</caption><thead><tr><td>Sample</td><td>Elastic spring back</td><td>Shape ratio</td></tr></thead><tbody><tr><td>T-30</td><td>1.35 ± 0.03</td><td>1.35 ± 0.08</td></tr><tr><td>T-45</td><td>1.15 ± 0.06</td><td>1.30 ± 0.07</td></tr><tr><td>A-30</td><td>1.15 ± 0.01</td><td>1.12 ± 0.12</td></tr><tr><td>A-45</td><td>1.14 ± 0.01</td><td>1.10 ± 0.11</td></tr></tbody></table>

From these preliminary tests, the following conclusions can be drawn:

* The cell design procedure leads to structures which can be obtained through a standard FDM process.

* The structure resistance mainly depends on the volume density of the workpiece and is not dependent on the thickness of the vertical elements.

* Auxetic structures show a series of peaks, in the compression tests, corresponding to the number of cell lines under the load direction.

* Although the max stress value of the auxetic structures is lower than the traditional ones, they continue to bear the collapse and report almost the same values for each element collapsing. This behaviour is also reported in other research work [28].

* After the compression peak is reached, the mechanism of resistance is completely different. The increase in the strength value for the traditional samples is due to the influence of the testing metal plates; on the other hand, the increase in the auxetic structure is due to a major compaction of the whole structure. This is evident from the shape ratio.

* The influence of the auxetic geometry is evident in the reduction of the shape ratio. This value shows a decrease between 15% and 17% in auxetic samples compared to traditional ones.

* More tests are useful to validate the cell design and the advantages against the traditional printing infill.

This study is a preliminary analysis for a more complete future work where, following a complete characterization of the different auxetic structures, the authors will try to propose a numerical model from which it is possible to evaluate the most appropriate cell to obtain the desired mechanical performances.

## References

[1] V. Birman, G.A. Kardomateas, Review of current trends in research and applications of sandwich structures, Compos. Part B Eng. 142 (2018). https://doi.org/10.1016/j.compositesb.2018.01.027

[2] C. Yang, H.D. Vora, Y. Chang, Behavior of auxetic structures under compression and impact forces, Smart Mater. Struct. 27 (2018). https://doi.org/10.1088/1361-665X/aaa3cf

[3] O. Duncan, T. Shepherd, C. Moroney, L. Foster, P.D. Venkatraman, K. Winwood, T. Allen, A. Alderson, Review of auxetic materials for sports applications: Expanding options in comfort and protection, Appl. Sci. 8 (2018). https://doi.org/10.3390/app8060941

[4] T. Li, Y. Chen, X. Hu, Y. Li, L. Wang, Exploiting negative Poisson's ratio to design 3D-printed composites with enhanced mechanical properties, Mater. Des. 142 (2018). https://doi.org/10.1016/j.matdes.2018.01.034
The shape ratio is evaluated by considering the ratio between the final in-plane dimensions of the sample and the initial dimensions. It is reported a significant reduction in the shape ratio between traditional samples and auxetic ones.

<table><caption>Table 6. Elastic spring back and the shape ratio of the tested samples.</caption><thead><tr><td>Sample</td><td>Elastic spring back</td><td>Shape ratio</td></tr></thead><tbody><tr><td>T-30</td><td>1.35 ± 0.03</td><td>1.35 ± 0.08</td></tr><tr><td>T-45</td><td>1.15 ± 0.06</td><td>1.30 ± 0.07</td></tr><tr><td>A-30</td><td>1.15 ± 0.01</td><td>1.12 ± 0.12</td></tr><tr><td>A-45</td><td>1.14 ± 0.01</td><td>1.10 ± 0.11</td></tr></tbody></table>

From these preliminary tests, the following conclusions can be drawn:

* The cell design procedure leads to structures which can be obtained through a standard FDM process.

* The structure resistance mainly depends on the volume density of the workpiece and is not dependent on the thickness of the vertical elements.

* Auxetic structures show a series of peaks, in the compression tests, corresponding to the number of cell lines under the load direction.

* Although the max stress value of the auxetic structures is lower than the traditional ones, they continue to bear the collapse and report almost the same values for each element collapsing. This behaviour is also reported in other research work [28].

* After the compression peak is reached, the mechanism of resistance is completely different. The increase in the strength value for the traditional samples is due to the influence of the testing metal plates; on the other hand, the increase in the auxetic structure is due to a major compaction of the whole structure. This is evident from the shape ratio.

* The influence of the auxetic geometry is evident in the reduction of the shape ratio. This value shows a decrease between 15% and 17% in auxetic samples compared to traditional ones.

* More tests are useful to validate the cell design and the advantages against the traditional printing infill.

This study is a preliminary analysis for a more complete future work where, following a complete characterization of the different auxetic structures, the authors will try to propose a numerical model from which it is possible to evaluate the most appropriate cell to obtain the desired mechanical performances.

## References

[1] V. Birman, G.A. Kardomateas, Review of current trends in research and applications of sandwich structures, Compos. Part B Eng. 142 (2018). https://doi.org/10.1016/j.compositesb.2018.01.027

[2] C. Yang, H.D. Vora, Y. Chang, Behavior of auxetic structures under compression and impact forces, Smart Mater. Struct. 27 (2018). https://doi.org/10.1088/1361-665X/aaa3cf

[3] O. Duncan, T. Shepherd, C. Moroney, L. Foster, P.D. Venkatraman, K. Winwood, T. Allen, A. Alderson, Review of auxetic materials for sports applications: Expanding options in comfort and protection, Appl. Sci. 8 (2018). https://doi.org/10.3390/app8060941

[4] T. Li, Y. Chen, X. Hu, Y. Li, L. Wang, Exploiting negative Poisson's ratio to design 3D-printed composites with enhanced mechanical properties, Mater. Des. 142 (2018). https://doi.org/10.1016/j.matdes.2018.01.034
[5] K.E. Evans, A. Alderson, Auxetic materials: Functional materials and structures from lateral thinking!, Adv. Mater. 12 (2000). https://doi.org/10.1002/(SICI)1521-4095(200005)12:9<617::AID-ADMA617>3.0.CO;2-3

[6] D.J. Gunton, G.A. Saunders, The Young's modulus and Poisson's ratio of arsenic, antimony and bismuth, J. Mater. Sci. 7 (1972). https://doi.org/10.1007/BF00550070

[7] Y. Li, The anisotropic behavior of Poisson's ratio, Young's modulus, and shear modulus in hexagonal materials, Phys. Status Solidi 38 (1976). https://doi.org/10.1002/pssa.2210380119

[8] A. Yeganeh-Haeri, D.J. Weidner, J.B. Parise, Elasticity of α-cristobalite: A silicon dioxide with a negative poisson's ratio, Science (80-. ). 257 (1992). https://doi.org/10.1126/science.257.5070.650

[9] L.N.G. F., A Treatise on the Mathematical Theory of Elasticity, Nature 105 (1920). https://doi.org/10.1038/105511a0

[10] R.H. Baughman, J.M. Shacklette, A.A. Zakhidov, S. Stafström, Negative poisson's ratios as a common feature of cubic metals, Nature 392 (1998). https://doi.org/10.1038/32842

[11] D.R. Veronda, R.A. Westmann, Mechanical characterization of skin-Finite deformations, J. Biomech. 3 (1970). https://doi.org/10.1016/0021-9290(70)90055-2

[12] L.M. Frolich, M. LaBarbera, W.P. Stevens, Poisson's ratio of a crossed fibre sheath: the skin of aquatic salamanders, J. Zool. 232 (1994). https://doi.org/10.1111/j.1469-7998.1994.tb01571.x

[13] C. Lees, J.F.V. Vincent, J.E. Hillerton, Poisson's ratio in skin, Biomed. Mater. Eng. 1 (1991). https://doi.org/10.3233/BME-1991-1104

[14] J.L. Williams, J.L. Lewis, Properties and an anisotropic model of cancellous bone from the proximal tibial epiphysis, J. Biomech. Eng. 104 (1982). https://doi.org/10.1115/1.3138303

[15] R. Lakes, Foam structures with a negative poisson's ratio, Science (80-. ). 235 (1987). https://doi.org/10.1126/science.235.4792.1038

[16] A. Alderson, K.L. Alderson, Auxetic materials, Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng. 221 (2007) 565-575. https://doi.org/10.1243/09544100JAERO185

[17] H.S. Gill, Mechanical and structure properties of cellular auxetic materials, in: Mater. Today Proc., 2020.

[18] An isotropic three-dimensional structure with Poisson's ratio =-1, J. Elast. 15 (1985). https://doi.org/10.1007/BF00042531

[19] A.G. Kolpakov, Determination of the average characteristics of elastic frameworks, J. Appl. Math. Mech. 49 (1985). https://doi.org/10.1016/0021-8928(85)90011-5

[20] A. V. Chentsov, D.S. Lisovenko, Experimental study of auxetic behavior of cellular structure, J. Phys. Conf. Ser. 991 (2018). https://doi.org/10.1088/1742-6596/991/1/012017

[21] D. Li, L. Dong, R.S. Lakes, A unit cell structure with tunable Poisson's ratio from positive to negative, Mater. Lett. 164 (2016). https://doi.org/10.1016/j.matlet.2015.11.037

[22] Y. Schneider, V. Guski, S. Schmauder, J. Kadkhodapour, J. Hufert, A. Grebhardt, C. Bonten, Deformation Behavior Investigation of Auxetic Structure Made of Poly(butylene adipate-co-terephthalate) Biopolymers Using Finite Element Method, Polymers (Basel). 15 (2023). https://doi.org/10.3390/polym15071792

[23] H. Cho, D. Seo, D.N. Kim, Mechanics of auxetic materials, in: Handb. Mech. Mater., 2019.
[5] K.E. Evans, A. Alderson, Auxetic materials: Functional materials and structures from lateral thinking!, Adv. Mater. 12 (2000). https://doi.org/10.1002/(SICI)1521-4095(200005)12:9<617::AID-ADMA617>3.0.CO;2-3

[6] D.J. Gunton, G.A. Saunders, The Young's modulus and Poisson's ratio of arsenic, antimony and bismuth, J. Mater. Sci. 7 (1972). https://doi.org/10.1007/BF00550070

[7] Y. Li, The anisotropic behavior of Poisson's ratio, Young's modulus, and shear modulus in hexagonal materials, Phys. Status Solidi 38 (1976). https://doi.org/10.1002/pssa.2210380119

[8] A. Yeganeh-Haeri, D.J. Weidner, J.B. Parise, Elasticity of α-cristobalite: A silicon dioxide with a negative poisson's ratio, Science (80-. ). 257 (1992). https://doi.org/10.1126/science.257.5070.650

[9] L.N.G. F., A Treatise on the Mathematical Theory of Elasticity, Nature 105 (1920). https://doi.org/10.1038/105511a0

[10] R.H. Baughman, J.M. Shacklette, A.A. Zakhidov, S. Stafström, Negative poisson's ratios as a common feature of cubic metals, Nature 392 (1998). https://doi.org/10.1038/32842

[11] D.R. Veronda, R.A. Westmann, Mechanical characterization of skin-Finite deformations, J. Biomech. 3 (1970). https://doi.org/10.1016/0021-9290(70)90055-2

[12] L.M. Frolich, M. LaBarbera, W.P. Stevens, Poisson's ratio of a crossed fibre sheath: the skin of aquatic salamanders, J. Zool. 232 (1994). https://doi.org/10.1111/j.1469-7998.1994.tb01571.x

[13] C. Lees, J.F.V. Vincent, J.E. Hillerton, Poisson's ratio in skin, Biomed. Mater. Eng. 1 (1991). https://doi.org/10.3233/BME-1991-1104

[14] J.L. Williams, J.L. Lewis, Properties and an anisotropic model of cancellous bone from the proximal tibial epiphysis, J. Biomech. Eng. 104 (1982). https://doi.org/10.1115/1.3138303

[15] R. Lakes, Foam structures with a negative poisson's ratio, Science (80-. ). 235 (1987). https://doi.org/10.1126/science.235.4792.1038

[16] A. Alderson, K.L. Alderson, Auxetic materials, Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng. 221 (2007) 565-575. https://doi.org/10.1243/09544100JAERO185

[17] H.S. Gill, Mechanical and structure properties of cellular auxetic materials, in: Mater. Today Proc., 2020.

[18] An isotropic three-dimensional structure with Poisson's ratio =-1, J. Elast. 15 (1985). https://doi.org/10.1007/BF00042531

[19] A.G. Kolpakov, Determination of the average characteristics of elastic frameworks, J. Appl. Math. Mech. 49 (1985). https://doi.org/10.1016/0021-8928(85)90011-5

[20] A. V. Chentsov, D.S. Lisovenko, Experimental study of auxetic behavior of cellular structure, J. Phys. Conf. Ser. 991 (2018). https://doi.org/10.1088/1742-6596/991/1/012017

[21] D. Li, L. Dong, R.S. Lakes, A unit cell structure with tunable Poisson's ratio from positive to negative, Mater. Lett. 164 (2016). https://doi.org/10.1016/j.matlet.2015.11.037

[22] Y. Schneider, V. Guski, S. Schmauder, J. Kadkhodapour, J. Hufert, A. Grebhardt, C. Bonten, Deformation Behavior Investigation of Auxetic Structure Made of Poly(butylene adipate-co-terephthalate) Biopolymers Using Finite Element Method, Polymers (Basel). 15 (2023). https://doi.org/10.3390/polym15071792

[23] H. Cho, D. Seo, D.N. Kim, Mechanics of auxetic materials, in: Handb. Mech. Mater., 2019.
https://doi.org/10.1007/978-981-10-6884-3_25

[24] A. Joseph, V. Mahesh, D. Harursampath, On the application of additive manufacturing methods for auxetic structures: a review, Adv. Manuf. 9 (2021). https://doi.org/10.1007/s40436-021-00357-y

[25] S. Hou, T. Li, Z. Jia, L. Wang, Mechanical properties of sandwich composites with 3d-printed auxetic and non-auxetic lattice cores under low velocity impact, Mater. Des. 160 (2018). https://doi.org/10.1016/j.matdes.2018.11.002

[26] L.J. Gibson, M.F. Ashby, Cellular solids: Structure and properties, second edition, 2014.

[27] ASTM, ASTM C365-03: Standard test method for flatwise compressive properties of sandwich cores, Annu. B. ASTM Stand. (2008).

[28] O. Tolochyna, N. Zgalat-Lozynska, Y. Podrezov, D. Verbylo, O. Tolochyn, O. Zgalat-Lozynskyy, The role of flexible polymer composite materials properties in energy absorption of three-dimensional auxetic lattice structures, Mater. Today Commun. 37 (2023) 107370. https://doi.org/10.1016/j.mtcomm.2023.107370
https://doi.org/10.1007/978-981-10-6884-3_25

[24] A. Joseph, V. Mahesh, D. Harursampath, On the application of additive manufacturing methods for auxetic structures: a review, Adv. Manuf. 9 (2021). https://doi.org/10.1007/s40436-021-00357-y

[25] S. Hou, T. Li, Z. Jia, L. Wang, Mechanical properties of sandwich composites with 3d-printed auxetic and non-auxetic lattice cores under low velocity impact, Mater. Des. 160 (2018). https://doi.org/10.1016/j.matdes.2018.11.002

[26] L.J. Gibson, M.F. Ashby, Cellular solids: Structure and properties, second edition, 2014.

[27] ASTM, ASTM C365-03: Standard test method for flatwise compressive properties of sandwich cores, Annu. B. ASTM Stand. (2008).

[28] O. Tolochyna, N. Zgalat-Lozynska, Y. Podrezov, D. Verbylo, O. Tolochyn, O. Zgalat-Lozynskyy, The role of flexible polymer composite materials properties in energy absorption of three-dimensional auxetic lattice structures, Mater. Today Commun. 37 (2023) 107370. https://doi.org/10.1016/j.mtcomm.2023.107370
# Characterization of recycled polyethylene terephthalate (rPET)
— impact of proportion of virgin and recycled PET on
mechanical strength

NGUYEN Thanh-Tung<sup>1,a*</sup>, LUO Yun-Mei<sup>1,b</sup>, CHEVALIER Luc<sup>1,c</sup>,
LESUEUR François<sup>2,d</sup>, DERRIEN Mikael<sup>2,e</sup>

**Keywords:** Recycled PET, Characterization, Mechanical Strength, Bi-Axial Test

**Abstract.** Polyethylene terephthalate (PET) is a standard plastic with the highest consumption in the food packaging industry. The large amount of bottles produced makes imperative the search for the recycled PET (rPET). However, the recycling procedure generates the modifications of the mechanical, thermal and optical properties from virgin PET to recycled one. In this study, Digital Image Correlation method associated to numerical simulation is used to identify the mechanical properties of the bottles. The effects of various recycled PET ratios on the mechanical properties of bottles are discussed.

**Introduction**

The massive production of PET plastic bottles generates very visible pollution problems around which negative communication endangers the future of the packaging industry. Recycling is certainly a response to this situation in order to improve the image of the polymer industry by working on the sorting and recycling sectors. Producing new bottles from old ones is a simple idea unless the properties of the product are not too degraded compared to a new bottle.

Numerous works have been carried out [1-5] to capture the mechanical and thermal behavior of rPET. In this work we look for the correlation between the rate of recycled and the induced mechanical behavior. It is well known that the blowing process reinforces the mechanical strength and rigidity of the material [6-9] and our purpose is to measure the induced elastic properties in order to answer whether if the recycled PET reduces or not these induced properties.

In the first part, we will present the material, the specimen and the method used to measure the wanted properties. Then, in the second part, we will present and discuss the obtained results.

**Material and identification methods**

The material used as the virgin PET is RAMAPET N180. Its glass transition temperature is 78+/-2°C and its intrinsic viscosity is 0.80+/-0.02 dl/g. The recycled PET is referenced MOPET and its intrinsic viscosity is 0.85+/-0.02 dl/g. Characteristics are detailed in the Tables of the Appendix.

Four types of bottles were blown by Sidel with different mass fraction of recycled PET (see Table 1). The geometry of the preform is illustrated on Fig.1a. The final diameter of the bottle set on the blowing apparatus shown on Fig.1b is 70mm and the measured thickness in the central region is 0.3mm.

