Recycled Low Density Polyethylene Reinforced with Deverra tortuosa Vegetable Fibers

Abstract

In this work, natural fibers extracted from the medicinal aromatic plant Deverra tortuosa, with different sizes (S1 = 2 mm and S2 = 500 μm), were incorporated into recycled low density polyethylene (rLDPE) to produce sustainable biocomposites. Compounding was performed with different fiber concentrations (0 to 30% wt.) via twin-screw extrusion followed by injection molding. Based on the samples obtained, a comprehensive series of characterization was conducted, encompassing morphological and mechanical (flexural, tensile, hardness, and impact) properties. Additionally, thermal properties were assessed via differential scanning calorimetry (DSC), while Fourier transform infrared spectroscopy (FTIR) was used to elucidate potential chemical interactions and changes with processing. Across the range of conditions investigated, substantial improvements were observed in the rLDPE properties, in particular for the tensile modulus (23% for S1 and 104% for S2), flexural modulus (47% for S1 and 61% for S2), and flexural strength (31% for S1 and 65% for S2). Nevertheless, the tensile strength decreased (15% for S1 and 46% for S2) due to poor fiber–matrix interfacial adhesion. These preliminary results can be used for further development in sustainable packaging materials.

1. Introduction

Polymers have made it possible to produce a wide range of goods that are both economical and effective, but this led to a large build-up of plastic waste as a result of the growing demand for these products. Polymers are known to stay in the environment for extended periods of time because of their stability and resistance, leading to serious environmental problems. Effective waste management is therefore now mandatory [1,2].

Previously, the majority of polymeric materials were disposed of in landfills, raising serious environmental issues and leading to stricter regulations. To this end, several recycling techniques are used nowadays to prolong the life cycle of these materials in an effort to decrease the amount of waste ending up in landfills or being burned [3,4].

In 2022, China dominated the recycled plastics market with 20.3% and the largest market share of recycled polyethylene consumed in the Asia–Pacific (APAC) region with 45.6% [5]. As a thermoplastic polymer having high versatility, polyethylene (PE) is widely used in bottles, storage tanks, containers, insulation products, packaging, sheets, films, and other applications such as foams, blends, composites, and multilayers [5]. Two of the most important polyethylene grades are high density polyethylene (HDPE) with 13% and low density polyethylene (LDPE) with 16% of the global plastic production [6], which are highly used for film and rotational molding applications since they are widely available in pellet and powder forms [7].

However, for more demanding applications, PE has limited mechanical properties (strength and rigidity) and a low continuous operating temperature, as well as being susceptible to creep and environmental stress cracking. To solve these issues, several solutions have been proposed over the years [8,9]. The use of crosslinking agents to improve the mechanical, chemical, and thermal properties is an option, but this makes the materials more difficult to recycle [4]. Another possibility is to add fillers/reinforcements, especially biobased ones. Although most of the work conducted on lignocellulosic fibers was performed on wood fibers [10], different natural fibers, including flax, jute, hemp, sisal, and others have also been investigated to reinforce PE and other polymers. Unfortunately, the resulting composite’s mechanical properties are generally inferior to those of the neat matrix because of poor fiber dispersion and adhesion to the polymer matrix associated with the highly hydrophilic nature of most biobased fibers due to their high hydroxyl group content (cellulose), compared to most polymers which are mainly hydrophobic. Again, some solutions were proposed such as fiber surface treatments and coupling agents addition [11,12]. Although better mechanical performance was observed, this also makes the biocomposites more difficult to produce (more steps/components) and more expensive (higher number of raw materials and production costs). Thus, a cost/performance ratio must be determined [13,14].

When compared to other commonly used natural fibers, such as hemp and flax, these materials have competitive mechanical and thermal properties. They have promising elasticity and tensile resistance, among other properties, making them especially well-suited to reinforce recycled polymers such as low/high density polyethylene [15,16].

In this study, D. tortuosa was selected as a biobased reinforcement seldom reported in the literature, offering several distinctive advantages. Their abundant/availability in arid regions makes them an accessible and inexpensive local resource. Their use promotes a circular economy and reduces dependence on fibers such as flax or hemp, which are more resource-intensive. In addition to having a lower environmental impact, these fibers constitute an ecological and sustainable alternative to reinforce recycled polymers, thus opening new perspectives for industrial applications.

