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. 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:
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 mm
3 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 mm
3). The samples were left to relax for 24 h before measurement. The tests (room temperature) include a minimum of ten repetitions [
30].
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.