<figure><img src="image_2457.png" /></figure>

<footnote n="1"><sup>1</sup>Université Gustave Eiffel, Laboratoire Modélisation et Simulation Multi Echelle, MSME UMR
8208 CNRS, 5 bd Descartes, 77545 Marne-la-Vallée, France</footnote>

<footnote n="2"><sup>2</sup>SIDEL Blowing & Services, Avenue de la Patrouille de France, C5 60627 Octeville sur mer,
France</footnote>

<footnote><sup>a</sup>thanh-tung.nguyen@univ-eiffel.fr, <sup>b</sup>yun-mei.luo@univ-eiffel.fr, <sup>c</sup>luc.chevalier@univ-eiffel.fr,</footnote>

<footnote><sup>d</sup>francois.lesueur@sidel.com, <sup>e</sup>mikael.derrien@sidel.com</footnote>
# Characterization of recycled polyethylene terephthalate (rPET)
— impact of proportion of virgin and recycled PET on
mechanical strength

NGUYEN Thanh-Tung<sup>1,a*</sup>, LUO Yun-Mei<sup>1,b</sup>, CHEVALIER Luc<sup>1,c</sup>,
LESUEUR François<sup>2,d</sup>, DERRIEN Mikael<sup>2,e</sup>

**Keywords:** Recycled PET, Characterization, Mechanical Strength, Bi-Axial Test

**Abstract.** Polyethylene terephthalate (PET) is a standard plastic with the highest consumption in the food packaging industry. The large amount of bottles produced makes imperative the search for the recycled PET (rPET). However, the recycling procedure generates the modifications of the mechanical, thermal and optical properties from virgin PET to recycled one. In this study, Digital Image Correlation method associated to numerical simulation is used to identify the mechanical properties of the bottles. The effects of various recycled PET ratios on the mechanical properties of bottles are discussed.

**Introduction**

The massive production of PET plastic bottles generates very visible pollution problems around which negative communication endangers the future of the packaging industry. Recycling is certainly a response to this situation in order to improve the image of the polymer industry by working on the sorting and recycling sectors. Producing new bottles from old ones is a simple idea unless the properties of the product are not too degraded compared to a new bottle.

Numerous works have been carried out [1-5] to capture the mechanical and thermal behavior of rPET. In this work we look for the correlation between the rate of recycled and the induced mechanical behavior. It is well known that the blowing process reinforces the mechanical strength and rigidity of the material [6-9] and our purpose is to measure the induced elastic properties in order to answer whether if the recycled PET reduces or not these induced properties.

In the first part, we will present the material, the specimen and the method used to measure the wanted properties. Then, in the second part, we will present and discuss the obtained results.

**Material and identification methods**

The material used as the virgin PET is RAMAPET N180. Its glass transition temperature is 78+/-2°C and its intrinsic viscosity is 0.80+/-0.02 dl/g. The recycled PET is referenced MOPET and its intrinsic viscosity is 0.85+/-0.02 dl/g. Characteristics are detailed in the Tables of the Appendix.

Four types of bottles were blown by Sidel with different mass fraction of recycled PET (see Table 1). The geometry of the preform is illustrated on Fig.1a. The final diameter of the bottle set on the blowing apparatus shown on Fig.1b is 70mm and the measured thickness in the central region is 0.3mm.

<figure><img src="image_2450.png" /></figure>

<footnote n="1"><sup>1</sup>Université Gustave Eiffel, Laboratoire Modélisation et Simulation Multi Echelle, MSME UMR
8208 CNRS, 5 bd Descartes, 77545 Marne-la-Vallée, France</footnote>

<footnote n="2"><sup>2</sup>SIDEL Blowing & Services, Avenue de la Patrouille de France, C5 60627 Octeville sur mer,
France</footnote>

<footnote><sup>a</sup>thanh-tung.nguyen@univ-eiffel.fr, <sup>b</sup>yun-mei.luo@univ-eiffel.fr, <sup>c</sup>luc.chevalier@univ-eiffel.fr,</footnote>

<footnote><sup>d</sup>francois.lesueur@sidel.com, <sup>e</sup>mikael.derrien@sidel.com</footnote>
<figure><img src="image_2458.png" /><figcaption>Figure 1. (a) The preform geometry, (b) the final bottle during the internal pressure test</figcaption></figure>

Consequently, one can evaluate the radial elongation $\lambda_r=5.15$ and the thickness contraction $\lambda_e=0.093$. By assuming the volume conservation, one can estimate the longitudinal elongation in the central region $\lambda_z=2.09$.

<table><caption>Table 1: Four different types of bottle</caption><thead><tr><th></th><th>MOPET (recycled)</th><th>N180 (virgin)</th></tr></thead><tbody><tr><th>Bottle 1</th><td>0%</td><td>100%</td></tr><tr><th>Bottle 2</th><td>25%</td><td>75%</td></tr><tr><th>Bottle 3</th><td>50%</td><td>50%</td></tr><tr><th>Bottle 4</th><td>100%</td><td>0%</td></tr></tbody></table>

Two different methods are used to identify the mechanical properties of the blown bottles. Both are based on measuring strains with a digital image correlation (DIC) technique [10]. For the nondestructive method, a blowing apparatus developed in laboratory MSME is used in order to put the bottle under pressure (Fig.1b). We focus on a region of interest (ROI) where the DIC provides the components of the displacement field as shown on Fig.2.

This method has already been detailed in previous paper of authors [11] for example. To make sure of the linearity, we increase the pressure by step of 0.5 bar from 0 to 5bar and the strains and stresses are obtained at each step. Considering the cylindrical central zone, one can evaluate the stresses $\sigma_r$ and $\sigma_z$ from the internal pressure $P$:

$$ \sigma_z = \frac{PR}{2e}, \sigma_\theta = \frac{PR}{e} \qquad (1) $$

where $R$ the radius in the cylindrical part of the bottle (35mm) and $e$ the thickness of the same part (0.3mm).

This leads to two relations from where one can evaluate the modulus $E_\theta$ and $E_z$ if the Poisson's ratio is known.
<figure><img src="image_2451.png" /><figcaption>Figure 1. (a) The preform geometry, (b) the final bottle during the internal pressure test</figcaption></figure>

Consequently, one can evaluate the radial elongation $\lambda_r=5.15$ and the thickness contraction $\lambda_e=0.093$. By assuming the volume conservation, one can estimate the longitudinal elongation in the central region $\lambda_z=2.09$.

<table><caption>Table 1: Four different types of bottle</caption><thead><tr><th></th><th>MOPET (recycled)</th><th>N180 (virgin)</th></tr></thead><tbody><tr><th>Bottle 1</th><td>0%</td><td>100%</td></tr><tr><th>Bottle 2</th><td>25%</td><td>75%</td></tr><tr><th>Bottle 3</th><td>50%</td><td>50%</td></tr><tr><th>Bottle 4</th><td>100%</td><td>0%</td></tr></tbody></table>

Two different methods are used to identify the mechanical properties of the blown bottles. Both are based on measuring strains with a digital image correlation (DIC) technique [10]. For the nondestructive method, a blowing apparatus developed in laboratory MSME is used in order to put the bottle under pressure (Fig.1b). We focus on a region of interest (ROI) where the DIC provides the components of the displacement field as shown on Fig.2.

This method has already been detailed in previous paper of authors [11] for example. To make sure of the linearity, we increase the pressure by step of 0.5 bar from 0 to 5bar and the strains and stresses are obtained at each step. Considering the cylindrical central zone, one can evaluate the stresses $\sigma_r$ and $\sigma_z$ from the internal pressure $P$:

$$ \sigma_z = \frac{PR}{2e}, \sigma_\theta = \frac{PR}{e} \qquad (1) $$

where $R$ the radius in the cylindrical part of the bottle (35mm) and $e$ the thickness of the same part (0.3mm).

This leads to two relations from where one can evaluate the modulus $E_\theta$ and $E_z$ if the Poisson's ratio is known.
$$ \epsilon_z = \frac{PR}{2eE_z} \left(1 - 2\frac{\nu_{\theta z}}{E_{\theta}}E_z\right), \epsilon_{\theta} = \frac{PR}{eE_{\theta}} \left(1 - \frac{\nu_{\theta z}}{2}\right) \qquad (2) $$

<figure><img src="image_2459.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_2460.png" /><figcaption>Figure 2. (a) The region of interest (ROI) measures in the central part of bottle that can be considered almost plane; (b) Visualization of the components U and V of the ROI under 2 bar pressure</figcaption></figure>

The other method, a destructive one, is managed on a biaxial tensile machine also developed at MSME shown on Fig.3 and described in a previous paper of authors [11]. The central zone of the bottle is cut and machined in a water jet machine to produce a cross specimen. To break the symmetry and to generate an heterogeneous strain field, a hole is done on the specimen. The location of the hole has been studied in order to optimize the identification process. This process has been detailed in a previous publication of authors [11] and enables to determine all elastic components of the orthotropic material.
$$ \epsilon_z = \frac{PR}{2eE_z} \left( 1 - 2 \frac{\nu_{\theta z}}{E_{\theta}} E_z \right), \epsilon_{\theta} = \frac{PR}{eE_{\theta}} \left( 1 - \frac{\nu_{\theta z}}{2} \right) \qquad (2) $$

<figure><img src="image_2452.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_2453.png" /><figcaption>(b)</figcaption></figure>

<caption>Figure 2. (a) The region of interest (ROI) measures in the central part of bottle that can be considered almost plane; (b) Visualization of the components U and V of the ROI under 2 bar pressure</caption>

The other method, a destructive one, is managed on a biaxial tensile machine also developed at MSME shown on Fig.3 and described in a previous paper of authors [11]. The central zone of the bottle is cut and machined in a water jet machine to produce a cross specimen. To break the symmetry and to generate an heterogeneous strain field, a hole is done on the specimen. The location of the hole has been studied in order to optimize the identification process. This process has been detailed in a previous publication of authors [11] and enables to determine all elastic components of the orthotropic material.
<figure><img src="image_2461.png" /><figcaption>Figure 3. (a) Biaxial testing machine, (b) specimen cut on a water jet machine and (c) biaxial specimen with hole and speckle placed on biaxial machine.</figcaption></figure>

Once again, the DIC method coupled with finite element meshing of the ROI (see Fig.4a) are used but instead of using the virtual field method presented in [11], we carried out a two steps identification. First, we guess some realistic values from literature [9,11] for the material parameters and a finite element calculation is managed imposing as Dirichlet boundary conditions, the DIC displacements on the 4 arms.

After the calculation, the computed global tensile loads $F_x$ and $F_y$ are compared with the measured ones and then 2 correcting factors $\alpha_x$ and $\alpha_y$ are applied to the loads distribution. The second step consists to apply Neumann boundary conditions on the top and left arms with the corrected distribution. We still impose the Dirichlet boundary condition on the bottom and right ones.

From then the set of parameters can be optimized in order to minimize the difference between the DIC displacements measured inside the specimen (Fig.4b) and the finite element displacements depending on the parameter values. This minimization is carried out using the Nelder-Mead simplex algorithm as described in Lagarias et al. [12]. in Matlab. This adaptation of the Finite Element Model Updating (FEMU) method is presented in [13,14] and gives all the material parameters from a single test. In this method, the unknown properties are determined through minimization of $\Phi$ with respect to material parameters that is the square difference between the displacement measured (DIC) and the displacement calculated from finite element method (FE) using DIC as boundary conditions.
<figure><img src="image_2454.png" /><figcaption>Figure 3. (a) Biaxial testing machine, (b) specimen cut on a water jet machine and (c) biaxial specimen with hole and speckle placed on biaxial machine.</figcaption></figure>

Once again, the DIC method coupled with finite element meshing of the ROI (see Fig.4a) are used but instead of using the virtual field method presented in [11], we carried out a two steps identification. First, we guess some realistic values from literature [9,11] for the material parameters and a finite element calculation is managed imposing as Dirichlet boundary conditions, the DIC displacements on the 4 arms.

After the calculation, the computed global tensile loads $F_x$ and $F_y$ are compared with the measured ones and then 2 correcting factors $\alpha_x$ and $\alpha_y$ are applied to the loads distribution. The second step consists to apply Neumann boundary conditions on the top and left arms with the corrected distribution. We still impose the Dirichlet boundary condition on the bottom and right ones.

From then the set of parameters can be optimized in order to minimize the difference between the DIC displacements measured inside the specimen (Fig.4b) and the finite element displacements depending on the parameter values. This minimization is carried out using the Nelder-Mead simplex algorithm as described in Lagarias et al. [12]. in Matlab. This adaptation of the Finite Element Model Updating (FEMU) method is presented in [13,14] and gives all the material parameters from a single test. In this method, the unknown properties are determined through minimization of $\Phi$ with respect to material parameters that is the square difference between the displacement measured (DIC) and the displacement calculated from finite element method (FE) using DIC as boundary conditions.
<figure><img src="image_2462.png" /><figcaption>Figure 4. (a) Mesh of the biaxial specimen with hole and (b) displacement components after an equal-biaxial stretching of the arms.</figcaption></figure>

## Results and discussions

First, we will focus on results of the virgin PET N180. Late 90's, Chevalier *et al.* [15] proposed a simple and empiric model to predict the induced modulus from the elongations. The longitudinal modulus $E_z$ was modelled by a 3rd order polynomial function of $\lambda_z$ while the hoop modulus $E_{\theta}$ was represented by a 2nd order polynomial function of $\lambda_r$ based on the low number of experimental values higher than $\lambda_r=4$. With the value of $\lambda_r$ equals to 5.15, the predicted hoop modulus would rise to more than 6000MPa which seems to be more than what the measure gives.

Let us first examine the results given by the modified FEMU method from the biaxial tensile test on the cross specimen with a hole. Table 2 summarizes the identified values obtained from the 3 tests managed under identical conditions.
<figure><img src="image_2455.png" /><figcaption>Figure 4. (a) Mesh of the biaxial specimen with hole and (b) displacement components after an equal-biaxial stretching of the arms.</figcaption></figure>

## Results and discussions

First, we will focus on results of the virgin PET N180. Late 90's, Chevalier *et al.* [15] proposed a simple and empiric model to predict the induced modulus from the elongations. The longitudinal modulus $E_z$ was modelled by a 3rd order polynomial function of $\lambda_z$ while the hoop modulus $E_{\theta}$ was represented by a 2nd order polynomial function of $\lambda_r$ based on the low number of experimental values higher than $\lambda_r=4$. With the value of $\lambda_r$ equals to 5.15, the predicted hoop modulus would rise to more than 6000MPa which seems to be more than what the measure gives.

Let us first examine the results given by the modified FEMU method from the biaxial tensile test on the cross specimen with a hole. Table 2 summarizes the identified values obtained from the 3 tests managed under identical conditions.
<table><caption>Table 2: Characteristics of virgin N180 samples</caption><thead><tr><th>Characteristics</th><th>E<sub>θ</sub> (MPa)</th><th>υ<sub>θz</sub></th><th>E<sub>z</sub> (MPa)</th><th>υ<sub>zθ</sub></th><th>G<sub>θz</sub> (MPa)</th></tr></thead><tbody><tr><td>Mean value</td><td>3230</td><td>0.43</td><td>1790</td><td>0.25</td><td>1420</td></tr><tr><td>Standard deviation</td><td>1025</td><td>0.05</td><td>540</td><td>0.16</td><td>320</td></tr><tr><td>Dispersion</td><td>29%</td><td>12%</td><td>29%</td><td>63%</td><td>23%</td></tr></tbody></table>

It is interesting to notice that:

- the ratio between the two modulus is conform to what is usually measured [9,11]. Here the ratio is equal to 1.8 between the hoop modulus and the longitudinal one.

- the dispersion is very important for all parameters and this is probably more a consequence of the method (or the measure) rather than a real dispersion from one sample to another. Consequently, we managed the DIC correlation in the arms that supposed to be nearly in simple tension to confirm the modulus and Poisson's ratio.

Figure 5a shows the ROI chosen for this confirmation and Fig.5b confirms that the arm is almost in simple tension because of the regular spacing of the contour lines. One can see that these regular straight lines are only located in the left side of the ROI, so the mean values of the strains must be measured there.

<figure><img src="image_2463.png" /><figcaption>Figure 5. (a) ROI in the arm of the biaxial specimen with hole and (b) displacement component after an equibiaxial stretching of the arms.</figcaption></figure>

Assuming the Poisson's ratio $υ_{θz}$ is equal to 0.43, one can manage the series of measure on bottles under pressure. From the displacement fields, we calculate the average deformations in the ROI. 30 rectangle sub-regions are chosen in order to obtain the strains $ε_z$ and $εθ$ and their dispersion.

The typical evolution of $ε_z$ and $εθ$ as a function of the pressure ($P(i) = 0.05$ times $i$ in MPa) are illustrated in Fig. 6a. Values are well aligned but fitted lines do not pass exactly through 0 when the pressure is zero. This is often the case because the bottles are never perfectly cylindrical but this geometrical defect disappears after the pressurization. We proceed to a shift (passage of the lines by 0) and proceed to the identification of the elastic modulus at each step as shown on Fig.6b.
<table><caption>Table 2: Characteristics of virgin N180 samples</caption><thead><tr><th>Characteristics</th><th>E<sub>θ</sub> (MPa)</th><th>υ<sub>θz</sub></th><th>E<sub>z</sub> (MPa)</th><th>υ<sub>zθ</sub></th><th>G<sub>θz</sub> (MPa)</th></tr></thead><tbody><tr><td>Mean value</td><td>3230</td><td>0.43</td><td>1790</td><td>0.25</td><td>1420</td></tr><tr><td>Standard deviation</td><td>1025</td><td>0.05</td><td>540</td><td>0.16</td><td>320</td></tr><tr><td>Dispersion</td><td>29%</td><td>12%</td><td>29%</td><td>63%</td><td>23%</td></tr></tbody></table>

It is interesting to notice that:

- the ratio between the two modulus is conform to what is usually measured [9,11]. Here the ratio is equal to 1.8 between the hoop modulus and the longitudinal one.

- the dispersion is very important for all parameters and this is probably more a consequence of the method (or the measure) rather than a real dispersion from one sample to another. Consequently, we managed the DIC correlation in the arms that supposed to be nearly in simple tension to confirm the modulus and Poisson's ratio.

Figure 5a shows the ROI chosen for this confirmation and Fig.5b confirms that the arm is almost in simple tension because of the regular spacing of the contour lines. One can see that these regular straight lines are only located in the left side of the ROI, so the mean values of the strains must be measured there.

<figure><img src="image_2456.png" /><figcaption>Figure 5. (a) ROI in the arm of the biaxial specimen with hole and (b) displacement component after an equibiaxial stretching of the arms.</figcaption></figure>

Assuming the Poisson's ratio $υ_{θz}$ is equal to 0.43, one can manage the series of measure on bottles under pressure. From the displacement fields, we calculate the average deformations in the ROI. 30 rectangle sub-regions are chosen in order to obtain the strains $ε_z$ and $εθ$ and their dispersion.

The typical evolution of $ε_z$ and $εθ$ as a function of the pressure ($P(i) = 0.05$ times $i$ in MPa) are illustrated in Fig. 6a. Values are well aligned but fitted lines do not pass exactly through 0 when the pressure is zero. This is often the case because the bottles are never perfectly cylindrical but this geometrical defect disappears after the pressurization. We proceed to a shift (passage of the lines by 0) and proceed to the identification of the elastic modulus at each step as shown on Fig.6b.
<figure><img src="image_2464.png" /><figcaption>Figure 6. (a) Linear strain evolution versus pressure (b) Induced modulus in virgin N180 PET bottles for $v_{\theta z}=0.43$</figcaption></figure>

Table 3 shows the values of the longitudinal and circumferential Young moduli in the case of virgin PET N180. Several bottles of the same grade of PET were tested both under pressure and by the arms of the biaxial tensile test. The mean values of $E_{\theta}$ is less than 8% far from the one identified from the global modified FEMU, the $E_{z}$ modulus is 14% lower. Consequently, the ratio decreases to 1.45, which is smaller than what is usually observed in bottles.