A synonym for D. tortuosa (Desf.) DC is Pituranthos tortuosus (Desf.), frequently referred to as “Guezzah” in Arabic. The Apiaceae family woody perennial shrub of D. tortuosa (DC) is found in desert places with varying climates, which affects the plant’s ability to produce its bioactive natural compounds [17]. Different compounds extracted from D. tortuosa are reported in

Table 1. Typical compounds extracted from D. tortuosa as a function of their location.

Fiber Origin	Compounds of D. tortuosa (%)	References
Degla valley protectorate, Elmaadi, Cairo, Egypt [29°56′16.5559″ N, 32°10′52.5384″ E]	9-octadecenoic acid (Z)-, methyl ester (28.39)	[18]
Wadi El-Rashrash, Eastern Desert, Egypt (29°26′44.1″ N, 31°29′54.1″ E)	β-phellandrene (10.49) α-terpinene (6.21) m-cymene (4.65) p-cymenene (3.16) α-terpinolene (2.78)	[19]
Widyian region (Wadi Abalkour, (30°0.83′02″ N, 41°0.24′53″ E), 53 km east of Arar City, Saudi Arabia	dillapiole (41.6) elemicin (7.3) myristicin (5.1) sabinene (4.2)	[20]
Djerissa, Kef region, located in the northwest of Tunisia (35°50′40″ N, 8°37′40″ E)	α-pinene (28.8) sabinene (18.67) β-pinene (6.2) cis-ocimene (7.85)	[21]

. As for any lignocellulosic material, the composition depends on the climatic conditions and location.

The shrub of D. tortuosa (Figure 1) is glabrous and has a strong perfume. It has an average height of 30–80 cm with striate stems and caducous leaves. The basal leaves are 2–3 cm long and have linear-subulate sharp lobes. The petiole is sheathed and has a wide scarious edge. The aerial sections of D. tortuosa are used for fuel and have several medicinal uses in addition to being edible [22]. The plant is also used as a medication for hypertension and to prevent conception, while antioxidant, allelopathic, and antifungal activity are reported [8,9].

This study represents a preliminary step towards the valorization of the residues of this significant natural resource. In order to examine the impact of gradually increasing reinforcement on the mechanical and thermal properties of the composite, low concentrations (10%, 20%, and 30%) were selected. In the literature, these values are frequently used to assess the reinforcement threshold [23,24,25]. Additionally, our initial findings show that these contents enable a good compromise between the material’s ductility and stiffness. Furthermore, the effect of fiber size (S1 = 2 mm and S2 = 500 μm) is investigated via morphological, mechanical, thermal, and chemical properties. S1 (2 mm) preserves enough length for load transfer while ensuring good fiber dispersion. In contrast, S2 (500 μm) was examined to investigate how a smaller size would affect the processability and reinforcing effect. These choices are in line with the recommendations of several studies on composites reinforced with natural fibers [26,27].

2. Materials and Methods

2.1. Materials

LDPE films were obtained from the computer department of Laval University (Canada). They were collected from electronics packaging (computers, tablets, printers, etc.). Since the materials were not contaminated, the films were directly extruded (Leistritz ZSE-27 twin-screw extruder with D = 27 mm and L/D = 40) to homogenize and obtain pellets. The temperature profile was 120 °C in the feeding zone, 135 °C in the compression zone, 140 °C in the metering zone, and 160 °C at the die. Other extrusion conditions include a screw rotational speed of 100 rpm and a die diameter of 2 mm. The pellets were then dried in an oven (overnight, 85 °C) to remove any moisture/volatiles. The melt flow index (MFI) was found to be 3.2 g/10 min (2.6 kg and 190 °C) with a density of 0.920 g/cm³. The pellets were then pulverized (Lab Millmodel PKA-18 (Powder King, Phoenix, AZ, USA) into a fine powder (less than 1 mm) to dry-blend with the fibers.

