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Article

Biodegradable Polymer Composites Based on Polypropylene and Hybrid Fillers for Applications in the Automotive Industry

by
Alina Ruxandra Caramitu
1,
Magdalena Valentina Lungu
1,*,
Romeo Cristian Ciobanu
2,
Ioana Ion
1,
Delia Pătroi
1,
Beatrice Gabriela Sbârcea
1,
Virgil Emanuel Marinescu
1 and
Doina Constantinescu
3
1
National Institute for Research and Development in Electrical Engineering (ICPE-CA), 030138 Bucharest, Romania
2
Department of Electrical Measurements and Materials, Gheorghe Asachi Technical University, 700050 Iasi, Romania
3
MONOFIL SRL, 617410 Săvineşti, Piatra Neamț, Romania
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1078; https://doi.org/10.3390/pr13041078
Submission received: 7 March 2025 / Revised: 29 March 2025 / Accepted: 31 March 2025 / Published: 3 April 2025
(This article belongs to the Special Issue Development and Characterization of Advanced Polymer Nanocomposites)
Browse Figures
Versions Notes

Abstract

:
This study focuses on the development and characterization of biodegradable polymer composites consisting of a polypropylene (PP) matrix, carbon black pigment, and hybrid fillers. The fillers incorporated into these composites consisted of a blend of fibers and particles derived from natural, biodegradable materials, such as flax fibers (FFs) and wood flour (WF) particles. The compositions of polymer material were expressed as PP/FF/WF weight ratios of 100/0/0, 70/5/25, and 70/10/20. The polymer materials were prepared using conventional plastic processing methods like extrusion to produce composite mixtures, followed by melt injection to manufacture the samples needed for characterization. The structural characterization of the polymer materials was conducted using optical microscopy and X-ray diffraction (XRD) analyses, while thermal, mechanical, and dielectric properties were also evaluated. Additionally, their biodegradation behavior under mold exposure was assessed over six months. The results were analyzed comparatively, and the optimal composition was identified as the polymer composite containing the highest flax fiber content, namely PP + 10 wt.% flax fiber + 20 wt.% wood flour.

1. Introduction

A new approach to polymer composites involves natural hybrid fillers, composed of at least two types of fillers in particle or fiber form, or a combination of both. Hybrid composites exhibit superior physical, mechanical, and thermal properties compared to single-filler composites due to the synergy between different fillers. They also provide economic benefits, as some inorganic fillers can be costly [1,2,3,4,5,6]. Natural-filler-reinforced composites have potential applications in furniture, automotive components, construction, and other industries [1,7,8,9] due to their recoverability and biodegradability.
Hybrid polymer composites with natural fiber fillers have gained significant attention as a more sustainable alternative to polymer composites with single fillers [6,10]. However, a key challenge in manufacturing these composites is the incompatibility between hydrophilic natural fibers (due to the hydroxyl (-OH) groups in cellulose) and the hydrophobic polymer matrix [5,11,12], leading to weak interfacial adhesion and reduced mechanical strength, which is a major disadvantage of these composites [13]. To address this incompatibility, chemical, physical, biological, and nanotechnology-based fiber treatment methods have been explored to enhance filler adhesion to the polymer matrix. Various researchers have studied filler adhesion by treating fiber-reinforced polymer composites using alkali treatments [14], or other advanced methods [15,16].
The compatibilization process involves introducing a third component, known as a coupling agent, to improve interfacial adhesion and compatibility between the fillers and the polymer matrix. Commonly used coupling agents include silanes, isocyanates, anhydrides, and anhydride-modified copolymers [17,18]. These agents are applied to the surface of fillers, polymer matrices, or both, using various techniques such as soaking, blending, compounding, or spraying [17]. Compatibilization is typically needed in polymer composites, as natural and synthetic polymers often exhibit poor interfacial bonding.
Polymer composites have advanced applications across various industries, particularly in the automotive sector, where strong, lightweight materials are essential. These materials are commonly used in both interior and exterior automotive components [19]. In contrast, metal matrix composites are selected for specific components, where their superior mechanical properties justify the higher material and processing costs [20].
In this study, unfunctionalized polypropylene/flax fiber/wood flour (PP/FF/WF) polymer materials were produced with weight ratios of 100/0/0, 70/5/25, and 70/10/20. The developed polymer materials were characterized using optical microscopy, XRD, thermal, mechanical, dielectric, and biodegradability tests to determine the optimal formulation for applications in the automotive industry.
Polypropylene (PP) was chosen as the polymer matrix for PP/FF/WF composites due to its extensive industrial significance in the automotive sector. PP is one of the most commonly used polymers in vehicle manufacturing due to its superior chemical resistance, low density, good mechanical properties, thermal stability, and cost-effectiveness [21]. Furthermore, PP can incorporate natural fillers without substantially increasing the weight of the composites or final automotive parts, promoting sustainability by reducing overall polymer content and dependence on synthetic materials.
Flax, a flowering plant from the Linaceae family, is highly versatile and requires minimal care. It is cultivated in temperate climates, reaching maturity in about 100 days [22]. Widely used in food, pharmaceuticals, textiles, and biodegradable composites [23], flax fibers have been adopted by the automotive industry for their strength and low density, reducing fuel consumption. Flax fiber-reinforced PP materials are 40–60% less dense than glass-fiber-reinforced PP composites [12]. Another major advantage of incorporating natural fibers into polymer composites for the automotive industry is that, upon impact, they break rather than splinter, unlike glass-fiber-reinforced polymer composites [22,24,25,26].
Wood flour is a natural filler widely used in the production of eco-friendly, polypropylene-based composites [11,27,28]. However, there are limited studies in the literature on the use of hybrid fillers composed of flax fibers and wood flour in polymer composites with polypropylene or polyethylene as the polymer matrix [29,30,31].
Recent research in the field [32] has led to the development of flax and ramie fiber-reinforced chemically functionalized polypropylene (CF-PP) through extrusion and injection molding using commercial raw materials. This includes PP functionalized by grafting with 1.5% maleic anhydride (MA), as well as untreated short fibers, with an average length of 5 mm and an average diameter of about 16.15 μm for flax fibers and 120 μm for ramie fibers. Unlike these polymer composites, which were reinforced with 10 wt.%, 20 wt.%, or 30 wt.% of a single flax or ramie fiber filler [32], the development of PP/FF/WF polymer materials involved unfunctionalized PP, a low amount of carbon black pigment, and 30 wt.% natural hybrid fillers (flax fiber and wood flour) with distinct characteristics. This combination increased processing difficulty due to having higher melt viscosity.
Miled et al. [33] developed composite materials with a polypropylene matrix and natural fiber concentrations of 10 wt.%, 20 wt.%, and 30 wt.%. Flax and hemp fibers, ranging in length from 0.4 mm to 1 mm, were processed via extrusion. Mechanical characterization showed that hybrid composites degraded more than non-hybrid ones due to weaker fiber–matrix bonds. Scanning electron microscopy (SEM) analysis revealed crack progression and micro-voids around fiber tips in the PP matrix.
Similar to the studies by Miled et al. [33] and Pant et al. [32], the development of PP/FF/WF polymer materials did exceed 30 wt.% natural filler content to avoid processing difficulties and potential material weaknesses.
Following structural, thermal, mechanical, and dielectric characterizations, along with assessing the biodegradation behavior of the polymer materials under mold exposure over six months, the optimal polymer composite was selected as the one with the highest flax fiber content, specifically PP + 10 wt.% flax fiber + 20 wt.% wood flour.
The M3 composite presents several advantages over conventional thermoplastics and metals used in automotive components. It features improved lightweight properties, high thermal stability, enhanced hardness, good resistance to surface charge accumulation, and biodegradability, making it a more environmentally friendly alternative to traditional materials like pure polypropylene or metals. Unlike metals, polymer composites are corrosion-resistant, and the M3 composite provides competitive mechanical properties for applications with lower strength requirements. It is also a cost-effective, sustainable alternative to traditional materials in automotive applications.