The dispersion of this second series of identification is significantly lower than with the global method: less than 4% for the circumferential module while we reach 12% for the longitudinal module. In literature [15], one observes that there is a less dispersion on the longitudinal modulus than on the circumferential one. Because the preform is stretched longitudinally by a rod, the longitudinal elongation is fixed.

<table><caption>Table 3: Modulus of virgin N180 samples confirmed from the tension in the arms and the bottle under pressure</caption><thead><tr><th></th><th><i>E</i><sub>θ</sub> (MPa)</th><th><i>E</i><sub>z</sub> (MPa)</th></tr></thead><tbody><tr><td>Mean value</td><td>2990</td><td>2065</td></tr><tr><td>Standard deviation</td><td>110</td><td>240</td></tr><tr><td>Dispersion</td><td>4%</td><td>12%</td></tr></tbody></table>

Considering this identification, we propose to modify the empirical function used in [15] and to choose a 3rd order polynomial function for the modulus $E_{\theta}$ so it varies by passing by a maximum which defines an optimal elongation as one could already see for $E_{z}$. The evolution are plotted on Fig.7.

$$ 
\begin{aligned}
E_z &= -36.66\lambda_z^3 + 296.9\lambda_z^2 - 396.0\lambda_z + 1313.5 \\
E_{\theta} &= -83.39\lambda_{\theta}^3 + 565.2\lambda_{\theta}^2 - 395.14\lambda_{\theta} + 1206.5
\end{aligned} 
\qquad (3) $$
<figure><img src="image_2457.png" /><figcaption>Figure 6. (a) Linear strain evolution versus pressure (b) Induced modulus in virgin N180 PET bottles for $v_{\theta z}=0.43$</figcaption></figure>

Table 3 shows the values of the longitudinal and circumferential Young moduli in the case of virgin PET N180. Several bottles of the same grade of PET were tested both under pressure and by the arms of the biaxial tensile test. The mean values of $E_{\theta}$ is less than 8% far from the one identified from the global modified FEMU, the $E_{z}$ modulus is 14% lower. Consequently, the ratio decreases to 1.45, which is smaller than what is usually observed in bottles.

The dispersion of this second series of identification is significantly lower than with the global method: less than 4% for the circumferential module while we reach 12% for the longitudinal module. In literature [15], one observes that there is a less dispersion on the longitudinal modulus than on the circumferential one. Because the preform is stretched longitudinally by a rod, the longitudinal elongation is fixed.

<table><caption>Table 3: Modulus of virgin N180 samples confirmed from the tension in the arms and the bottle under pressure</caption><thead><tr><th></th><th><i>E</i><sub>θ</sub> (MPa)</th><th><i>E</i><sub>z</sub> (MPa)</th></tr></thead><tbody><tr><td>Mean value</td><td>2990</td><td>2065</td></tr><tr><td>Standard deviation</td><td>110</td><td>240</td></tr><tr><td>Dispersion</td><td>4%</td><td>12%</td></tr></tbody></table>

Considering this identification, we propose to modify the empirical function used in [15] and to choose a 3rd order polynomial function for the modulus $E_{\theta}$ so it varies by passing by a maximum which defines an optimal elongation as one could already see for $E_{z}$. The evolution are plotted on Fig.7.

$$ 
\begin{aligned}
E_z &= -36.66\lambda_z^3 + 296.9\lambda_z^2 - 396.0\lambda_z + 1313.5 \\
E_{\theta} &= -83.39\lambda_{\theta}^3 + 565.2\lambda_{\theta}^2 - 395.14\lambda_{\theta} + 1206.5
\end{aligned} 
\quad (3) $$
<figure><img src="image_2465.png" /><figcaption>Figure 7. Evolution of modulus *E*<sub>z</sub> and *E*<sub>θ</sub> together with previous experimental data from [15] and values predicted or identified for N180 PET.</figcaption></figure>

Through inflation tests, we carried out a modulus measurement campaign for the three MOPET recycled rates, i.e. 25%, 50% and 100%. Table 4 and Fig. 8 show that there is no very clear influence of the recycling rate on the induced modules. It seems that recycled MOPET reinforces the virgin N180 in terms of rigidity: all the modulus of rPET are higher than that of virgin N180.

When we analyze the material sheets (Fig. 9 in the appendix), we see that the density and intrinsic viscosity of MOPET are greater than those of N180 ($ρ_{MOPET}$=0.9kg/dm³ versus $ρ_{N180}$=0.83kg/dm³ and $IV_{MOPET}$ = 0.82-0.87dl/g in front of $IV_{N180}$ = 0.78-0.82dl/g). This is certainly the explanation of why the MOPET bottles are more rigid than the virgin PET N180 one.

<figure><img src="image_2466.png" /><figcaption>Figure 8: Induced modulus versus virgin N180 PET rate</figcaption></figure>
<figure><img src="image_2458.png" /><figcaption>Figure 7. Evolution of modulus $E_z$ and $E_\theta$ together with previous experimental data from [15] and values predicted or identified for N180 PET.</figcaption></figure>

Through inflation tests, we carried out a modulus measurement campaign for the three MOPET recycled rates, i.e. 25%, 50% and 100%. Table 4 and Fig. 8 show that there is no very clear influence of the recycling rate on the induced modules. It seems that recycled MOPET reinforces the virgin N180 in terms of rigidity: all the modulus of rPET are higher than that of virgin N180.

When we analyze the material sheets (Fig. 9 in the appendix), we see that the density and intrinsic viscosity of MOPET are greater than those of N180 ($\rho_{MOPET}$=0.9kg/dm³ versus $\rho_{N180}$=0.83kg/dm³ and $\overline{V_{MOPET}}$ = 0.82-0.87dl/g in front of $\overline{V_{N180}}$ = 0.78-0.82dl/g). This is certainly the explanation of why the MOPET bottles are more rigid than the virgin PET N180 one.

<figure><img src="image_2459.png" /><figcaption>Figure 8: Induced modulus versus virgin N180 PET rate</figcaption></figure>
<table><caption>Table 4: Modulus of rPET samples – influence of the rate of recycled PET</caption><thead><tr><th></th><th>E<sub>θ</sub> (MPa) and<br>dispersion</th><th>E<sub>z</sub> (MPa) and<br>dispersion</th></tr></thead><tbody><tr><td>Bottle 1 – 100% N180, 0%<br>MOPET</td><td>2990<br>4%</td><td>2065<br>12%</td></tr><tr><td>Bottle 2 – 75% N180, 25%<br>MOPET</td><td>3595<br>9%</td><td>2580<br>5%</td></tr><tr><td>Bottle 3 – 50% N180, 50%<br>MOPET</td><td>3430<br>3%</td><td>2560<br>8%</td></tr><tr><td>Bottle 4 – 0% N180, 100%<br>MOPET</td><td>3330<br>8%</td><td>2680<br>15%</td></tr></tbody></table>

## Conclusion

The measure of the mechanical elastic properties induced by the stretch blow molding process has been carried out on rPET bottles by two different ways: from heterogeneous biaxial tensile tests and from the growth of the bottle under pressure. The second method provides experimental results with less variation and has been used to test the influence of the rate of rPET in the initial preform.

The specific geometry of the blown bottle enriches the data in order to provide a better interpolation of the predicted modulus versus elongation during the process. The influence of the rate of recycled material has been studied. Finally, it appears that in our case, the recycled part reinforce the rigidity in both longitudinal and hoop directions.

## Appendix: material characteristics

<table><thead><tr><th>Property</th><th>Unit</th><th>Test method</th><th>RAMAPET N180 value</th></tr></thead><tbody><tr><td>Intrinsic Viscosity</td><td>dl/g</td><td>(K) SDP-15<br>(R) IND-A-PE-G-IV-01<br>(Q) CQ-V-1</td><td>0.80 ± 0.02</td></tr><tr><td>Acetaldehyde</td><td>ppm</td><td>(K) SDP-21<br>(R) IND-A-PE-GC-01<br>(Q) ASTM F 2013</td><td>Max 1</td></tr><tr><td>Colour L*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>Min 78</td></tr><tr><td>Colour a*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>-3.8 to -0.8</td></tr><tr><td>Colour b*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>-3.5 to 1</td></tr><tr><td>Pellet weight</td><td>Pieces/gram</td><td>(K) SDP-18<br>(R) IND-A-PE-PTM-09<br>(Q) CQ-GA-2</td><td>60 ± 5</td></tr><tr><td>Fines</td><td>Wt%</td><td>(K) SDP-60<br>(R) IND-A-PE-GA-04<br>(Q) CQ-GA-4</td><td>Max 0.05</td></tr></tbody></table>

(a)
<table><caption>Table 4: Modulus of rPET samples – influence of the rate of recycled PET</caption><thead><tr><th></th><th>E<sub>θ</sub> (MPa) and<br>dispersion</th><th>E<sub>z</sub> (MPa) and<br>dispersion</th></tr></thead><tbody><tr><td>Bottle 1 – 100% N180, 0%<br>MOPET</td><td>2990<br>4%</td><td>2065<br>12%</td></tr><tr><td>Bottle 2 – 75% N180, 25%<br>MOPET</td><td>3595<br>9%</td><td>2580<br>5%</td></tr><tr><td>Bottle 3 – 50% N180, 50%<br>MOPET</td><td>3430<br>3%</td><td>2560<br>8%</td></tr><tr><td>Bottle 4 – 0% N180, 100%<br>MOPET</td><td>3330<br>8%</td><td>2680<br>15%</td></tr></tbody></table>

## Conclusion

The measure of the mechanical elastic properties induced by the stretch blow molding process has been carried out on rPET bottles by two different ways: from heterogeneous biaxial tensile tests and from the growth of the bottle under pressure. The second method provides experimental results with less variation and has been used to test the influence of the rate of rPET in the initial preform.

The specific geometry of the blown bottle enriches the data in order to provide a better interpolation of the predicted modulus versus elongation during the process. The influence of the rate of recycled material has been studied. Finally, it appears that in our case, the recycled part reinforce the rigidity in both longitudinal and hoop directions.

## Appendix: material characteristics

<table><thead><tr><th>Property</th><th>Unit</th><th>Test method</th><th>RAMAPET N180 value</th></tr></thead><tbody><tr><td>Intrinsic Viscosity</td><td>dl/g</td><td>(K) SDP-15<br>(R) IND-A-PE-G-IV-01<br>(Q) CQ-V-1</td><td>0.80 ± 0.02</td></tr><tr><td>Acetaldehyde</td><td>ppm</td><td>(K) SDP-21<br>(R) IND-A-PE-GC-01<br>(Q) ASTM F 2013</td><td>Max 1</td></tr><tr><td>Colour L*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>Min 78</td></tr><tr><td>Colour a*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>-3.8 to -0.8</td></tr><tr><td>Colour b*</td><td>-</td><td>(K) SDP-05<br>(R) IND-A-PE-COL-01<br>(Q) ASTM 6290</td><td>-3.5 to 1</td></tr><tr><td>Pellet weight</td><td>Pieces/gram</td><td>(K) SDP-18<br>(R) IND-A-PE-PTM-09<br>(Q) CQ-GA-2</td><td>60 ± 5</td></tr><tr><td>Fines</td><td>Wt%</td><td>(K) SDP-60<br>(R) IND-A-PE-GA-04<br>(Q) CQ-GA-4</td><td>Max 0.05</td></tr></tbody></table>

(a)
<table><thead><tr><th>Property<sup>a</sup></th><th>Test<sup>b</sup><br>Method</th><th>Typical Value, Units<sup>c</sup></th></tr></thead><tbody><tr><td><strong>Pellet Properties</strong></td><td></td><td></td></tr><tr><td>Crystalline Density Solid Stated pellets</td><td>D 1505</td><td>1.39 - 1.4 g/cm<sup>3</sup></td></tr><tr><td>Bulk Density Poured</td><td>D 1895</td><td>830 ± 30 kg/m<sup>3</sup></td></tr><tr><td>Bulk Density Vibrated</td><td>D 1895</td><td>880 ± 30 kg/m<sup>3</sup></td></tr><tr><td>Melt Density @ 285°C</td><td>D 1238 (Note A- Table 2)</td><td>1.2 g/cm<sup>3</sup></td></tr><tr><td>Crystalline Peak Melting Point (T<sub>m</sub>)<sup>d</sup></td><td>D 3418</td><td>245 ± 5°C</td></tr><tr><td>Glass transition point (T<sub>g</sub>) (dry)</td><td>D 3418</td><td>78 ± 2°C</td></tr><tr><td>Heat of Fusion<sup>e</sup></td><td>E 793</td><td>56 kJ/kg (13 cal/g)</td></tr><tr><td>% crystallinity solid Stated pellets</td><td></td><td>50 ± 5%</td></tr><tr><td>Pellet Size</td><td></td><td>2.5 mm</td></tr><tr><td>Pellet Shape</td><td></td><td>Cylindrical</td></tr><tr><td>Moisture content pellets</td><td></td><td>&lt;0.3%</td></tr></tbody></table>

(b)

<table><thead><tr><th>Rheology</th><th>Value</th><th>Unity</th><th>Test std.</th></tr></thead><tbody><tr><td>Viscosity (IV)</td><td>0.82 - 0.87</td><td>dl/g</td><td>ASTM D1238 / ISO 1133)</td></tr><tr><td><strong>Other properties</strong></td><td><strong>Value</strong></td><td><strong>Unity</strong></td><td><strong>Test std.</strong></td></tr><tr><td>Bulk Density</td><td>0.90±0.50</td><td>kg/dm<sup>3</sup></td><td>Internal</td></tr><tr><td>Melt filtration</td><td>40</td><td>Micron</td><td>-</td></tr><tr><td>Humidity</td><td>&lt; 0.4</td><td>%</td><td>Internal</td></tr><tr><td>Pelletsize (M50)</td><td>1 ± 0.2</td><td>g</td><td>Internal</td></tr><tr><td>Black spots (0.5 - 1.0 mm)</td><td>&lt; 10</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Black spots (1.0 - 2.0 mm)</td><td>&lt; 3</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Black spots (&gt; 2.0 mm)</td><td>0</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Acetaldehyde</td><td>&lt; 1.0</td><td>ppm on chip</td><td>ASTM 2013</td></tr><tr><td>Dust</td><td>&lt; 0.03</td><td>%</td><td>-</td></tr><tr><td rowspan="3">Colour margins Crystallised (specification)</td><td>(66) - (74)</td><td>L</td><td rowspan="3">CieLAB</td></tr><tr><td>(-4.5) - (-1)</td><td>a</td></tr><tr><td>(-7) - (-3)</td><td>b</td></tr></tbody></table>

(c)

<caption>Figure 9. (a) and (b) RAMAPET N180 properties (c) MOPET properties</caption>

## References

[1] F. Welle, Twenty years of PET bottle to bottle recycling—an overview, Resour. Conserv. Recycl. 55 (11) (Sep. 2011) 865–875, https://doi.org/10.1016/j.resconrec.2011.04.009

[2] F. Awaja, D. Pavel. (2005). Injection stretch blow moulding process of reactive extruded recycled PET and virgin PET blends, European Polymer Journal, Volume 41, Issue 11, Pages 2614-2634, ISSN 0014-3057. https://doi.org/10.1016/j.eurpolymj.2005.05.036

[3] N. Torres, J.J. Robin, B. Boutevin. (2000). Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding, European Polymer Journal, Volume 36, Issue 10, Pages 2075-2080, ISSN 0014-3057. https://doi.org/10.1016/S0014-3057(99)00301-8

[4] V. Rohart, C. Combeaud. Stretchability above glass transition of mechanically recycled poly(ethylene terephthalate), PET, Polymer, Vol. 257, 2022. https://doi.org/10.1016/j.polymer.2022.125218
<table><thead><tr><th>Property<sup>a</sup></th><th>Test<sup>b</sup><br>Method</th><th>Typical Value, Units<sup>c</sup></th></tr></thead><tbody><tr><td><strong>Pellet Properties</strong></td><td></td><td></td></tr><tr><td>Crystalline Density Solid Stated pellets</td><td>D 1505</td><td>1.39 - 1.4 g/cm<sup>3</sup></td></tr><tr><td>Bulk Density Poured</td><td>D 1895</td><td>830 ± 30 kg/m<sup>3</sup></td></tr><tr><td>Bulk Density Vibrated</td><td>D 1895</td><td>880 ± 30 kg/m<sup>3</sup></td></tr><tr><td>Melt Density @ 285°C</td><td>D 1238 (Note A- Table 2)</td><td>1.2 g/cm<sup>3</sup></td></tr><tr><td>Crystalline Peak Melting Point (T<sub>m</sub>)<sup>d</sup></td><td>D 3418</td><td>245 ± 5°C</td></tr><tr><td>Glass transition point (T<sub>g</sub>) (dry)</td><td>D 3418</td><td>78 ± 2°C</td></tr><tr><td>Heat of Fusion<sup>e</sup></td><td>E 793</td><td>56 kJ/kg (13 cal/g)</td></tr><tr><td>% crystallinity solid Stated pellets</td><td></td><td>50 ± 5%</td></tr><tr><td>Pellet Size</td><td></td><td>2.5 mm</td></tr><tr><td>Pellet Shape</td><td></td><td>Cylindrical</td></tr><tr><td>Moisture content pellets</td><td></td><td>&lt;0.3%</td></tr></tbody></table>

(b)

<table><thead><tr><th>Rheology</th><th>Value</th><th>Unity</th><th>Test std.</th></tr></thead><tbody><tr><td>Viscosity (IV)</td><td>0.82 - 0.87</td><td>dl/g</td><td>ASTM D1238 / ISO 1133)</td></tr><tr><td><strong>Other properties</strong></td><td><strong>Value</strong></td><td><strong>Unity</strong></td><td><strong>Test std.</strong></td></tr><tr><td>Bulk Density</td><td>0.90±0.50</td><td>kg/dm³</td><td>Internal</td></tr><tr><td>Melt filtration</td><td>40</td><td>Micron</td><td>-</td></tr><tr><td>Humidity</td><td>&lt; 0.4</td><td>%</td><td>Internal</td></tr><tr><td>Pelletsize (M50)</td><td>1 ± 0.2</td><td>g</td><td>Internal</td></tr><tr><td>Black spots (0.5 - 1.0 mm)</td><td>&lt; 10</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Black spots (1.0 - 2.0 mm)</td><td>&lt; 3</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Black spots (&gt; 2.0 mm)</td><td>0</td><td>%</td><td>Internal / DVS AM356</td></tr><tr><td>Acetaldehyde</td><td>&lt; 1.0</td><td>ppm on chip</td><td>ASTM 2013</td></tr><tr><td>Dust</td><td>&lt; 0.03</td><td>%</td><td>-</td></tr><tr><td rowspan="3">Colour margins Crystallised (specification)</td><td>(66) - (74)</td><td>L</td><td rowspan="3">CieLAB</td></tr><tr><td>(-4.5) - (-1)</td><td>a</td></tr><tr><td>(-7) - (-3)</td><td>b</td></tr></tbody></table>

(c)

<caption>Figure 9. (a) and (b) RAMAPET N180 properties (c) MOPET properties</caption>

## References

[1] F. Welle, Twenty years of PET bottle to bottle recycling—an overview, Resour. Conserv. Recycl. 55 (11) (Sep. 2011) 865–875, https://doi.org/10.1016/j.resconrec.2011.04.009

[2] F. Awaja, D. Pavel. (2005). Injection stretch blow moulding process of reactive extruded recycled PET and virgin PET blends, European Polymer Journal, Volume 41, Issue 11, Pages 2614-2634, ISSN 0014-3057. https://doi.org/10.1016/j.eurpolymj.2005.05.036

[3] N. Torres, J.J. Robin, B. Boutevin. (2000). Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding, European Polymer Journal, Volume 36, Issue 10, Pages 2075-2080, ISSN 0014-3057. https://doi.org/10.1016/S0014-3057(99)00301-8

[4] V. Rohart, C. Combeaud. Stretchability above glass transition of mechanically recycled poly(ethylene terephthalate), PET, Polymer, Vol. 257, 2022. https://doi.org/10.1016/j.polymer.2022.125218
[5] A.-D. Le, R. Gilblas, V. Lucin, Y. Le Maoult, F. Schmidt. (2022). Infrared heating modeling of recycled PET preforms in injection stretch blow molding process, International Journal of Thermal Sciences, Vol. 181, 107762, ISSN 1290-0729. https://doi.org/10.1016/j.ijthermalsci.2022.107762

[6] Luo Y.-M., Chevalier L. On induced properties and self heating during free blowing of PET preform. (2019) International Polymer Processing, 34 (3), p. 330 – 338. https://doi.org/10.3139/217.3759

[7] Alvarado Chacon, F., Brouwer, M. T., & Thoden van Velzen, «Effect of recycled content and rPET quality on the properties of PET bottles, part I: Optical and mechanical properties,» Packaging Technology and Science, vol. 33:9, p. 347–357, 2020. https://doi.org/10.1002/pts.2490

[8] Y.M. Luo, T.T. Nguyen, H. Attar, L. Chevalier, F. Lesueur. A new biaxial apparatus for tensile tests on Poly Ethylene Terephthalate optimized specimen at stretch blow molding conditions. Polymer Testing, Elsevier, 2022, pp.107676. https://doi.org/10.1016/j.polymertesting.2022.107676

[9] Anumula, V. S. N., «Prediction of Process-induced Mechanical Properties for Stretch Blow Moulding of PET Bottles,» Queen's University Belfast. Faculty of Engineering and Physical Sciences, 2018.