The D. tortuosa (aerial parts) were obtained in the winter of 2022 from the Bir Lahmar region in southern Tunisia, a natural ecosystem (33°10′45.4″ N 10°24′07.4″ E) of the Tataouine governorate (Tunisia). The plant’s aerial portion was taken out, cleaned, and dried for 9 days at 20 °C in the dark to preserve its mechanical and chemical properties. Following drying, the material was crushed using an industrial mechanical crusher, HM600P, to produce a homogeneous powder. The crusher’s electric power was 7.5 kW, and its mass flow rate ranges from 100 to 1000 kg/h. The ground material was then separated into two fractions (S1 = 2 mm and S2 = 500 μm) using a rotary screening machine from Hoskin Scientific Ltd., featuring fine mesh screens to determine the particle size and evaluate the effect of this parameter on the final properties. Several steps were used in this screening procedure to ensure accurate classification and reliable fiber separation.

2.2. Sample Preparation

The rLDPE powder was combined with a range of D. tortuosa fiber contents (10, 20, and 30% by weight) for both fiber sizes (S1 and S2). The materials were manually dry-blended in a plastic bag before being introduced in a Leistritz ZSE-27 twin-screw extruder (L/D = 40 with D = 27 mm), operated in a corotating mode with a screw speed of 100 rpm and a temperature profile of 120, 135, 140, 160, 160, 160, 160, 160, 160, and 160 °C (feed to die). The extrudate passed through a 2 mm diameter die before being cooled (water bath, room temperature) and being pelletized. Finally, the pellets were dried overnight in an oven at 100 °C to remove any moisture before being fed into an injection molding machine (PN60, NISSEI, Tokyo, Japan) with a temperature profile of 160, 192, 190, and 190 °C from the back to the nozzle. The mold temperature was constant (30 °C) to directly prepare samples for characterization. For comparison purposes, the neat rLDPE (matrix) was also produced under the same processing conditions.

2.3. Characterization

2.3.1. Morphology

Scanning electron microscopy (SEM) was used to determine the fiber morphology and to examine their distribution and dispersion in rLDPE. For the biocomposites, the cross-sections were exposed via cryogenic fractures in liquid nitrogen. A Zeiss Crossbeam 540 FIB/SEM (Carl Zeiss, Oberkochen, Germany) scanning electron microscope was used to examine the samples under various magnifications at 1.5 kV after being previously coated with gold.

2.3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Using an ABB (Zurich, Switzerland) Bomem FTLA 2000-102 spectrometer (ATR: Specac Golden Gate), the FTIR spectra of the fibers and the biocomposites were acquired. The spectra were obtained by accumulating 16 scans in the spectral range of 600–4000 cm⁻¹ with a resolution of 4 cm⁻¹, and the results were analyzed via OMNIC software.

2.3.3. Differential Scanning Calorimetry (DSC)

Using differential scanning calorimetry (DSC25, TA Instruments, New Castle, DE, USA), the heating and cooling curves were investigated. Data extraction and analysis were performed using TRIOS software. Aluminum pans were used with samples between 5 and 10 mg. The samples were heated in a nitrogen environment from 20 to 180 °C at 10 °C/min, then cooled back to 20 °C and heated up again to 180 °C. The endothermic and exothermic peaks were used to determine the melting temperature (Tm), crystallization temperature (Tc), and enthalpy of melting (ΔHm) of the neat rLDPE and rLDPE with different fiber contents (10, 20, and 30% wt.) for both fiber sizes S1 (2 mm) and S2 (500 μm). Furthermore, the crystallinity (Xc) was calculated as:

$$\mathrm{Xc} (\%)={\displaystyle \frac{\mathsf{\Delta }\mathrm{H}\mathrm{m}}{\mathsf{\Delta }\mathrm{H}\mathrm{m}0(1-\mathsf{\alpha })}}$$

(1)

where ΔHm0 is the melting enthalpy of fully crystalline LDPE (285 J/g) [28], and α is the fiber fraction in the biocomposite.

2.3.4. Hardness

According to ASTM D2240, model 306 L (Shore A) and 307 L (Shore D) hardness testers from PTC Instruments (Los Angeles, CA, USA) were used to determine the surface hardness. A total of 10 measurements were used for each specimen to report the average values with their standard deviations [29].

2.3.5. Tensile Properties

A 500 N loadcell on an Instron model 5565 (Instron, Norwood, MA, USA) apparatus was used to obtain the tensile properties. The tests were conducted using type IV dog bone samples (3.1 mm thickness) at room temperature with a rate of 10 mm/min (ASTM D638). The average and standard deviation of the tensile strength, Young’s modulus, and elongation at break were measured on five specimens [30].