[10] Leclerc H., Périé J-N., Stephane R., François H., «Integrated Digital Image Correlation for the Identification of Mechanical Properties,» Lectures Notes in Computer Sciences, vol. 5496, pp. 161-171, 2009. https://doi.org/10.1007/978-3-642-01811-4_15

[11] H. Attar, Y.M. Luo, L. Chevalier, T.T. Nguyen, F. Detrez. Identification of anisotropic properties of polymer sheets from heterogeneous biaxial tests. Polymer Testing, Elsevier, 2022, 115, pp.107721. https://doi.org/10.1016/j.polymertesting.2022.107721

[12] Lagarias, J. C., J. A. Reeds, M. H. Wright, and P. E. Wright, Convergence Properties of the Nelder-Mead Simplex Method in Low Dimensions, SIAM Journal of Optimization, Vol. 9, Number 1, pp. 112–147, 1998. https://doi.org/10.1137/S1052623496303470

[13] J. Réthoré, Muhibullah, T. Elguedj, M. Coret, P. Chaudet, et A. Combescure. Robust Identification of Elasto-Plastic Constitutive Law Parameters from Digital Images Using 3D Kinematics. International Journal of Solids and Structures 50, no 1 (2013): 73-85. https://doi.org/10.1016/j.ijsolstr.2012.09.002

[14] T. Marwala, I. Boulkaibet, S. Adhikari. Probabilistic Finite Element Model Updating using Bayesian Statistics. Application to aeronautical and mechanical engineering. 1st edition, John Wiley & Sons Ltd, Chichester, 2017. https://doi.org/10.1002/9781119153023

[15] L. Chevalier, Y. Marco, G. Regnier. Modification des propriétés durant le soufflage des bouteilles plastiques en PET, Mécanique & Industries, vol. 2(3), pp. 229-248, 2001. https://doi.org/10.1016/S1296-2139(01)01094-6
[5] A.-D. Le, R. Gilblas, V. Lucin, Y. Le Maoult, F. Schmidt. (2022). Infrared heating modeling of recycled PET preforms in injection stretch blow molding process, International Journal of Thermal Sciences, Vol. 181, 107762, ISSN 1290-0729. https://doi.org/10.1016/j.ijthermalsci.2022.107762

[6] Luo Y.-M., Chevalier L. On induced properties and self heating during free blowing of PET preform. (2019) International Polymer Processing, 34 (3), p. 330 – 338. https://doi.org/10.3139/217.3759

[7] Alvarado Chacon, F., Brouwer, M. T., & Thoden van Velzen, «Effect of recycled content and rPET quality on the properties of PET bottles, part I: Optical and mechanical properties,» Packaging Technology and Science, vol. 33:9, p. 347–357, 2020. https://doi.org/10.1002/pts.2490

[8] Y.M. Luo, T.T. Nguyen, H. Attar, L. Chevalier, F. Lesueur. A new biaxial apparatus for tensile tests on Poly Ethylene Terephthalate optimized specimen at stretch blow molding conditions. Polymer Testing, Elsevier, 2022, pp.107676. https://doi.org/10.1016/j.polymertesting.2022.107676

[9] Anumula, V. S. N., «Prediction of Process-induced Mechanical Properties for Stretch Blow Moulding of PET Bottles,» Queen's University Belfast. Faculty of Engineering and Physical Sciences, 2018.

[10] Leclerc H., Périé J-N., Stephane R., François H., «Integrated Digital Image Correlation for the Identification of Mechanical Properties,» Lectures Notes in Computer Sciences, vol. 5496, pp. 161-171, 2009. https://doi.org/10.1007/978-3-642-01811-4_15

[11] H. Attar, Y.M. Luo, L. Chevalier, T.T. Nguyen, F. Detrez. Identification of anisotropic properties of polymer sheets from heterogeneous biaxial tests. Polymer Testing, Elsevier, 2022, 115, pp.107721. https://doi.org/10.1016/j.polymertesting.2022.107721

[12] Lagarias, J. C., J. A. Reeds, M. H. Wright, and P. E. Wright, Convergence Properties of the Nelder-Mead Simplex Method in Low Dimensions, SIAM Journal of Optimization, Vol. 9, Number 1, pp. 112–147, 1998. https://doi.org/10.1137/S1052623496303470

[13] J. Réthoré, Muhibullah, T. Elguedj, M. Coret, P. Chaudet, et A. Combescure. Robust Identification of Elasto-Plastic Constitutive Law Parameters from Digital Images Using 3D Kinematics. International Journal of Solids and Structures 50, no 1 (2013): 73-85. https://doi.org/10.1016/j.ijsolstr.2012.09.002

[14] T. Marwala, I. Boulkaibet, S. Adhikari. Probabilistic Finite Element Model Updating using Bayesian Statistics. Application to aeronautical and mechanical engineering. 1st edition, John Wiley & Sons Ltd, Chichester, 2017. https://doi.org/10.1002/9781119153023

[15] L. Chevalier, Y. Marco, G. Regnier. Modification des propriétés durant le soufflage des bouteilles plastiques en PET, Mécanique & Industries, vol. 2(3), pp. 229-248, 2001. https://doi.org/10.1016/S1296-2139(01)01094-6
# Development of novel composites of incompatible barrier polymers and their optimal processing conditions for recyclable thermoformable food packaging sheet

AKIKI Alain¹,²,*, EL RASSY Elissa², CHALLITA Georges³, NACCOUL Rouaida Abou¹, BAILLEUL Jean-Luc²

¹INDEVCO PACT, Christ le Roi, Industrial Zone, Zouk Mosbeh P.O.BOX :11-2354, Lebanon

²Nantes Université, UMR CNRS 6607, LTEN, Rue Christian Pauc, 44300 Nantes, France

³Lebanese University, Faculty of Engineering, Roumieh, Lebanon

ªalain.akiki@etu.univ-nantes.fr

**Keywords:** Dispersed Nanophase, MXD6, Compatibilizer, Thermoplastic Composite, Laminar Morphology, Permeability

**Abstract.** In this work, a high barrier polymer MXD6 (m-xylene diamine) has been incorporated in a PE (polyethylene) matrix to create an extensible, thermoplastic composite system. MXD6 is a high barrier and glassy polymer. However, being polar, it is incompatible with a non-polar PE. To this date, there were no significant attempts neither in compatibilizing a binary blend of PE/MXD6 nor in exploring the benefits of orientation on such a mixed system. The main value proposition in this work resides in reducing the dispersed MXD6 phase close to a submicron scale in the PE matrix by designing an adequate mixing process/compatibilizer method. Thermal characterization of the composites and the scanning electron microscopy have confirmed that the MXD6 dispersed phase in the compatibilized blends has been physically and thermally confined; its crystallization during cooling stage has been inhibited. The compatibilized blend shows a creation of an interface adhesion between phases. The dispersed phase average particle size is stabilized at around 0.5 µm enhancing the recyclability of the final product.

## Introduction

Extensive research has been conducted to incorporate nano-minerals in a polyolefin matrix to improve its gas barrier properties in food packaging [1]. Nano minerals such as MMT (montmorillonite) and graphene can produce a long tortuous path to gas molecules if exfoliated efficiently. Although many studies yielded promising findings, such nanocomposites have limited extensibility due to the brittleness of the minerals, making it obsolete in applications such as thermally stretched films/sheets. In addition, there has been a reserved approach from food packers towards using nano-minerals in packaging due to food contamination risks [2].

A new concept has emerged recently in dispersing high barrier polymers in a polyolefin continuous matrix. The idea is to reduce the particle size of the dispersed phase to reach the order of nano-minerals, and then convert the thermoplastic composite into films or sheets by extrusion. In a scope extending beyond this paper and interpreted in Fig.1, we propose stretching such sheets to realize a laminar morphology and thus create a long tortuous path for gas molecules.

<figure><img src="image_2467.png" /></figure>
# Development of novel composites of incompatible barrier polymers and their optimal processing conditions for recyclable thermoformable food packaging sheet

AKIKI Alain¹,²,*, EL RASSY Elissa², CHALLITA Georges³, NACCOUL Rouaida Abou¹, BAILLEUL Jean-Luc²

¹INDEVCO PACT, Christ le Roi, Industrial Zone, Zouk Mosbeh P.O.BOX :11-2354, Lebanon

²Nantes Université, UMR CNRS 6607, LTEN, Rue Christian Pauc, 44300 Nantes, France

³Lebanese University, Faculty of Engineering, Roumieh, Lebanon

ªalain.akiki@etu.univ-nantes.fr

**Keywords:** Dispersed Nanophase, MXD6, Compatibilizer, Thermoplastic Composite, Laminar Morphology, Permeability

**Abstract.** In this work, a high barrier polymer MXD6 (m-xylene diamine) has been incorporated in a PE (polyethylene) matrix to create an extensible, thermoplastic composite system. MXD6 is a high barrier and glassy polymer. However, being polar, it is incompatible with a non-polar PE. To this date, there were no significant attempts neither in compatibilizing a binary blend of PE/MXD6 nor in exploring the benefits of orientation on such a mixed system. The main value proposition in this work resides in reducing the dispersed MXD6 phase close to a submicron scale in the PE matrix by designing an adequate mixing process/compatibilizer method. Thermal characterization of the composites and the scanning electron microscopy have confirmed that the MXD6 dispersed phase in the compatibilized blends has been physically and thermally confined; its crystallization during cooling stage has been inhibited. The compatibilized blend shows a creation of an interface adhesion between phases. The dispersed phase average particle size is stabilized at around 0.5 µm enhancing the recyclability of the final product.

## Introduction

Extensive research has been conducted to incorporate nano-minerals in a polyolefin matrix to improve its gas barrier properties in food packaging [1]. Nano minerals such as MMT (montmorillonite) and graphene can produce a long tortuous path to gas molecules if exfoliated efficiently. Although many studies yielded promising findings, such nanocomposites have limited extensibility due to the brittleness of the minerals, making it obsolete in applications such as thermally stretched films/sheets. In addition, there has been a reserved approach from food packers towards using nano-minerals in packaging due to food contamination risks [2].

A new concept has emerged recently in dispersing high barrier polymers in a polyolefin continuous matrix. The idea is to reduce the particle size of the dispersed phase to reach the order of nano-minerals, and then convert the thermoplastic composite into films or sheets by extrusion. In a scope extending beyond this paper and interpreted in Fig.1, we propose stretching such sheets to realize a laminar morphology and thus create a long tortuous path for gas molecules.

<figure><img src="image_2460.png" /></figure>
<figure><img src="image_2468.png" /><figcaption>Fig. 1 Realization model concept of the extended study</figcaption></figure>

In this study, we have selected a special polyamide called MXD6 (m-xylene diamine) as the dispersed phase. This polymer has a very high barrier to gas, specifically oxygen, even at high relative humidity (RH %). The chemical structure in Fig.2 illustrates a molecule with an aromatic ring which is the reason behind its high glass transition temperature T<sub>g</sub> around 87 °C. MXD6 is a glassy polymer, therefore is only stretchable above its T<sub>g</sub>.

<figure><img src="image_2469.png" /><figcaption>Fig. 2 Molecular structure of MXD6</figcaption></figure>

We used a linear low-density polyethylene (LLDPE) as the continuous phase (matrix). LLDPE is known for its extensibility, toughness, good thermal welding at relatively low temperatures (90-120 °C). LLDPE is also known to be a medium barrier to moisture.

MXD6 and LLDPE have been used in the past in multilayer extrusion technology to produce high barrier films [3]. However, such films are non-recyclable, and the production technology requires huge investments and has relatively high operational challenges compared to our concept that requires fewer numbers of layers.

The novelty in our study resides in investigating the following aspects:

* The thermoplastic composites can be extruded in simple existing technologies of 1 to 3 layers.

* To this date, no significant attempts neither in compatibilizing a binary blend of PE/MXD6 nor in exploring the benefits of orientation on such a mixed system. The main value proposition in this work resides in achieving a dispersed MXD6 nanophase in a PE matrix by designing an adequate mixing process-compatibilizer method.

* A compatibilized thermoplastic composite can be recycled.

In this paper, we will focus on the thermal characterization of the thermoplastic composites. We will also show the efficiency of compatibilization as well as its impact on the thermal transitions and the morphology of the composites.

## Materials

A LLDPE having a density 0.918 g/cm³ and a melt flow index of 3.5 g/10 min was selected as the continuous matrix. This polymer has a relatively high toughness and stretchability compared to other semi-crystalline polymers. However most importantly, it is thermally weldable at relatively low temperatures ranging between 100 and 120 °C. The needed properties were excerpted from the material specification sheets provided by the suppliers and have been tabulated in table 1.
<figure><img src="image_2461.png" /><figcaption>Fig. 1 Realization model concept of the extended study</figcaption></figure>

In this study, we have selected a special polyamide called MXD6 (m-xylene diamine) as the dispersed phase. This polymer has a very high barrier to gas, specifically oxygen, even at high relative humidity (RH %). The chemical structure in Fig.2 illustrates a molecule with an aromatic ring which is the reason behind its high glass transition temperature T<sub>g</sub> around 87 °C. MXD6 is a glassy polymer, therefore is only stretchable above its T<sub>g</sub>.

<figure><img src="image_2462.png" /><figcaption>Fig. 2 Molecular structure of MXD6</figcaption></figure>

We used a linear low-density polyethylene (LLDPE) as the continuous phase (matrix). LLDPE is known for its extensibility, toughness, good thermal welding at relatively low temperatures (90-120 °C). LLDPE is also known to be a medium barrier to moisture.

MXD6 and LLDPE have been used in the past in multilayer extrusion technology to produce high barrier films [3]. However, such films are non-recyclable, and the production technology requires huge investments and has relatively high operational challenges compared to our concept that requires fewer numbers of layers.

The novelty in our study resides in investigating the following aspects:

* The thermoplastic composites can be extruded in simple existing technologies of 1 to 3 layers.

* To this date, no significant attempts neither in compatibilizing a binary blend of PE/MXD6 nor in exploring the benefits of orientation on such a mixed system. The main value proposition in this work resides in achieving a dispersed MXD6 nanophase in a PE matrix by designing an adequate mixing process-compatibilizer method.

* A compatibilized thermoplastic composite can be recycled.

In this paper, we will focus on the thermal characterization of the thermoplastic composites. We will also show the efficiency of compatibilization as well as its impact on the thermal transitions and the morphology of the composites.

## Materials

A LLDPE having a density 0.918 g/cm³ and a melt flow index of 3.5 g/10 min was selected as the continuous matrix. This polymer has a relatively high toughness and stretchability compared to other semi-crystalline polymers. However most importantly, it is thermally weldable at relatively low temperatures ranging between 100 and 120 °C. The needed properties were excerpted from the material specification sheets provided by the suppliers and have been tabulated in table 1.
<table><caption>Table 1. Major properties of selected LLDPE & MXD6 grades</caption><thead><tr><th>Material</th><th>Melt flow index<br>[g/10 min]</th><th>Melt Viscosity<br>[Pa.s]</th><th>Density<br>[g/cm³]</th><th>Melting peak<br>[°C]</th><th>Tg<br>[°C]</th></tr></thead><tbody><tr><td>LLDPE</td><td>3.5*</td><td>NA</td><td>0.918</td><td>118</td><td>-</td></tr><tr><td>MXD6</td><td>2.0**</td><td>770/300 ***</td><td>1.22</td><td>237</td><td>85</td></tr></tbody></table>

Thermoplastic composites made of LLDPE and different concentrations of MXD6 were prepared. The composites were prepared using a lab scale co-rotating twin screw extruder with L/D=44 and screws diameter of 22 mm. The temperatures profile was set at 240-260 °C to exceed the peak melting temperature of MXD6.

A highly grafted maleic anhydride grafted LLDPE (MAH-g-PE) thereof called MAH-GC was added at different weight % to compatibilize the continuous LLDPE matrix with the MXD6 dispersed phase. The formulations of the thermoplastic composites, with and without compatibilizers, and their respective names are detailed in table 2.

<table><caption>Table 2. Thermoplastic composites studied in this article.</caption><thead><tr><th>Compatibilizer</th><th>Compatibilizer weight %</th><th>Composite Name</th><th>Formulation PE/MXD6/Comp</th></tr></thead><tbody><tr><td rowspan="3">None</td><td rowspan="3">0</td><td>MX10</td><td>90/10/0</td></tr><tr><td>MX20</td><td>80/20/0</td></tr><tr><td>MX30</td><td>70/30/0</td></tr><tr><td rowspan="3">MAH-g-LLDPE</td><td>3</td><td>MX20-GC3</td><td>77/20/3</td></tr><tr><td>5</td><td>MX20-GC5</td><td>75/20/5</td></tr><tr><td>7</td><td>MX20-GC7</td><td>73/20/7</td></tr></tbody></table>

## Methods & Experimental Characterization

Before preparing the compounds in the twin screw extruder, we dried the MXD6 at 110 °C for 3 hours under atmospheric pressure. The compatibilizers were dried at 50 °C for 4 hours. LLDPE being hydrophobic, does not absorb humidity and therefore did not require pre-drying. Dried samples were kept in a concealed container until extrusion.

Before characterization, the samples of thermoplastic composites were dried at 90 °C for 4 hours under atmospheric pressure. The pre-dried samples of thermoplastic composites were thermally characterized using differential scanning calorimetry (DSC). The cycles consisted of a first heating ramp from 25 °C to 300 °C at 10 °C/min, an isotherm at 300 °C for 5 min, a cooling ramp down from 300 °C to 25 °C at different cooling rates, a second isotherm at 25 °C for 5 min and a second heating ramp up from 25 °C to 300 °C at 10 °C/min. The first heating stage serves as an annealing process for the extruded samples. The cooling stage was done at 2, 3, 5, 10 and 20 °C/min cooling rates. The thermal transitions of the latter stage and the second heating stage were reported and measured in this study.

Pre-dried composites pellets were pressed at 250 °C to produce a film having a thickness average of 200 ± 5 µm. 5 sheets were prepared from each composition for repeatability. The identification of the functional groups in each sample was done by Fourier Transform Infrared (FTIR). The spectroscopy was conducted using Bruker equipment in transmission mode (FTIR-TR) with 32 scans and wavelengths ranging from 4000 to 400 cm⁻¹. Scanning electron microscopy (SEM) was used to determine the dimensions of the dispersed phase, and to optically characterize the interfacial interaction between the MXD6 dispersed phase and PE continuous phase.

A thermogravimetric analyzer (TGA) was used to test the thermal stability of all raw material granules. Samples of (20 ± 1) mg were prepared and tested in non-isothermal mode as per test method ASTM E1131-20 at 10 °C/min heating rate in the range of 25 to 900 °C.

<footnote>* @190 °C/2.16 kg ** @275 °C/0.325 kg *** @260 °C 100/1000 s⁻¹</footnote>
<table><caption>Table 1. Major properties of selected LLDPE & MXD6 grades</caption><thead><tr><th>Material</th><th>Melt flow index<br>[g/10 min]</th><th>Melt Viscosity<br>[Pa.s]</th><th>Density<br>[g/cm³]</th><th>Melting peak<br>[°C]</th><th>Tg<br>[°C]</th></tr></thead><tbody><tr><td>LLDPE</td><td>3.5*</td><td>NA</td><td>0.918</td><td>118</td><td>-</td></tr><tr><td>MXD6</td><td>2.0**</td><td>770/300 ***</td><td>1.22</td><td>237</td><td>85</td></tr></tbody></table>

Thermoplastic composites made of LLDPE and different concentrations of MXD6 were prepared. The composites were prepared using a lab scale co-rotating twin screw extruder with L/D=44 and screws diameter of 22 mm. The temperatures profile was set at 240-260 °C to exceed the peak melting temperature of MXD6.

A highly grafted maleic anhydride grafted LLDPE (MAH-g-PE) thereof called MAH-GC was added at different weight % to compatibilize the continuous LLDPE matrix with the MXD6 dispersed phase. The formulations of the thermoplastic composites, with and without compatibilizers, and their respective names are detailed in table 2.

<table><caption>Table 2. Thermoplastic composites studied in this article.</caption><thead><tr><th>Compatibilizer</th><th>Compatibilizer weight %</th><th>Composite Name</th><th>Formulation PE/MXD6/Comp</th></tr></thead><tbody><tr><td rowspan="3">None</td><td rowspan="3">0</td><td>MX10</td><td>90/10/0</td></tr><tr><td>MX20</td><td>80/20/0</td></tr><tr><td>MX30</td><td>70/30/0</td></tr><tr><td rowspan="3">MAH-g-LLDPE</td><td>3</td><td>MX20-GC3</td><td>77/20/3</td></tr><tr><td>5</td><td>MX20-GC5</td><td>75/20/5</td></tr><tr><td>7</td><td>MX20-GC7</td><td>73/20/7</td></tr></tbody></table>

## Methods & Experimental Characterization

Before preparing the compounds in the twin screw extruder, we dried the MXD6 at 110 °C for 3 hours under atmospheric pressure. The compatibilizers were dried at 50 °C for 4 hours. LLDPE being hydrophobic, does not absorb humidity and therefore did not require pre-drying. Dried samples were kept in a concealed container until extrusion.

Before characterization, the samples of thermoplastic composites were dried at 90 °C for 4 hours under atmospheric pressure. The pre-dried samples of thermoplastic composites were thermally characterized using differential scanning calorimetry (DSC). The cycles consisted of a first heating ramp from 25 °C to 300 °C at 10 °C/min, an isotherm at 300 °C for 5 min, a cooling ramp down from 300 °C to 25 °C at different cooling rates, a second isotherm at 25 °C for 5 min and a second heating ramp up from 25 °C to 300 °C at 10 °C/min. The first heating stage serves as an annealing process for the extruded samples. The cooling stage was done at 2, 3, 5, 10 and 20 °C/min cooling rates. The thermal transitions of the latter stage and the second heating stage were reported and measured in this study.

Pre-dried composites pellets were pressed at 250 °C to produce a film having a thickness average of 200 ± 5 µm. 5 sheets were prepared from each composition for repeatability. The identification of the functional groups in each sample was done by Fourier Transform Infrared (FTIR). The spectroscopy was conducted using Bruker equipment in transmission mode (FTIR-TR) with 32 scans and wavelengths ranging from 4000 to 400 cm⁻¹. Scanning electron microscopy (SEM) was used to determine the dimensions of the dispersed phase, and to optically characterize the interfacial interaction between the MXD6 dispersed phase and PE continuous phase.

A thermogravimetric analyzer (TGA) was used to test the thermal stability of all raw material granules. Samples of (20 ± 1) mg were prepared and tested in non-isothermal mode as per test method ASTM E1131-20 at 10 °C/min heating rate in the range of 25 to 900 °C.

<footnote>* @190 °C/2.16 kg ** @275 °C/0.325 kg *** @260 °C 100/1000 s⁻¹</footnote>
## Results and discussion

The DSC exothermal thermogram of MXD6 presented in Fig.3A shows that the polymer has a peak crystallization temperature at 152 °C and a Tg at 87 °C. The second heating stage shows a cold crystallization peak thermal crystallization at 145 °C which is an important thermal transition to track throughout the compounding stage. The peak melting temperature of the polymer was detected at 231 °C.

In Fig.3B LLDPE shows a peak crystallization temperature at 98 °C and two melting modes with respective peak melting temperatures at 106 °C and 116 °C. The lowest one being related to the low molecular weight portion rich in short chain branches resulting from higher distribution of C6 copolymer [7].

The thermograms of the MAH-GC compatibilizer shows that it has a peak crystallization temperature at 52 °C and a peak melting temperature at 70 °C.

<figure><img src="image_2470.png" /><figcaption>Fig 3. DSC thermograms of (A)MXD6 (B)LLDPE & MAH-GC</figcaption></figure>

The crystallinity of MXD6 can be calculated from the DSC thermogram using the following formula [8]:

$$X_c = \frac{(\Delta H_f - \Delta H_{cc})}{\Delta H_f^o} = 20.2 \% \qquad (1)$$

Where $\Delta H_f=16.39$ J/g is the measured enthalpy of fusion of MXD6, $\Delta H_{cc}= 51.7$ J/g is the measured enthalpy of cold crystallization, and $\Delta H_f^o = 175$ J/g is the heat of fusion of a fully crystalline MXD6 [9].

## Thermal stability of sensitive components.

The temperature of the twin screw extrusion (TSE) mixing process and in subsequent film extrusion should exceed the end melting point of MXD6 which is Tend~240 °C as measured by DSC. We will show in a later section that the compatibilization between MXD6 and PE using a MAH-GC is a reactive process which leads to grafting the MXD6 to the MAH-GC. Accordingly, an increase in the molecular weight of the component is expected resulting in an increase in extrusion torque and therefore overheating in the extruder. Thus, instead of isothermal stability tests, we put the LLDPE and the MAH-GAC compatibilizer in a non-isothermal TGA ramp to confirm their thermal stability in the expected processing range between 250 °C and 270 °C. Indeed, Fig.4 and Table 3. show that LLDPE has undergone zero thermal decomposition whereas the compatibilizer lost less than 0.5 % of its weight making it relatively stable in this range. To note that the degradation of LLDPE due to oxidation or crosslinking has been tested by parallel plate rheology which confirmed its stability. However, this extended study remains out of the scope of this paper.
## Results and discussion

The DSC exothermal thermogram of MXD6 presented in Fig.3A shows that the polymer has a peak crystallization temperature at 152 °C and a Tg at 87 °C. The second heating stage shows a cold crystallization peak thermal crystallization at 145 °C which is an important thermal transition to track throughout the compounding stage. The peak melting temperature of the polymer was detected at 231 °C.

In Fig.3B LLDPE shows a peak crystallization temperature at 98 °C and two melting modes with respective peak melting temperatures at 106 °C and 116 °C. The lowest one being related to the low molecular weight portion rich in short chain branches resulting from higher distribution of C6 copolymer [7].

The thermograms of the MAH-GC compatibilizer shows that it has a peak crystallization temperature at 52 °C and a peak melting temperature at 70 °C.

<figure><img src="image_2463.png" /><figcaption>Fig 3. DSC thermograms of (A)MXD6 (B)LLDPE & MAH-GC</figcaption></figure>

The crystallinity of MXD6 can be calculated from the DSC thermogram using the following formula [8]:

$$X_c = \frac{(\Delta H_f - \Delta H_{cc})}{\Delta H_f^o} = 20.2 \% \qquad (1)$$

Where $\Delta H_f$=16.39 J/g is the measured enthalpy of fusion of MXD6, $\Delta H_{cc}$= 51.7 J/g is the measured enthalpy of cold crystallization, and $\Delta H_f^o$ = 175 J/g is the heat of fusion of a fully crystalline MXD6 [9].

### Thermal stability of sensitive components.

The temperature of the twin screw extrusion (TSE) mixing process and in subsequent film extrusion should exceed the end melting point of MXD6 which is Tend~240 °C as measured by DSC. We will show in a later section that the compatibilization between MXD6 and PE using a MAH-GC is a reactive process which leads to grafting the MXD6 to the MAH-GC. Accordingly, an increase in the molecular weight of the component is expected resulting in an increase in extrusion torque and therefore overheating in the extruder. Thus, instead of isothermal stability tests, we put the LLDPE and the MAH-GAC compatibilizer in a non-isothermal TGA ramp to confirm their thermal stability in the expected processing range between 250 °C and 270 °C. Indeed, Fig.4 and Table 3. show that LLDPE has undergone zero thermal decomposition whereas the compatibilizer lost less than 0.5 % of its weight making it relatively stable in this range. To note that the degradation of LLDPE due to oxidation or crosslinking has been tested by parallel plate rheology which confirmed its stability. However, this extended study remains out of the scope of this paper.
<figure><img src="image_2471.png" /><figcaption>Fig 4. TGA thermogram of LLDPE & MAH-GC</figcaption></figure>

<table><caption>Table 3. Thermal weight loss of the compatibilizer and the LLDPE</caption><thead><tr><th></th><th>MAH-GC</th><th>PE</th></tr></thead><tbody><tr><td>Weight loss % @ 250 °C</td><td>0.44</td><td>0</td></tr><tr><td>Weight loss % @ 270 °C</td><td>0.49</td><td>0</td></tr><tr><td>Loss % in the processing range</td><td>0.05</td><td>0</td></tr></tbody></table>

## Study of non-compatibilized composites.

The DSC thermograms of the thermoplastic composites at 10, 20 and 30 % of MXD6 without compatibilizers have been overlaid in Fig.5. MX10 shows only crystallization temperature related to LLDPE at around 101 °C and 65 °C but no crystallization related to MXD6 in the exothermal scan. However, a cold crystallization temperature of MXD6 at 133 °C in the endothermal scan. At the other hand, crystallization peaks of MX20 & MX30 have been detected of respectively at 178 °C and 181 °C. The increase of the crystallization peaks has been accompanied with an increase in respective melting peaks and enthalpy of fusion in the endotherm as seen in Table 4 and Fig 6.

As a first outcome, MXD6 having low and slow crystallization as shown in the previous paragraph, does not undergo crystallization in cooling stage even at a relatively slow 10 °C/min cooling rate.

At higher %, the MXD6 phase shows a separate crystallization peak from the LLDPE phase which confirms the incompatibility between phases.

<figure><img src="image_2472.png" /><figcaption>Fig 5. DSC Thermograms of MX10, MX20, MX30</figcaption></figure>

<figure><img src="image_2473.png" /><figcaption>Fig 6. MXD6 Heat of fusion with MXD6 %</figcaption></figure>
<figure><img src="image_2464.png" /><figcaption>Fig 4. TGA thermogram of LLDPE & MAH-GC</figcaption></figure>

<table><caption>Table 3. Thermal weight loss of the compatibilizer and the LLDPE</caption><thead><tr><th></th><th>MAH-GC</th><th>PE</th></tr></thead><tbody><tr><td>Weight loss % @ 250 °C</td><td>0.44</td><td>0</td></tr><tr><td>Weight loss % @ 270 °C</td><td>0.49</td><td>0</td></tr><tr><td>Loss % in the processing range</td><td>0.05</td><td>0</td></tr></tbody></table>

## Study of non-compatibilized composites.

The DSC thermograms of the thermoplastic composites at 10, 20 and 30 % of MXD6 without compatibilizers have been overlaid in Fig.5. MX10 shows only crystallization temperature related to LLDPE at around 101 °C and 65 °C but no crystallization related to MXD6 in the exothermal scan. However, a cold crystallization temperature of MXD6 at 133 °C in the endothermal scan. At the other hand, crystallization peaks of MX20 & MX30 have been detected of respectively at 178 °C and 181 °C. The increase of the crystallization peaks has been accompanied with an increase in respective melting peaks and enthalpy of fusion in the endotherm as seen in Table 4 and Fig 6.

As a first outcome, MXD6 having low and slow crystallization as shown in the previous paragraph, does not undergo crystallization in cooling stage even at a relatively slow 10 °C/min cooling rate.

At higher %, the MXD6 phase shows a separate crystallization peak from the LLDPE phase which confirms the incompatibility between phases.

<figure><img src="image_2465.png" /><figcaption>Fig 5. DSC Thermograms of MX10, MX20, MX30</figcaption></figure>

<figure><img src="image_2466.png" /><figcaption>Fig 6. MXD6 Heat of fusion with MXD6 %</figcaption></figure>
<table><caption>Table 4. Thermal transitions of non-compatibilized composites</caption><thead><tr><th></th><th>T<sub>cp</sub>[°C]</th><th>T<sub>cc</sub>[°C]</th><th>T<sub>mp</sub>[°C]</th><th>ΔH<sub>f</sub><br>[J·g<sup>-1</sup>]</th></tr></thead><tbody><tr><td>MX10</td><td>ND</td><td>133</td><td>225</td><td>4.7</td></tr><tr><td>MX20</td><td>178</td><td>132</td><td>227</td><td>6.2</td></tr><tr><td>MX30</td><td>181</td><td>ND</td><td>230</td><td>10.8</td></tr></tbody></table>

The FTIR spectrum of the non-compatibilized composites have been overlaid in Fig.7. The relative absorption of the MXD6 phase increases in the same direction of the level of MXD6 in the blend. We have not detected any new absorption wavelength which means that there was no chemical reaction between phases. Fig 7B shows that the linear relation between the absorption units and the MXD6 weight % in the composite with a correlation factor of 0.9925. The latter confirms the precise weight % of MXD6 in the blends.

<figure><img src="image_2474.png" /><figcaption>Fig 7. (A) FTIR-TR of MX10, MX20 and MX30
(B) Absorption of N-H(MXD6) in the range 3065-3070 cm⁻¹ vs MXD6 %</figcaption></figure>

## Study of compatibilized composites.

The DSC thermograms of the compatibilized composites are overlaid in Fig 9. For MAH-GC at 5 & 7 % the crystallization peak temperatures in exothermal cycle have disappeared completely. However, the related heat of fusion in the endothermal cycle are clearly detected meaning that the MXD6 dispersed phase has now a slower crystallization rate than with the non-compatibilized one. The MXD6 phase has been thermally confined, the reason behind it is the geometrical confinement which will be elaborated in later section in this study. One of the most interesting findings in this study is the existence of 2 modes of compatibilization:

*   **Mode 1:** Transitional confinement of the dispersed phase at low level of MAH-GC such as 3 %. The thermogram of MX20-GC3 shows a multiple peak of crystallization in exothermal scan. In fact, such a behavior is called fractionated crystallization in semi crystalline polymers. Arnal et al. [5] have first studied this mode when mixing incompatible polymers such as polyethylene in atactic polystyrene. In fact, the heterogeneity of the microdomains and the dispersed phase sizes have been found to be the reason behind this matter. Indeed, Wang et al. [6] have made a detailed elaboration on this behavior and they found as we quote:

> "When the crystallizing polymer is divided into numerous MDs (microdomains), such as droplets dispersed in a liquid medium or in a matrix of an immiscible polymer, fractionated crystallization can arise."
<table><caption>Table 4. Thermal transitions of non-compatibilized composites</caption><thead><tr><th></th><th>T<sub>cp</sub>[°C]</th><th>T<sub>cc</sub>[°C]</th><th>T<sub>mp</sub>[°C]</th><th>ΔH<sub>f</sub><br>[J·g<sup>-1</sup>]</th></tr></thead><tbody><tr><td>MX10</td><td>ND</td><td>133</td><td>225</td><td>4.7</td></tr><tr><td>MX20</td><td>178</td><td>132</td><td>227</td><td>6.2</td></tr><tr><td>MX30</td><td>181</td><td>ND</td><td>230</td><td>10.8</td></tr></tbody></table>

The FTIR spectrum of the non-compatibilized composites have been overlaid in Fig.7. The relative absorption of the MXD6 phase increases in the same direction of the level of MXD6 in the blend. We have not detected any new absorption wavelength which means that there was no chemical reaction between phases. Fig 7B shows that the linear relation between the absorption units and the MXD6 weight % in the composite with a correlation factor of 0.9925. The latter confirms the precise weight % of MXD6 in the blends.