2.3.6. Flexion Properties

Following ASTM D790, flexural characterization was carried out with a speed of 2 mm/min on an Instron model 5565 (Instron, Norwood, MA, USA). A 50 N loadcell was used and the tests were performed at room temperature. Rectangular bars measuring 125 × 12.7 × 3 mm³ were used with a 60 mm span. To determine the average and standard deviation, a minimum of five samples were examined [30].

2.3.7. Impact Strength

Charpy impact strength testing was conducted using a 242 g (1.22 J) pendulum weight on a Tinius Olsen (Horsham, PA, USA) model Impact 104 apparatus. With an arm length of 279 mm, the impact speed was 3.3 m/s. An automatic sample notcher (ASN 120 m, Dynisco, Franklin, MA, USA) was used to V-notch samples (125 × 12.7 × 3 mm³). The samples were left to relax for 24 h before measurement. The tests (room temperature) include a minimum of ten repetitions [30].

3. Results and Discussion

3.1. Morphology

Scanning electron microscopy (SEM) was used to examine the adhesion and dispersion states of the fibers inside the rLDPE matrix. Figure 2a shows the neat rLDPE matrix, where no defect is visible indicating that good molding conditions were selected. Figure 2b,c present typical micrographs for the biocomposites with 10% wt. of S1 (2 mm) and S2 (500 μm) at 500× magnification, respectively. Both biocomposites show some holes related to fiber pull-out. This is a typical behavior for composites due to the low adhesion, dispersion and limited wettability of natural fibers in the matrix, leading to fiber pull-out and mechanical removal under stress [31,32]. Figure 2d,e display the surface of the rLDPE composite containing 20% wt. of S1 (2 mm) and S2 (500 μm) at a higher magnification (800×), respectively. It can be observed that smaller fiber sizes leads to a higher surface area, producing more interfacial physical interaction/contact. This improvement has a direct effect on the biocomposites’ macroscopic properties. Additionally, Figure 2f,g show the biocomposites with 30% wt. of S1 (2 mm) and S2 (500 μm) at a lower magnification (300×), respectively. In this case, good fibre distribution is observed indicating that the processing conditions were well selected to produce materials having consistent quality and properties [33].

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 3 presents an overview of the FTIR spectra for the biocomposites compared to the neat matrix (rLDPE). The characteristic peaks of rLDPE are easily distinguished from the five main signals (2918, 2851, 1468, 1373, and 718 cm−¹) [34]. Figure 3a shows the symmetric and asymmetric C-H stretching of the methylene groups (CH₂) at 2848 and 2915 cm⁻¹, respectively [35]. The peaks in Figure 3b at 1476, 1373, and 718 cm−¹ are bending deformation, while a symmetric deformation vibration of CH₃ is responsible for the band and rocking deformation [35,36], respectively. All the samples of neat rLDPE and rLDPE with different fiber contents and sizes, S1 (2 mm) and S2 (500 μm), present the same peaks, but in Figure 3b a new peak at 1051 cm−¹ is detected, which is the matrix reinforced with D. tortuosa. This signal corresponds to C-O stretching or an alkyl-substituted ether [35]. The crystallinity of rLDPE may change as a result of fiber inclusion (heterogeneous nucleation effect). Peaks associated with the crystalline regions can have higher intensity when the crystallinity level increases, as reported next.

3.3. Differential Scanning Calorimetry (DSC)

The DSC results (heating and cooling) are reported in Figure 4, while

Table 2. Melting and crystallization temperatures, enthalpy of melting, and degree of crystallinity of the neat rLDPE and rLDPE with different fiber contents (10%, 20%, and 30% wt.) and sizes S1 (2 mm) and S2 (500 μm).

Samples	Tm (°C)	Tc (°C)	ΔHm (J/g)	Xc (%)
rLDPE	122.7	110.0	99.6	34.9
S1 10%	122.7	109.9	89.8	35.0
S1 20%	125.9	108.3	78.6	34.5
S1 30%	125.6	108.3	58.9	29.5
S2 10%	123.6	109.7	93.3	36.3
S2 20%	122.2	109.9	86.5	37.9
S2 30%	123.6	108.9	76.7	38.4

presents the parameters of interest. All the biocomposites have a complex thermal behavior related to their composition, as evidenced by two melting and three crystallization peaks, as illustrated in Figure 4a,b, respectively. This behavior is associated with heterogeneity, as well as different crystalline phases, but the heating and cooling curves can be modified by adding a fiber [37]. Table 2 shows that fiber addition leads to a higher degree of crystallinity. For example, the degree of crystallinity of rLDPE alone is 34.9%, which slightly increases with 10% wt. of fiber: 35.0% for S1 (2 mm) and 36.3% for S2 (500 μm). This shows that the heterogenous nucleation effect depends on their sizes (contact area and polymer mobility restriction). However, at a higher fiber content (30% wt.), S1 leads to lower crystallinity (29.5%) compared to S2 (38.4%). For S1, this can be explained by a dilution effect caused by the fibers decreasing the amount of polymeric chains that might otherwise undergo thermodynamic changes during a melting process [38]. Additionally, there is a spatial restriction limiting the polymer chains’ mobility during the crystallization step, disrupting their rearrangement and preventing them from crystallizing. For S2, the increased degree of crystallinity is explained by more surface area created by smaller fibers acting as nucleating agents [39].

Table 3. Crystallization temperatures for the neat rLDPE and rLDPE with different fiber contents (10%, 20%, and 30% wt.) and sizes S1 (2 mm) and S2 (500 μm).

Samples	Crystallization Temperature (°C)
	Peak 1	Peak 2	Peak 3
rLDPE	58.6	94.3	110.1
S1 10%	58.2	93.8	110.0
S1 20%	57.1	92.2	108.8
S1 30%	57.7	91.5	108.7
S2 10%	58.2	94.2	109.9
S2 20%	58.0	94.4	109.9
S2 30%	57.9	93.6	109.1

reports on the three crystallization peaks observed in Figure 4b. The crystallization temperatures of neat rLDPE are 58.6, 94.3, and 110.1 °C, but the crystallization temperature continuously dropped with increasing fiber addition. For S1 (2 mm), the temperatures for peak 1 are 58.2 °C at 10% wt. and 57.7 °C at 30% wt. Similarly, for peak 2, the temperatures are 93.8 °C and 91.5 °C, while the temperatures for peak 3 are 110.0 °C and 108.7 °C, respectively. The same trend is observed for S2 (500 μm), but the crystallization temperature for S1 (2 mm) decreases more than for S2 (500 μm).

3.4. Hardness

Figure 5 reports the hardness for all the samples. The initial values (neat rLDPE) are 95 Shore A and 52 Shore D. However, increasing the fiber concentration led to higher values. For instance, the Shore A hardness increased by 3 points for S1 (30% wt.) and S2 (30% wt.), but the Shore D hardness increased even more: 15 points for S1 (30% wt.) and 19 points for S2 (30% wt.). Higher hardness is attributed to fiber reinforcement and higher crystallinity of the polymer matrix (Figure 4). The literature reports that more crystalline phases contribute to harder and more resilient physical properties and that fiber reinforcement enhances hardness and decreases the strain at break [40,41,42]. Finally, the fibre size (S1 vs. S2) does not seem to have an effect on hardness, except maybe at a higher content (30% wt.), where some agglomeration may occur.

3.5. Tensile Properties

The tensile modulus is presented in Figure 6a. The tensile modulus of rLDPE is 96 MPa for comparison. It can be seen that S1 (2 mm) has a negligible effect on the tensile modulus (within experimental error) for the range investigated. On the other hand, S2 (500 μm) presents a continuous modulus increase up to 104% at 30% wt. Adding a rigid phase (fibers) to the matrix is known to improve the modulus [35], but the fibre size, distribution, and orientation also have an effect [43,44,45].

Figure 6b reports the tensile strength where the initial value for neat rLDPE is 13 MPa. When the fibers are added, the values decrease by 15% for S1 (2 mm) and by 46% for S2 (500 μm). Decreasing values are the result of limited fiber adhesion with the matrix (Figure 3), as well as more defects (void, agglomeration, etc.) created with increasing fiber content [46]. Again, a significant difference is observed between both fibers as longer fibers (S1) perform better than shorter ones (S2) due to their higher aspect ratio.