<figure><img src="image_2467.png" /><figcaption>Fig 7. (A) FTIR-TR of MX10, MX20 and MX30
(B) Absorption of N-H(MXD6) in the range 3065-3070 cm⁻¹ vs MXD6 %</figcaption></figure>

## Study of compatibilized composites.

The DSC thermograms of the compatibilized composites are overlaid in Fig 9. For MAH-GC at 5 & 7 % the crystallization peak temperatures in exothermal cycle have disappeared completely. However, the related heat of fusion in the endothermal cycle are clearly detected meaning that the MXD6 dispersed phase has now a slower crystallization rate than with the non-compatibilized one. The MXD6 phase has been thermally confined, the reason behind it is the geometrical confinement which will be elaborated in later section in this study. One of the most interesting findings in this study is the existence of 2 modes of compatibilization:

*   **Mode 1:** Transitional confinement of the dispersed phase at low level of MAH-GC such as 3 %. The thermogram of MX20-GC3 shows a multiple peak of crystallization in exothermal scan. In fact, such a behavior is called fractionated crystallization in semi crystalline polymers. Arnal et al. [5] have first studied this mode when mixing incompatible polymers such as polyethylene in atactic polystyrene. In fact, the heterogeneity of the microdomains and the dispersed phase sizes have been found to be the reason behind this matter. Indeed, Wang et al. [6] have made a detailed elaboration on this behavior and they found as we quote:

> "When the crystallizing polymer is divided into numerous MDs (microdomains), such as droplets dispersed in a liquid medium or in a matrix of an immiscible polymer, fractionated crystallization can arise."
The illustration of the concept is present in Fig 8 where A, B, C& D represent different nucleating sites in a polymer blend containing microdomains such as MXD6 in our case.

<figure><img src="image_2475.png" /><figcaption>Fig 8. Concept of fractionated crystallization illustrated by Wang et al. [6]</figcaption></figure>

* Mode2: Steady-state confinement where the dispersed phase is confined homogeneously, in a later section we will confirm that this is related to a narrow distribution of a reduced dispersed particle sizes. At 5 % & 7 % of MAH-GC, we can see the disappearance of crystallization peaks and the $\Delta H_f$ of the MXD6 phases is in both cases around 9 J/g. The latter is described in Fig 10.

<figure><img src="image_2476.png" /><figcaption>Fig 9. DSC Thermograms of MX20, MX20-GC3, MX20-GC5 & MX20GC7</figcaption></figure>

<figure><img src="image_2477.png" /><figcaption>Fig 10. MXD6 Heat of fusion with MAH-GC %</figcaption></figure>

<table><caption>Table 5. Thermal transitions of compatibilized composites</caption><thead><tr><th></th><th>T<sub>cp</sub><br>[°C]</th><th>T<sub>cc</sub><br>[°C]</th><th>T<sub>mp</sub><br>[°C]</th><th>ΔH<sub>f</sub><br>[J/g]</th></tr></thead><tbody><tr><td>MX20</td><td>178</td><td>132</td><td>227</td><td>6.2</td></tr><tr><td>MX20-GC3</td><td>179/186</td><td>127</td><td>231</td><td>8.6</td></tr><tr><td>MX20-GC5</td><td>ND</td><td>128</td><td>231</td><td>9.0</td></tr><tr><td>MX20-GC7</td><td>ND</td><td>128</td><td>231</td><td>8.9</td></tr></tbody></table>
The illustration of the concept is present in Fig 8 where A, B, C& D represent different nucleating sites in a polymer blend containing microdomains such as MXD6 in our case.

<figure><img src="image_2468.png" /><figcaption>Fig 8. Concept of fractionated crystallization illustrated by Wang et al. [6]</figcaption></figure>

* Mode2: Steady-state confinement where the dispersed phase is confined homogeneously, in a later section we will confirm that this is related to a narrow distribution of a reduced dispersed particle sizes. At 5 % & 7 % of MAH-GC, we can see the disappearance of crystallization peaks and the $\Delta H_f$ of the MXD6 phases is in both cases around 9 J/g. The latter is described in Fig 10.

<figure><img src="image_2469.png" /><figcaption>Fig 9. DSC Thermograms of MX20, MX20-GC3, MX20-GC5 & MX20GC7</figcaption></figure>

<figure><img src="image_2470.png" /><figcaption>Fig 10. MXD6 Heat of fusion with MAH-GC %</figcaption></figure>

<table><caption>Table 5. Thermal transitions of compatibilized composites</caption><thead><tr><th></th><th>T<sub>cp</sub><br>[°C]</th><th>T<sub>cc</sub><br>[°C]</th><th>T<sub>mp</sub><br>[°C]</th><th>ΔH<sub>f</sub><br>[J/g]</th></tr></thead><tbody><tr><td>MX20</td><td>178</td><td>132</td><td>227</td><td>6.2</td></tr><tr><td>MX20-GC3</td><td>179/186</td><td>127</td><td>231</td><td>8.6</td></tr><tr><td>MX20-GC5</td><td>ND</td><td>128</td><td>231</td><td>9.0</td></tr><tr><td>MX20-GC7</td><td>ND</td><td>128</td><td>231</td><td>8.9</td></tr></tbody></table>
The FTIR spectrum of the compatibilized composites are overlaid in Fig.11. We have detected a new functional group at 1787 cm⁻¹ which was not present in the non-compatibilized MX20 (cf. Fig.11A). This wavelength is related to the asymmetric C=O stretching in the MAH group present in the compatibilizer [13]. The absorption of this functional group increases with the % of MAH-GC confirming the incremental level of the compatibilizer as intended in the blend as shown in Fig.11B. The latter confirms the precise weight % of MAH-GC in the blend.

<figure><img src="image_2478.png" /><figcaption>Fig 11. (A) Overlay FTIR-TR of wavelength range 1800-1780 cm⁻¹
(B) Absorption units @ 1788 cm⁻¹ (MAH) vs MAH-GC %</figcaption></figure>

Effect of compatibilization on the morphology of the composites

The morphology tested by SEM of MX20 and MX20-GC7 blends are displayed in Fig.12. The compatibilized blend shows a significant reduction of the dispersed MXD6 size. MXD6 phase has been dispersed in MX20 with sizes reaching 4 µm whereas the droplets size in the MX20-GC7 is mainly below 1 µm. In addition, the MXD6 droplets in MX20 is completely separated from the LLDPE phase, whereas there is a significant interface adhesion (a typical adhesion area in red circle) between the droplets and LLDPE in MX20-GC7.
The FTIR spectrum of the compatibilized composites are overlaid in Fig.11. We have detected a new functional group at 1787 cm⁻¹ which was not present in the non-compatibilized MX20 (cf. Fig.11A). This wavelength is related to the asymmetric C=O stretching in the MAH group present in the compatibilizer [13]. The absorption of this functional group increases with the % of MAH-GC confirming the incremental level of the compatibilizer as intended in the blend as shown in Fig.11B. The latter confirms the precise weight % of MAH-GC in the blend.

<figure><img src="image_2471.png" /><figcaption>Fig 11. (A) Overlay FTIR-TR of wavelength range 1800-1780 cm⁻¹
(B) Absorption units @ 1788 cm⁻¹ (MAH) vs MAH-GC %</figcaption></figure>

Effect of compatibilization on the morphology of the composites

The morphology tested by SEM of MX20 and MX20-GC7 blends are displayed in Fig.12. The compatibilized blend shows a significant reduction of the dispersed MXD6 size. MXD6 phase has been dispersed in MX20 with sizes reaching 4 µm whereas the droplets size in the MX20-GC7 is mainly below 1 µm. In addition, the MXD6 droplets in MX20 is completely separated from the LLDPE phase, whereas there is a significant interface adhesion (a typical adhesion area in red circle) between the droplets and LLDPE in MX20-GC7.
<figure><img src="image_2479.png" /><figcaption>Fig 12. SEM microscopy of MX20(Left), MX20-GC7(Right)</figcaption></figure>

It is very important to note that this interface adhesion is the reason behind the physical and thermal confinement. The droplet size is directly related to the interfacial tension between phases as per the Taylor's formula [10,11,12]

$$R_{min} = \frac{2^{-\frac{1}{2}}Ca_{crit}\cdot\Gamma}{\eta_m\cdot\dot{\gamma}} \quad (2)$$

Where Rmin is the minimum theoretical droplet radius, Γ is the interfacial tension between phases, ηm is the matrix viscosity and γ̇ is the mixing shear rate. the critical capillary number Ca crit can be retrieved from Grace's curve [10,11,12]. We deduce that the compatibilizer has created an interfacial bond between phase therefore reducing the interfacial tension which was the reason behind the confinement of the MXD6 droplets in the compatibilized composites. The second finding is that selected MAH-GC is an efficient compatibilizer between LLDPE & MXD6.

## Summary

An important outcome of this study is that compatibilization of LLDPE and MXD6 mixed in a twin-screw extruder using 5 to 7 % MAH-g-PE leads to a confinement of the crystallization and the droplet size of the MXD6 dispersed phase. More importantly, the confinement happens after annealing the composites in the first heating cycle in DSC, this leads to a conclusion that the droplets size will remain stable in subsequent extrusion processes which will be confirmed at a later project phase. Thus, the composite acts as a sub-micro filled LLDPE improving its recyclability in LLDPE stream.

However, additional works are still needed to complete this work, namely detecting the fractionated crystals as well as the ones in steady-state mode. The latter can be done using WAXS and XRD techniques.
<figure><img src="image_2472.png" /><figcaption>Fig 12. SEM microscopy of MX20(Left), MX20-GC7(Right)</figcaption></figure>

It is very important to note that this interface adhesion is the reason behind the physical and thermal confinement. The droplet size is directly related to the interfacial tension between phases as per the Taylor's formula [10,11,12]

$$R_{min} = \frac{2^{-\frac{1}{2}}Ca_{crit}\cdot\Gamma}{\eta_m\cdot\dot{\gamma}} \quad (2)$$

Where Rmin is the minimum theoretical droplet radius, Γ is the interfacial tension between phases, ηm is the matrix viscosity and γ is the mixing shear rate. the critical capillary number Ca crit can be retrieved from Grace's curve [10,11,12]. We deduce that the compatibilizer has created an interfacial bond between phase therefore reducing the interfacial tension which was the reason behind the confinement of the MXD6 droplets in the compatibilized composites. The second finding is that selected MAH-GC is an efficient compatibilizer between LLDPE & MXD6.

## Summary

An important outcome of this study is that compatibilization of LLDPE and MXD6 mixed in a twin-screw extruder using 5 to 7 % MAH-g-PE leads to a confinement of the crystallization and the droplet size of the MXD6 dispersed phase. More importantly, the confinement happens after annealing the composites in the first heating cycle in DSC, this leads to a conclusion that the droplets size will remain stable in subsequent extrusion processes which will be confirmed at a later project phase. Thus, the composite acts as a sub-micro filled LLDPE improving its recyclability in LLDPE stream.

However, additional works are still needed to complete this work, namely detecting the fractionated crystals as well as the ones in steady-state mode. The latter can be done using WAXS and XRD techniques.
## Acknowledgement

Authors want to thank IMN laboratory for allowing us the use of their SEM. We also thank INDEVCO for its financial and material support.

## References

[1] J. Luna and A. Vílchez, "Polymer nanocomposites for food packaging," *Emerging Nanotechnologies in Food Science*, pp. 119-147, 2017. https://doi.org/10.1016/B978-0-323-42980-1.00007-8

[2] H. Onyeaka, P. Passaretti, T. Miri, and Z. T. Al-Sharify, "The safety of nanomaterials in food production and packaging," *Current Research in Food Science*, vol. 5, pp. 763-774, 2022. https://doi.org/10.1016/j.crfs.2022.04.005

[3] Fereydoon, M. (2014). Development of High Barrier Nylon Based Multilayer Films [Ph.D. thesis, École Polytechnique de Montréal]. PolyPublie.

[4] Seif, S. (2009). Temporally programmed stretching of polymer films: Influence of nanoparticles. The University of Akron.

[5] Arnal, M. L., Matos, M. E., Morales, R. A., Santana, O. O., & Müuller, A. J. (1998). Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix. *Macromolecular Chemistry and Physics*, 199(10), 2275-2288. https://doi.org/10.1002/(SICI)1521-3935(19981001)199:10<2275::AID-MACP2275>3.0.CO;2-#

[6] Sangroniz, L., Wang, B., Su, Y., Liu, G., Cavallo, D., Wang, D., & Müller, A. J. (2021). Fractionated crystallization in semicrystalline polymers. *Progress in Polymer Science*, 115, 101376. https://doi.org/10.1016/j.progpolymsci.2021.101376

[7] Mariangela Camargo, Maria Madalena Camargo Forte & Carlos Rodolfo Wolf (2008): Linear Low Density Polyethylene Thermal Fractionation by DSC Technique, *International Journal of Polymer Analysis and Characterization*, 13:1, 49-65. https://doi.org/10.1080/10236660701802593

[8] Kong, Y., & Hay, J. N. (2002). The measurement of the crystallinity of polymers by DSC. *Polymer*, *43*(14), 3873-3878. https://doi.org/10.1016/S0032-3861(02)00235-5

[9] Doudou, B., Dargent, E., & Grenet, J. (2006). Crystallization and melting behaviour of poly (m-xylene adipamide). *Journal of thermal analysis and calorimetry*, *85*(2), 409-415. https://doi.org/10.1007/s10973-005-7299-y

[10] Fortelný, I., & Jůza, J. (2021). The effects of copolymer compatibilizers on the phase structure evolution in polymer blends—a review. *Materials*, *14*(24), 7786. https://doi.org/10.3390/ma14247786

[11] Araujo, J. R., Vallim, M. R., Spinacé, M. A. S., & De Paoli, M. A. (2008). Use of postconsumer polyethylene in blends with polyamide 6: effects of the extrusion method and the compatibilizer. *Journal of applied polymer science*, *110*(3), 1310-1317. https://doi.org/10.1002/app.28441

[12] Chloé Épinat. Morphology development and rheological properties of reactively compatibilized Polyamide 6 / High Density Polyethylene blends. *Material chemistry*. Université Claude Bernard - Lyon I, 2014.

[13] Karakus, G., Polat, Z. A., Yenidunya, A. F., Zengin, H. B., & Karakus, C. B. (2013). Synthesis, characterization and cytotoxicity of novel modified poly [(maleic anhydride)-co-(vinyl acetate)]/noradrenaline conjugate. *Polymer international*, *62*(3), 492-500. https://doi.org/10.1002/pi.4341
Acknowledgement

Authors want to thank IMN laboratory for allowing us the use of their SEM. We also thank INDEVCO for its financial and material support.

References

[1] J. Luna and A. Vílchez, "Polymer nanocomposites for food packaging," *Emerging Nanotechnologies in Food Science*, pp. 119-147, 2017. https://doi.org/10.1016/B978-0-323-42980-1.00007-8

[2] H. Onyeaka, P. Passaretti, T. Miri, and Z. T. Al-Sharify, "The safety of nanomaterials in food production and packaging," *Current Research in Food Science*, vol. 5, pp. 763-774, 2022. https://doi.org/10.1016/j.crfs.2022.04.005

[3] Fereydoon, M. (2014). Development of High Barrier Nylon Based Multilayer Films [Ph.D. thesis, École Polytechnique de Montréal]. PolyPublie.

[4] Seif, S. (2009). Temporally programmed stretching of polymer films: Influence of nanoparticles. The University of Akron.

[5] Arnal, M. L., Matos, M. E., Morales, R. A., Santana, O. O., & Müuller, A. J. (1998). Evaluation of the fractionated crystallization of dispersed polyolefins in a polystyrene matrix. *Macromolecular Chemistry and Physics*, 199(10), 2275-2288. https://doi.org/10.1002/(SICI)1521-3935(19981001)199:10<2275::AID-MACP2275>3.0.CO;2-#

[6] Sangroniz, L., Wang, B., Su, Y., Liu, G., Cavallo, D., Wang, D., & Müller, A. J. (2021). Fractionated crystallization in semicrystalline polymers. *Progress in Polymer Science*, 115, 101376. https://doi.org/10.1016/j.progpolymsci.2021.101376

[7] Mariangela Camargo, Maria Madalena Camargo Forte & Carlos Rodolfo Wolf (2008): Linear Low Density Polyethylene Thermal Fractionation by DSC Technique, *International Journal of Polymer Analysis and Characterization*, 13:1, 49-65. https://doi.org/10.1080/10236660701802593

[8] Kong, Y., & Hay, J. N. (2002). The measurement of the crystallinity of polymers by DSC. *Polymer*, *43*(14), 3873-3878. https://doi.org/10.1016/S0032-3861(02)00235-5

[9] Doudou, B., Dargent, E., & Grenet, J. (2006). Crystallization and melting behaviour of poly (m-xylene adipamide). *Journal of thermal analysis and calorimetry*, *85*(2), 409-415. https://doi.org/10.1007/s10973-005-7299-y

[10] Fortelný, I., & Jůza, J. (2021). The effects of copolymer compatibilizers on the phase structure evolution in polymer blends—a review. *Materials*, *14*(24), 7786. https://doi.org/10.3390/ma14247786

[11] Araujo, J. R., Vallim, M. R., Spinacé, M. A. S., & De Paoli, M. A. (2008). Use of postconsumer polyethylene in blends with polyamide 6: effects of the extrusion method and the compatibilizer. *Journal of applied polymer science*, *110*(3), 1310-1317. https://doi.org/10.1002/app.28441

[12] Chloé Épinat. Morphology development and rheological properties of reactively compatibilized Polyamide 6 / High Density Polyethylene blends. *Material chemistry*. Université Claude Bernard - Lyon I, 2014.

[13] Karakus, G., Polat, Z. A., Yenidunya, A. F., Zengin, H. B., & Karakus, C. B. (2013). Synthesis, characterization and cytotoxicity of novel modified poly [(maleic anhydride)-co-(vinyl acetate)]/noradrenaline conjugate. *Polymer international*, *62*(3), 492-500. https://doi.org/10.1002/pi.4341
# Recycling of thermoplastic materials: Development of a self-adaptive process to the recycled materials

FARAH Elena<fnref n="1" />,<fnref n="2" />*, EL HAJJ SLEIMAN Ghinwa<fnref n="1" />,<fnref n="3" />, LE GAL LA SALLE Eric<fnref n="1" />,<fnref n="2" />, BAILLEUL Jean-Luc<fnref n="2" />

**Keywords:** Mechanical Recycling, Aging, Thermoplastics, Rheology, Inline Measurements

**Abstract.** Over the last few decades, thermoplastic polymers have become increasingly popular and widely used materials in our lives. However, these mass consumptions remain difficult to reuse. Although mechanical recycling is being seriously proposed as an alternative solution [1], until now many obstacles prevent its application to polymers. Undetermined initial properties and aging conditions of post-consumer waste, which are, in fact, extremely variable, result in an ignorance of the aged material rheological properties before their reprocessing. As a consequence, the adjustment of extrusion parameters becomes very difficult and can vary over time and between supply batches. In this context, the project RECYPLAST-DEMO was proposed aiming at developing a prototype able to characterize, in real-time [2], the rheological properties of extruded waste materials and self-adjust the operating parameters of the machine, with the help of artificial intelligence, to maintain products of required quality. Our work concerns inline rheological characterization of materials using an instrumented die numerically tested with the software POLYFLOW® for the viscosity range of a PP, designed and experimentally validated in a later step of the project. The goal is to obtain the rheological behavior of the material from pressure difference measurements along the length of the developed die and assess data compared to rheological characterization measurements performed in laboratory on virgin material and aged ones. This paper highlights the effect of the processing temperature and the residence time on the aging of the polymer. Moreover, the numerical work of the developed extrusion die is presented and explained.

**Introduction**

The global plastics market has experienced exponential growth over the past 70 years, due to the use of polymers across almost all sectors. Aware of its responsibilities toward solving climate and environmental-related problems, the European Union has been putting forward agreements [3] aiming to control end-of-life plastic products by means of imposing taxes, promoting eco-design and developing collection and recycling of plastic waste [4]. The purpose is to transform the linear “produce, consume, throw away” economy into a circular one, consisting of a “production, use, recycling” loop [5]. On the other side, plastic industry is undergoing major changes mainly in waste management to overcome economic challenges. It started with the reintegration of controlled amounts of post-industrial waste, also known as “clean” waste, into the process after demonstrating similar technical properties to virgin materials [6]. Although incineration maintains its originality as a solution for solid waste in general, mechanical recycling is a serious alternative solution to revalidate plastic waste following the rise in raw material prices. However, recycling of post-consumer products [1] remains difficult to put into action due to many obstacles preventing, until

<figure><img src="image_2480.png" /></figure>

<footnote n="1">¹Icam School of Engineering, 35 av. Champ de Manœuvres, 44470 Carquefou, France</footnote>

<footnote n="2">²Nantes Université, UMR CNRS 6607, Laboratoire LTeN, F-44000 Nantes, France</footnote>

<footnote n="3">³Nantes Université, UMR CNRS 6144, Laboratoire GEPEA, F-44000 Nantes, France</footnote>

<footnote>*elena.farah@icam.fr</footnote>
# Recycling of thermoplastic materials: Development of a self-adaptive process to the recycled materials

FARAH Elena<fnref n="1" />,<fnref n="2" />*, EL HAJJ SLEIMAN Ghinwa<fnref n="1" />,<fnref n="3" />, LE GAL LA SALLE Eric<fnref n="1" />,<fnref n="2" />, BAILLEUL Jean-Luc<fnref n="2" />

**Keywords:** Mechanical Recycling, Aging, Thermoplastics, Rheology, Inline Measurements

**Abstract.** Over the last few decades, thermoplastic polymers have become increasingly popular and widely used materials in our lives. However, these mass consumptions remain difficult to reuse. Although mechanical recycling is being seriously proposed as an alternative solution [1], until now many obstacles prevent its application to polymers. Undetermined initial properties and aging conditions of post-consumer waste, which are, in fact, extremely variable, result in an ignorance of the aged material rheological properties before their reprocessing. As a consequence, the adjustment of extrusion parameters becomes very difficult and can vary over time and between supply batches. In this context, the project RECYPLAST-DEMO was proposed aiming at developing a prototype able to characterize, in real-time [2], the rheological properties of extruded waste materials and self-adjust the operating parameters of the machine, with the help of artificial intelligence, to maintain products of required quality. Our work concerns inline rheological characterization of materials using an instrumented die numerically tested with the software POLYFLOW® for the viscosity range of a PP, designed and experimentally validated in a later step of the project. The goal is to obtain the rheological behavior of the material from pressure difference measurements along the length of the developed die and assess data compared to rheological characterization measurements performed in laboratory on virgin material and aged ones. This paper highlights the effect of the processing temperature and the residence time on the aging of the polymer. Moreover, the numerical work of the developed extrusion die is presented and explained.

**Introduction**

The global plastics market has experienced exponential growth over the past 70 years, due to the use of polymers across almost all sectors. Aware of its responsibilities toward solving climate and environmental-related problems, the European Union has been putting forward agreements [3] aiming to control end-of-life plastic products by means of imposing taxes, promoting eco-design and developing collection and recycling of plastic waste [4]. The purpose is to transform the linear “produce, consume, throw away” economy into a circular one, consisting of a “production, use, recycling” loop [5]. On the other side, plastic industry is undergoing major changes mainly in waste management to overcome economic challenges. It started with the reintegration of controlled amounts of post-industrial waste, also known as “clean” waste, into the process after demonstrating similar technical properties to virgin materials [6]. Although incineration maintains its originality as a solution for solid waste in general, mechanical recycling is a serious alternative solution to revalidate plastic waste following the rise in raw material prices. However, recycling of post-consumer products [1] remains difficult to put into action due to many obstacles preventing, until

<figure><img src="image_2473.png" /></figure>

<footnote n="1">¹Icam School of Engineering, 35 av. Champ de Manœuvres, 44470 Carquefou, France</footnote>

<footnote n="2">²Nantes Université, UMR CNRS 6607, Laboratoire LTeN, F-44000 Nantes, France</footnote>

<footnote n="3">³Nantes Université, UMR CNRS 6144, Laboratoire GEPEA, F-44000 Nantes, France</footnote>

<footnote>*elena.farah@icam.fr</footnote>
now, the processing of waste polymers, such as polypropylene (PP), which is the second most-
widely produced plastic after polyethylene.

Extrusion is one of the best processes for continuous plastics production. When working with a raw material, a characterization step precedes the production phase to determine the processing parameters, for instance processing temperature and screw speed. Once known, they are set for long runs. However, in many cases, adjustments to process parameters have to be made for each batch of material, especially in plastics waste recycling as we encounter a major material heterogeneity problem. First of all, the presence of several polymers, in most cases immiscible with each other, results in incompatible phase mixtures in the molten state [7]. Moreover, additives are added to polymer blends without clearly specifying the composition [8], which could complicate the recycling process and result in products with poor properties. Second, the properties of the product, before consumption and after aging, can't be determined and are, in fact, extremely variable. This results in an ignorance of the waste material rheological properties before its reprocessing. At this level, recycling is viable but the adjustment of extrusion parameters becomes very difficult and can vary over time and between supply batches requiring well-trained operators in control of the process. This could explain why industrials do not favor the shift towards reincorporating large amounts of recycled materials into their processes. Further, the effect of processing temperature and residence time on the aged material should be studied as it becomes more significant for reprocessed material. In their article on the effect of multiple extrusion, Hermanova et al. [9] confirmed the drop of material molecular weight due to PP chains scission. Thus, in terms of melt viscosities, the deterioration of the non-stabilized polymer was reflected by an increase in the melt flow index (MFI). Another study showed a reduction in the particle size and particle size distribution of the dispersed rubbery phase during reprocessing [10]. The processing stability is primarily dependent on variables such as additive concentration, mainly stabilizers, temperature and the origin and composition of polymer. It can be quantitatively assessed by a processing degradation index (PDI) defined as the plot of MFI vs the number of extrusions [11], [12].

Given the complexity of the plastic production process and its effect on the quality of the final product, it is obviously important to monitor it in order to understand and analyze the material's behavior in real time. The instrumentation of the extrusion process could be designed to detect flow instabilities and heat transfer of the material in its molten state using ultrasonic techniques, spectroscopy, pressure sensors, etc. There are several methods for characterizing extruded material, from on-line characterization, where the measurement is carried out on a bypass channel deviation, to the inline measurement known for real-time characterization on the main flow channel. This latter is more reliable as it operates under real conditions. One of the key properties of molten polymers is viscosity as it provides information about the molecular structure of the polymer, the processability and the final product properties. Previous studies show successful pressure drop measurements along the extrusion die converted into viscosity. In their research, M. Padmanabhan et al. [13] developed a flat die of constant geometry that worked as an inline rheometer, measuring viscosity at different shear rates by varying the screw speed of the extruder. Later on, P.X. Li et al. [14] developed the previous model as they found that varying the screw speed did not maintain a constant flow and therefore influenced the pressure measurements. In their model, they proposed a system of valves to control the flow rate. Given the limitations of pressure range, researchers began to look for innovative die designs. In 2010, F. Robin et al. [15] introduced the concept of adjustable twin slit die consisting of two independently controlled parallel rectangular channels modified during the process to vary the shear rate in the measuring channel. With a lower level of complexity, a single-channel rheometer was introduced, a few years later, by M. Horvat et al. [16] based on the concept of a fixed outer geometry with insertion modules of different rectangular cross sections which allowed the measuring of the material
now, the processing of waste polymers, such as polypropylene (PP), which is the second most-
widely produced plastic after polyethylene.

Extrusion is one of the best processes for continuous plastics production. When working with a raw material, a characterization step precedes the production phase to determine the processing parameters, for instance processing temperature and screw speed. Once known, they are set for long runs. However, in many cases, adjustments to process parameters have to be made for each batch of material, especially in plastics waste recycling as we encounter a major material heterogeneity problem. First of all, the presence of several polymers, in most cases immiscible with each other, results in incompatible phase mixtures in the molten state [7]. Moreover, additives are added to polymer blends without clearly specifying the composition [8], which could complicate the recycling process and result in products with poor properties. Second, the properties of the product, before consumption and after aging, can't be determined and are, in fact, extremely variable. This results in an ignorance of the waste material rheological properties before its reprocessing. At this level, recycling is viable but the adjustment of extrusion parameters becomes very difficult and can vary over time and between supply batches requiring well-trained operators in control of the process. This could explain why industrials do not favor the shift towards reincorporating large amounts of recycled materials into their processes. Further, the effect of processing temperature and residence time on the aged material should be studied as it becomes more significant for reprocessed material. In their article on the effect of multiple extrusion, Hermanova et al. [9] confirmed the drop of material molecular weight due to PP chains scission. Thus, in terms of melt viscosities, the deterioration of the non-stabilized polymer was reflected by an increase in the melt flow index (MFI). Another study showed a reduction in the particle size and particle size distribution of the dispersed rubbery phase during reprocessing [10]. The processing stability is primarily dependent on variables such as additive concentration, mainly stabilizers, temperature and the origin and composition of polymer. It can be quantitatively assessed by a processing degradation index (PDI) defined as the plot of MFI vs the number of extrusions [11], [12].

Given the complexity of the plastic production process and its effect on the quality of the final product, it is obviously important to monitor it in order to understand and analyze the material's behavior in real time. The instrumentation of the extrusion process could be designed to detect flow instabilities and heat transfer of the material in its molten state using ultrasonic techniques, spectroscopy, pressure sensors, etc. There are several methods for characterizing extruded material, from on-line characterization, where the measurement is carried out on a bypass channel deviation, to the inline measurement known for real-time characterization on the main flow channel. This latter is more reliable as it operates under real conditions. One of the key properties of molten polymers is viscosity as it provides information about the molecular structure of the polymer, the processability and the final product properties. Previous studies show successful pressure drop measurements along the extrusion die converted into viscosity. In their research, M. Padmanabhan et al. [13] developed a flat die of constant geometry that worked as an inline rheometer, measuring viscosity at different shear rates by varying the screw speed of the extruder. Later on, P.X. Li et al. [14] developed the previous model as they found that varying the screw speed did not maintain a constant flow and therefore influenced the pressure measurements. In their model, they proposed a system of valves to control the flow rate. Given the limitations of pressure range, researchers began to look for innovative die designs. In 2010, F. Robin et al. [15] introduced the concept of adjustable twin slit die consisting of two independently controlled parallel rectangular channels modified during the process to vary the shear rate in the measuring channel. With a lower level of complexity, a single-channel rheometer was introduced, a few years later, by M. Horvat et al. [16] based on the concept of a fixed outer geometry with insertion modules of different rectangular cross sections which allowed the measuring of the material
rheological behavior. Moreover, the interest in conducting measurements on a deviated channel still exists. In 2020, J.A. Covas proposed a twin-slit die design [2] that divided the initial flow of material between 2 parallel channels. By controlling the flow rate in the measuring channel and using two pressure sensors, measures of melt viscosity were taken.

Inspired from previous works, and in response to the crucial situation regarding the environment, the project RECYPLAST-DEMO is funded by the Pays de La Loire region and aims to set up, by 2025, a device able to self-adapt a production line according to the variability of the input plastic waste materials. Therefore, our work concerns developing an instrumented die with the objective of measuring the complete rheological behavior curve of the material during production. In parallel, the polymer aging phenomenon is studied to develop an understanding of the waste's behavior under multiple reprocessing. In the following parts, we will present the material used in the first phase of the project and the methods applied during the rheological characterization. Then, we will briefly introduce the numerical model of the instrumented die that will be machined and validated experimentally in a later step of the project. The last part shows results regarding material aging and the numerical simulation of the developed die.

## Materials and Methods

### Materials

In this study, three grades of extrusion PP are used (supplied by Gazechim Plastics, France). During the first phase of the project, the purpose is to experimentally test the device with a diversity of virgin PP materials. Later on, mixtures of two or more components will eventually take place, to imitate an example of real case recycling scenario. Table 1 summarizes the main properties of the three different PP grades.

<table><caption>Table 1. PP properties of 3 commercial grades.</caption><thead><tr><td>Material</td><td>Density (g/cm³)</td><td>Tg (°C)</td><td>Tm (°C)</td><td>MFI (g/10 min)</td></tr></thead><tbody><tr><td>PP089Y1E</td><td>0.85</td><td>-8.4</td><td>165.8</td><td>30.70</td></tr><tr><td>PP-040G1E</td><td>0.89</td><td>-9.6</td><td>164.7</td><td>3.26</td></tr><tr><td>PP-030G1E</td><td>0.86</td><td>-10.3</td><td>165.8</td><td>1.55</td></tr></tbody></table>

### Sample Preparation

Prior to characterization, PP samples were prepared under similar extrusion conditions. The three grades were extruded separately with a plate die to form plastic films of which circular cuts were taken.

### Rheological Characterization

Specimens are submitted to rheological characterization by a HAAKE Mars III rheology instrument using a parallel plate geometry with a diameter of 25mm and a fixed gap thickness of 1mm. Frequency sweep tests were carried out at temperatures ranging from 175°C to 210°C with an amplitude of $\gamma_0=1\%$ for frequencies in the range of 0.1 Hz to 100 Hz. By applying the Cox-Merz rule [17], we obtain the shear-rate-dependent viscosity $\eta(\dot{\gamma})$, converted from the oscillatory shear measurements $\eta^*(\omega)$ as follows in Eq. 1:

$$ \eta^*(\omega) = \eta(\dot{\gamma}) \qquad (1) $$

The PP melt results are then fitted by the nonlinear viscoelastic Cross law, given by Eq. 2:

$$ \eta = \eta_{\infty} + \frac{(\eta_{0} - \eta_{\infty})}{1 + (\lambda\dot{\gamma})^{n}} \qquad (2) $$

Where $\eta_0$ is the zero-shear rate viscosity, $\lambda$ is a time constant, and $n$ is the power law index.

### Aging Test
rheological behavior. Moreover, the interest in conducting measurements on a deviated channel still exists. In 2020, J.A. Covas proposed a twin-slit die design [2] that divided the initial flow of material between 2 parallel channels. By controlling the flow rate in the measuring channel and using two pressure sensors, measures of melt viscosity were taken.

Inspired from previous works, and in response to the crucial situation regarding the environment, the project RECYPLAST-DEMO is funded by the Pays de La Loire region and aims to set up, by 2025, a device able to self-adapt a production line according to the variability of the input plastic waste materials. Therefore, our work concerns developing an instrumented die with the objective of measuring the complete rheological behavior curve of the material during production. In parallel, the polymer aging phenomenon is studied to develop an understanding of the waste's behavior under multiple reprocessing. In the following parts, we will present the material used in the first phase of the project and the methods applied during the rheological characterization. Then, we will briefly introduce the numerical model of the instrumented die that will be machined and validated experimentally in a later step of the project. The last part shows results regarding material aging and the numerical simulation of the developed die.

## Materials and Methods

### Materials

In this study, three grades of extrusion PP are used (supplied by Gazechim Plastics, France). During the first phase of the project, the purpose is to experimentally test the device with a diversity of virgin PP materials. Later on, mixtures of two or more components will eventually take place, to imitate an example of real case recycling scenario. Table 1 summarizes the main properties of the three different PP grades.

<table><caption>Table 1. PP properties of 3 commercial grades.</caption><thead><tr><td>Material</td><td>Density (g/cm³)</td><td>Tg (°C)</td><td>Tm (°C)</td><td>MFI (g/10 min)</td></tr></thead><tbody><tr><td>PP089Y1E</td><td>0.85</td><td>-8.4</td><td>165.8</td><td>30.70</td></tr><tr><td>PP-040G1E</td><td>0.89</td><td>-9.6</td><td>164.7</td><td>3.26</td></tr><tr><td>PP-030G1E</td><td>0.86</td><td>-10.3</td><td>165.8</td><td>1.55</td></tr></tbody></table>

### Sample Preparation

Prior to characterization, PP samples were prepared under similar extrusion conditions. The three grades were extruded separately with a plate die to form plastic films of which circular cuts were taken.

### Rheological Characterization

Specimens are submitted to rheological characterization by a HAAKE Mars III rheology instrument using a parallel plate geometry with a diameter of 25mm and a fixed gap thickness of 1mm. Frequency sweep tests were carried out at temperatures ranging from 175°C to 210°C with an amplitude of $\gamma_0=1\%$ for frequencies in the range of 0.1 Hz to 100 Hz. By applying the Cox-Merz rule [17], we obtain the shear-rate-dependent viscosity $\eta(\dot{\gamma})$, converted from the oscillatory shear measurements $\eta^*(\omega)$ as follows in Eq. 1:

$$ \eta^*(\omega) = \eta(\dot{\gamma}) \qquad (1) $$

The PP melt results are then fitted by the nonlinear viscoelastic Cross law, given by Eq. 2:

$$ \eta = \eta_{\infty} + \frac{(\eta_{0} - \eta_{\infty})}{1 + (\lambda\dot{\gamma})^{n}} \qquad (2) $$

Where $\eta_0$ is the zero-shear rate viscosity, $\lambda$ is a time constant, and $n$ is the power law index.

### Aging Test
To develop an understanding of the material's aging conditions, samples of the three PP grades were tested at different temperatures, ranging from 180°C to 210°C, using the same previous rheometer. Three protocols were followed:

* Mode 1: rotational test at shear rate equal to 0.01 s⁻¹ for 15 minutes, repeated for 12 cycles with break gaps of 3 minutes.

* Mode 2: a periodic oscillating strain with an amplitude of γ₀=1% at a constant frequency w = 0.0628 rad/s, over 2.5 hours, conducted in air.

* Mode 3: a periodic oscillating strain with an amplitude of γ₀=1% at a constant frequency w = 0.0628 rad/s, over 3 hours tested under nitrogen gas.