Figure 6c shows the elongation at break. As expected, the neat rLDPE has the highest value (300%), which substantially decreases with increasing fiber content: down to 83% for S1 (2 mm) and 78% for S2 (500 μm). This trend is related to the presence of rigid fibres limiting the polymer chains’ mobility and capacity to deform/reorganize under external load. Furthermore, the polymer becomes more susceptible to failure under deformation due to the presence of another phase (more defects). Lower mechanical properties can also be associated with fiber agglomeration, limiting interactions with the LDPE matrix, i.e., lower interfacial adhesion [46,47]. In this case, smaller particles (S2) perform better than larger ones (S1) since there are fewer polymer molecular restrictions; i.e., it is easier for the polymer macromolecules to accommodate themselves around/in between smaller restrictions (rigid particles).

Figure 6d presents typical tensile stress–strain curves to compare the materials. It can be seen that S1 (2 mm) has different curves than S2 (500 μm), leading to the differences reported in Figure 6a–c, but these differences are also controlled by the crystallinity (Table 2), as the amorphous (more elastic) and crystalline (more rigid) regions, as well as their interface, behave differently under mechanical solicitations. There are also different fiber–matrix interactions because of the different interfacial contact areas.

3.6. Flexural Properties

Figure 7a indicates that the rLDPE flexural modulus is 144 MPa, while reinforcing the polymer with fibers resulted in an increase of up to 47% for S1 (2 mm) and 61% for S2 (500 μm). Figure 7b shows that D. tortuosa addition to rLDPE increased the flexural strength (26 MPa), with the larger size S1 (2 mm) by 31% being less effective than the smaller size of S2 (500 μm) by 65%. This difference is related to different fiber agglomeration and matrix interaction, leading to different interfacial stress transfer [48], but to a different extent compared to the tensile strength (Figure 6b). This can be related to a different fibre orientation and defect depending on the applied load (tension vs. flexion), but this would need more investigations to completely understand the structure–properties relationships for these biocomposites.

3.7. Impact Strength

Figure 8 reports the impact strength of the samples with and without fiber. The impact strength of rLDPE is 380 J/m, but negligible differences are observed between the compositions. The hydrophilic nature of natural fibers is the origin of poor adhesion with the matrix, reducing the impact strength, as seen by fiber pull-outs, agglomerations, detachments, and fractures, observed via SEM (Figure 2) [49]. Thus, the total energy to completely break the samples represents a balance between both effects, and similar results were reported for typical biocomposites [50].

4. Conclusions

This study evaluated the benefits of using D. tortuosa fibers to reinforce recycled low density polyethylene (rLDPE) coming from electronics packaging. Especially, the effect of fiber contents (10, 20, and 30% wt.) and sizes (S1 = 2 mm and S2 = 500 μm) was investigated. The samples were successfully prepared via extrusion compounding and injection molding.

Based on the samples produced, different characterizations were performed (SEM, DSC, and FTIR) to get some basic properties. Nevertheless, a focus on mechanical properties (flexion, tension, and impact) was conducted. The data obtained showed that improvements were observed, including tensile modulus increases by 23% for S1 (2 mm) and 104% for S2 (500 μm) and flexural modulus increases of 47% for S1 (2 mm) and 61% for S2 (500 μm). Nevertheless, the tensile strength was found to decrease, while the flexural strength increased, indicating different mechanical behavior depending on the type/direction of the stress/deformation applied.

From the results obtained, biocomposites based on D. tortuosa/rLDPE can be easily produced and can find application in several areas, including construction, material handling, packaging, and transport, but more importantly, these materials provide a way to support the circular economy via the creation of new applications reusing/recycling/valorizing natural resources such as D. tortuosa fibers and LDPE flexible packaging.

However, fiber variability, challenges associated with interfacial adhesion, and the high cost of the required treatments limit the practical use of these composites. Furthermore, additional research is needed to determine the complete environmental impact of plant fibers, including their capacity for being recycled and biodegradation, even if they are more ecologically friendly than most synthetic reinforcements.

Finally, it is proposed that future research investigates how to improve the surface state of the fibers and optimize the manufacturing conditions, as well as other processes including compression or injection molding to enable their large-scale production. It would be of interest to carry out a full life cycle analysis to completely understand the environmental impact and economic feasibility of these composites.