## Numerical Simulation

### Extrusion Die Model

Numerical simulations of the flow in the extrusion die were performed using the commercial CFD package POLYFLOW®, by ANSYS. The 2D-axisymmetric geometry was modeled and solved as a steady-state problem using finite-element method (FEM). The fluid package was set as a Generalized Newtonian isothermal flow problem, considering the experimental extrusion conditions for the PP089Y1E grade at 190°C. As for meshing, a simple configuration of quadratic elements was constructed. The boundary sets for the problem are shown in Fig. 1 and the conditions at the boundaries of the domain are:

* BS 1: flow inlet, for a diameter = 45 mm

* BS 2: outer wall, with a maximum shear = 165 s⁻¹

* BS 3: flow exit, for a diameter = 6 mm

* BS 4: symmetry axis

<figure><img src="image_2481.png" /><figcaption>Fig. 1. Boundary sets on 2D-axisymmetric extrusion die schematic.</figcaption></figure>

No-slipping was considered and the model was tested for the PP089Y1E grade material, at a flow value of Q =10 kg/h, using the Cross law obtained from the succeeding rheological characterization step.

### Mathematical Model

Pressure loss measurements along the length of different sections will be converted to shear stress data [18] at the boundary, given by Eq. 3:

$$ \tau_w = \frac{\Delta P - \Delta P_e}{4L/D} \qquad (3) $$

Where, τw is the shear stress, ΔP is the measured pressure loss, L is the distance between 2 measuring points, D is the diameter of the section and ΔP_e is the Bagley correction, it is used to correct a pressure reading measured before the die inlet.

From the Newtonian fluid formula, the apparent shear velocity, ˙γₐw, is defined according to Eq. 4. To convert it into the real shear velocity, ˙γw, a correction is proposed by Rabinowitsch applied as follows in Eq. 5 and Eq. 6:

$$ \dot{\gamma}_{aw} = \frac{32 Q}{\pi D^3} \qquad (4) $$

$$ \dot{\gamma}_w = \left( \frac{3n' + 1}{4n'} \right) \dot{\gamma}_{aw} \qquad (5) $$

With,
To develop an understanding of the material's aging conditions, samples of the three PP grades were tested at different temperatures, ranging from 180°C to 210°C, using the same previous rheometer. Three protocols were followed:

* Mode 1: rotational test at shear rate equal to 0.01 s⁻¹ for 15 minutes, repeated for 12 cycles with break gaps of 3 minutes.

* Mode 2: a periodic oscillating strain with an amplitude of γ₀=1% at a constant frequency w = 0.0628 rad/s, over 2.5 hours, conducted in air.

* Mode 3: a periodic oscillating strain with an amplitude of γ₀=1% at a constant frequency w = 0.0628 rad/s, over 3 hours tested under nitrogen gas.

## Numerical Simulation

### Extrusion Die Model

Numerical simulations of the flow in the extrusion die were performed using the commercial CFD package POLYFLOW®, by ANSYS. The 2D-axisymmetric geometry was modeled and solved as a steady-state problem using finite-element method (FEM). The fluid package was set as a Generalized Newtonian isothermal flow problem, considering the experimental extrusion conditions for the PP089Y1E grade at 190°C. As for meshing, a simple configuration of quadratic elements was constructed. The boundary sets for the problem are shown in Fig. 1 and the conditions at the boundaries of the domain are:

* BS 1: flow inlet, for a diameter = 45 mm

* BS 2: outer wall, with a maximum shear = 165 s⁻¹

* BS 3: flow exit, for a diameter = 6 mm

* BS 4: symmetry axis

<figure><img src="image_2474.png" /><figcaption>Fig. 1. Boundary sets on 2D-axisymmetric extrusion die schematic.</figcaption></figure>

No-slipping was considered and the model was tested for the PP089Y1E grade material, at a flow value of Q =10 kg/h, using the Cross law obtained from the succeeding rheological characterization step.

### Mathematical Model

Pressure loss measurements along the length of different sections will be converted to shear stress data [18] at the boundary, given by Eq. 3:

$$ \tau_w = \frac{\Delta P - \Delta P_e}{4L/D} \qquad (3) $$

Where, τw is the shear stress, ΔP is the measured pressure loss, L is the distance between 2 measuring points, D is the diameter of the section and ΔP_e is the Bagley correction, it is used to correct a pressure reading measured before the die inlet.

From the Newtonian fluid formula, the apparent shear velocity, ˙γₐw, is defined according to Eq. 4. To convert it into the real shear velocity, ˙γw, a correction is proposed by Rabinowitsch applied as follows in Eq. 5 and Eq. 6:

$$ \dot{\gamma}_{aw} = \frac{32 Q}{\pi D^3} \qquad (4) $$

$$ \dot{\gamma}_w = \left( \frac{3n' + 1}{4n'} \right) \dot{\gamma}_{aw} \qquad (5) $$

With,
$$n' = \frac{dlog\tau_w}{dlog\dot{\gamma}_{aw}} \qquad (6)$$

Once correction is applied, viscosity can be determined as follows using Eq. 7:

$$\eta = \frac{\tau_w}{\dot{\gamma}_w} \qquad (7)$$

## Results

### Rheological Characterization

Figures 2 and 3 present master curve plots of the complex modulus, G*, versus angular frequency, ω, for grades PP-030G1E and PP-040G1E, respectively. Further, the experimental viscosity-to-shear data at 190°C of the different grades of PP (Fig. 4) were fitted by the Cross model (Eq.2), with the parameters listed in Table 2. Clearly, results show that the Cross model fits the measured values and successfully describes the measurements. Therefore, these parameters will be used as entry data for the numerical model and later on, to validate the experimental extrusion measurements of the instrumented die.

<figure><img src="image_2482.png" /><figcaption>Fig. 2. Master curve of complex modulus for PP-030G1E grade at 190°C.</figcaption></figure>

<figure><img src="image_2483.png" /><figcaption>Fig. 3. Master curve of complex modulus for PP-040G1E grade at 190°C.</figcaption></figure>
$$n' = \frac{dlog\tau_w}{dlog\dot{\gamma}_{aw}} \qquad (6)$$

Once correction is applied, viscosity can be determined as follows using Eq. 7:

$$\eta = \frac{\tau_w}{\dot{\gamma}_w} \qquad (7)$$

## Results

### Rheological Characterization

Figures 2 and 3 present master curve plots of the complex modulus, G*, versus angular frequency, ω, for grades PP-030G1E and PP-040G1E, respectively. Further, the experimental viscosity-to-shear data at 190°C of the different grades of PP (Fig. 4) were fitted by the Cross model (Eq.2), with the parameters listed in Table 2. Clearly, results show that the Cross model fits the measured values and successfully describes the measurements. Therefore, these parameters will be used as entry data for the numerical model and later on, to validate the experimental extrusion measurements of the instrumented die.

<figure><img src="image_2475.png" /><figcaption>Fig. 2. Master curve of complex modulus for PP-030G1E grade at 190°C.</figcaption></figure>

<figure><img src="image_2476.png" /><figcaption>Fig. 3. Master curve of complex modulus for PP-040G1E grade at 190°C.</figcaption></figure>
<figure><img src="image_2484.png" /><figcaption>Fig. 4. Frequency sweep characterization test for the three grades of PP at 190°C, fitted by the Cross model.</figcaption></figure>

<table><caption>Table 2. Parameters for the PP melt at 190°C obeying the Cross model.</caption><thead><tr><th rowspan="3">Material</th><th colspan="4">Value</th></tr><tr><th>η<sub>0</sub></th><th>η<sub>∞</sub><sup>1</sup></th><th>λ</th><th>n</th></tr><tr><th>(Pa.s)</th><th>(Pa.s)</th><th>(s)</th><th>(-)</th></tr></thead><tbody><tr><td>PP089Y1E</td><td>993.1</td><td>12.06</td><td>0.0281</td><td>0.691</td></tr><tr><td>PP-040G1E</td><td>1.34e4</td><td>-121.8</td><td>0.768</td><td>0.607</td></tr><tr><td>PP-030G1E</td><td>2.56e4</td><td>-191.2</td><td>1.118</td><td>0.626</td></tr></tbody></table>

## Material Aging

As the waste material will be subjected to multiple reprocessing under heat and mechanical deformation, we were interested in examining the viscosity variation along this process. Therefore, we submitted a sample of PP-030G1E grade to four consecutive characterization tests at 200°C. All curves present the Newtonian plateau at low shear rates and a pseudoplastic behavior at higher shear rates. However, after each test, the viscosity value dropped as shown in Fig. 5, reaching an initial viscosity loss of nearly 90% between the first and the last test.

<figure><img src="image_2485.png" /><figcaption>Fig. 5. Frequency sweep test for a PP-030G1E grade sample at 200°C, repeated consecutively 4 times.</figcaption></figure>

<footnote n="1">¹ The minus sign of η_∞ is due to the fit of the measured points and has no physical meaning. It also means that the model can not be extrapolated out of the measuring range.</footnote>
<figure><img src="image_2477.png" /><figcaption>Fig. 4. Frequency sweep characterization test for the three grades of PP at 190°C, fitted by the Cross model.</figcaption></figure>

<table><caption>Table 2. Parameters for the PP melt at 190°C obeying the Cross model.</caption><thead><tr><th rowspan="3">Material</th><th colspan="4">Value</th></tr><tr><th>η<sub>0</sub></th><th>η<sub>∞</sub><sup>1</sup></th><th>λ</th><th>n</th></tr><tr><th>(Pa.s)</th><th>(Pa.s)</th><th>(s)</th><th>(-)</th></tr></thead><tbody><tr><td>PP089Y1E</td><td>993.1</td><td>12.06</td><td>0.0281</td><td>0.691</td></tr><tr><td>PP-040G1E</td><td>1.34e4</td><td>-121.8</td><td>0.768</td><td>0.607</td></tr><tr><td>PP-030G1E</td><td>2.56e4</td><td>-191.2</td><td>1.118</td><td>0.626</td></tr></tbody></table>

## Material Aging

As the waste material will be subjected to multiple reprocessing under heat and mechanical deformation, we were interested in examining the viscosity variation along this process. Therefore, we submitted a sample of PP-030G1E grade to four consecutive characterization tests at 200°C. All curves present the Newtonian plateau at low shear rates and a pseudoplastic behavior at higher shear rates. However, after each test, the viscosity value dropped as shown in Fig. 5, reaching an initial viscosity loss of nearly 90% between the first and the last test.

<figure><img src="image_2478.png" /><figcaption>Fig. 5. Frequency sweep test for a PP-030G1E grade sample at 200°C, repeated consecutively 4 times.</figcaption></figure>

<footnote n="1">¹ The minus sign of η_∞ is due to the fit of the measured points and has no physical meaning. It also means that the model can not be extrapolated out of the measuring range.</footnote>
To further investigate this property loss, we conducted tests at constant rotational speed relative to a low shear rate for the grade of PP-040G1E at two different temperatures. As expected, the viscosity values dropped with time, showing a faster decrease at higher temperatures (Fig. 6). Until now, this phenomenon can only be explained by the degradation of material and possibly the reorientation of the polymer chains at the micro-structural level as the specimen was exposed to a constant rotational deformation under heat. Moreover, we noticed that after a certain time of test, each curve reached a threshold and showed a nearly constant viscosity, which could be related further to the material's total residence time in the extruder under reprocessing.

<figure><img src="image_2486.png" /><figcaption>Fig. 6. Rotational time sweep test of PP-040G1E grade at different temperatures following mode 1 (section 2.4).</figcaption></figure>

To eliminate the effect of micro-structural modifications, oscillatory tests of low deformation were applied on the same grade of PP at different temperatures. A lower decrease in viscosity is observed in Fig. 7, putting ahead thermally activated chemical degradation, likely oxidation, over the mechanical one. Similarly, the threshold was observed. This information could be significant to the number of cycles a waste material could undergo while maintaining its properties.

<figure><img src="image_2487.png" /><figcaption>Fig. 7. Oscillatory time sweep test of PP-040G1E grade at different temperatures following mode 2 (section 2.4).</figcaption></figure>
To further investigate this property loss, we conducted tests at constant rotational speed relative to a low shear rate for the grade of PP-040G1E at two different temperatures. As expected, the viscosity values dropped with time, showing a faster decrease at higher temperatures (Fig. 6). Until now, this phenomenon can only be explained by the degradation of material and possibly the reorientation of the polymer chains at the micro-structural level as the specimen was exposed to a constant rotational deformation under heat. Moreover, we noticed that after a certain time of test, each curve reached a threshold and showed a nearly constant viscosity, which could be related further to the material's total residence time in the extruder under reprocessing.

<figure><img src="image_2479.png" /><figcaption>Fig. 6. Rotational time sweep test of PP-040G1E grade at different temperatures following mode 1 (section 2.4).</figcaption></figure>

To eliminate the effect of micro-structural modifications, oscillatory tests of low deformation were applied on the same grade of PP at different temperatures. A lower decrease in viscosity is observed in Fig. 7, putting ahead thermally activated chemical degradation, likely oxidation, over the mechanical one. Similarly, the threshold was observed. This information could be significant to the number of cycles a waste material could undergo while maintaining its properties.

<figure><img src="image_2480.png" /><figcaption>Fig. 7. Oscillatory time sweep test of PP-040G1E grade at different temperatures following mode 2 (section 2.4).</figcaption></figure>
Finally, to examine the effect of air on the oxidation of PP-040G1E, we tested samples under the inert nitrogen gas. Results (Fig. 8) show a decrease in the aging of the specimen under nitrogen compared to the ones in air, which promotes the use of inert gases during the recycling process to reduce the thermal degradation and maintain the viscosity of material as long as possible.

<figure><img src="image_2488.png" /><figcaption>Fig. 8. Oscillatory time sweep test of PP-040G1E grade at 200°C following mode 3 (section 2.4).</figcaption></figure>

## Numerical calculation

The RECYPLAST-DEMO die was designed and machined as shown in Fig. 9 based on numerical calculations. It is composed of 4 sections of independent heating zones. The extrusion die will be tested experimentally, in a later step, for validation of the numerical model.

<figure><img src="image_2489.png" /><figcaption>Fig. 9. RECYPLAST-DEMO extrusion die.</figcaption></figure>

As mentioned earlier, an isothermal simulation was calculated using the viscous Cross model of the PP089Y1E grade, at 190°C. The software shows 3D view (Fig. 10) and graphical representations (Fig. 11) of the flow stress distribution in the geometry, which helps us mount pressure sensors in the die.
Finally, to examine the effect of air on the oxidation of PP-040G1E, we tested samples under the inert nitrogen gas. Results (Fig. 8) show a decrease in the aging of the specimen under nitrogen compared to the ones in air, which promotes the use of inert gases during the recycling process to reduce the thermal degradation and maintain the viscosity of material as long as possible.

<figure><img src="image_2481.png" /><figcaption>Fig. 8. Oscillatory time sweep test of PP-040G1E grade at 200°C following mode 3 (section 2.4).</figcaption></figure>

## Numerical calculation

The RECYPLAST-DEMO die was designed and machined as shown in Fig. 9 based on numerical calculations. It is composed of 4 sections of independent heating zones. The extrusion die will be tested experimentally, in a later step, for validation of the numerical model.

<figure><img src="image_2482.png" /><figcaption>Fig. 9. RECYPLAST-DEMO extrusion die.</figcaption></figure>

As mentioned earlier, an isothermal simulation was calculated using the viscous Cross model of the PP089Y1E grade, at 190°C. The software shows 3D view (Fig. 10) and graphical representations (Fig. 11) of the flow stress distribution in the geometry, which helps us mount pressure sensors in the die.
<figure><img src="image_2490.png" /><figcaption>Fig. 10. Pressure distribution in the extrusion die simulated by POLYFLOW®.</figcaption></figure>

<figure><img src="image_2491.png" /><figcaption>Fig. 11. Pressure along the length L of the die.</figcaption></figure>

## Conclusion

The global plastic production has been growing rapidly since the 1950s, and correspondingly the plastic waste accumulation causing serious environmental damage. Moreover, as we are draining non-renewable resources, prices have been rising at an increasing rate. Hence, it became necessary to work for a circular economy and recover the value of the waste material by reprocessing it. However, this solution results in further degradation of the material. Therefore, our project proposed to create a device able to characterize the material in real time, to be able to monitor different properties of the material, namely viscosity, during the reprocessing. At first, we characterized three grades of virgin PP that will be used in this project and data was fitted by the Cross model to be applied in the numerical model. Then, we developed an instrumented extrusion die using the software POLYFLOW® to calculate the flow stress distribution. Moreover, we investigated the aging phenomenon of PP samples under heat and mechanical deformation. Results from the rotational mode tests showed that the material viscosity dropped drastically when subjected to mechanical deformation. Furthermore, to reduce the effect of thermal degradation, oscillatory tests of low deformation were conducted showing favorable results under nitrogen gas compared to tests under air.

Further investigation and testing are needed to validate, theoretically and experimentally, the device's suitability for mechanical recycling of PP while maintaining, for a maximum number of reprocessing cycles, the quality of material waste.

## References

[1] A. Tominaga, H. Sekiguchi, R. Nakano, S. Yao, and E. Takatori, "Advanced recycling process for waste plastics based on physical degradation theory and its stability," *J Mater Cycles Waste Manag*, vol. 21, 2019. https://doi.org/10.1007/s10163-018-0777-7

[2] J. A. Covas, "MONITORING OF POLYMER EXTRUSION AND COMPOUNDING PROCESSES," 2020.

[3] European Commission, "The European Green Deal: COM(2019) 640 Final," European Commission: Brussels, Belgium 2019.
<figure><img src="image_2483.png" /><figcaption>Fig. 10. Pressure distribution in the extrusion die simulated by POLYFLOW®.</figcaption></figure>

<figure><img src="image_2484.png" /><figcaption>Fig. 11. Pressure along the length L of the die.</figcaption></figure>

## Conclusion

The global plastic production has been growing rapidly since the 1950s, and correspondingly the plastic waste accumulation causing serious environmental damage. Moreover, as we are draining non-renewable resources, prices have been rising at an increasing rate. Hence, it became necessary to work for a circular economy and recover the value of the waste material by reprocessing it. However, this solution results in further degradation of the material. Therefore, our project proposed to create a device able to characterize the material in real time, to be able to monitor different properties of the material, namely viscosity, during the reprocessing. At first, we characterized three grades of virgin PP that will be used in this project and data was fitted by the Cross model to be applied in the numerical model. Then, we developed an instrumented extrusion die using the software POLYFLOW® to calculate the flow stress distribution. Moreover, we investigated the aging phenomenon of PP samples under heat and mechanical deformation. Results from the rotational mode tests showed that the material viscosity dropped drastically when subjected to mechanical deformation. Furthermore, to reduce the effect of thermal degradation, oscillatory tests of low deformation were conducted showing favorable results under nitrogen gas compared to tests under air.

Further investigation and testing are needed to validate, theoretically and experimentally, the device's suitability for mechanical recycling of PP while maintaining, for a maximum number of reprocessing cycles, the quality of material waste.

## References

[1] A. Tominaga, H. Sekiguchi, R. Nakano, S. Yao, and E. Takatori, "Advanced recycling process for waste plastics based on physical degradation theory and its stability," *J Mater Cycles Waste Manag*, vol. 21, 2019. https://doi.org/10.1007/s10163-018-0777-7

[2] J. A. Covas, "MONITORING OF POLYMER EXTRUSION AND COMPOUNDING PROCESSES," 2020.

[3] European Commission, "The European Green Deal: COM(2019) 640 Final," European Commission: Brussels, Belgium 2019.
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