2. Materials and Methods

2.1. Materials

The raw materials used for the production of polymer materials were as follows:
  • Polypropylene homopolymer PP J700 TEHNOLEN acquired by MONOFIL SRL, Săvineşti, Piatra Neamț, Romania from ROMPETROL RAFINARIE SA, Năvodari, Constanța, Romania. The characteristics of the PP used are presented in Table 1;
  • Wood powder, obtained from industrial waste, specifically beech, poplar, and pine wood residues. These waste materials, used in a weight ratio of 1:1:1, were dried at 40 °C, ground, and sieved to achieve a particle size of up to 200 μm;
  • Short (chopped) flax fibers (MONOFIL SRL, Săvineşti, Piatra Neamț, Romania) with an outer diameter of about 10–30 μm and an average length of 2–4 mm, a density of 1.4 ± 0.1 g/cm3, a modulus of elasticity of 60 ± 3 GPa, a tensile strength of 1.2 ± 0.2 GPa, and an elongation at fracture of 2.5 ± 0.5%;
  • Carbon black (CB) powder of Fast Extruding Furnace (FEF) type, with an average specific surface area of 45 m2/g (SC Arpechim SA, Piteşti-, Argeș, Romania).

2.2. Methods and Equipment

2.2.1. Obtaining Polymer Composite Materials

The polymer composite materials were obtained in two stages: first, composite pellets were produced by extrusion using a twin screw extruder of EC52 type (Useon Technology Ltd., Jurong, China) with an L/D ratio of 40, a screw diameter of 51.4 mm, a maximum screw speed of 600 rpm, and nine heating zones. The processing temperatures on the extruder ranged between 220 °C and 240 °C. After extrusion, the composites were cooled in a water bath to form the composite strand, which was then pelletized into cylindrical form with a diameter × height of 3 mm × 3 mm, and dried for 2–4 h at 80 °C in a Memmert UNE 500 oven (Schwabach, Germany) with air circulation. For obtaining the required test specimens, an injection molding machine of MI TP 100/50 type (SC IMATEX, Târgu Mureş, Romania) was used, with a screw diameter of 35 mm, a melting capacity of 96 cm3, and four heating zones (three on the barrel and one on the injection nozzle). The processing temperatures on the injection molding machine ranged between 210 °C and 230 °C, with a mold temperature of 60 °C and an injection pressure of 110 MPa.
The materials studied in this paper are presented in Table 2. The PP matrix of the M2 and M3 polymer composites contains 2 wt.% CB pigment, relative to pure PP, resulting in 1.4 wt.% CB in the PP/FF/WF composites.

2.2.2. Characterization Methods

  • − Optical Microscopy
The structural aspects of the polymer materials in cross-sectional cuts were highlighted using images obtained with a Carl Zeiss NU 2 optical microscope (Jena, Germany) using the AxioVision v.4.4 imaging software (Zeiss, Jena, Germany) at a magnification of 200× for the PP (M1) and 500× for the M2 and M3 composites.
  • − X-Ray Diffraction (XRD) Analysis
XRD analysis was conducted using a Bruker AXS D8 ADVANCE X-ray diffractometer (Bruker, Karlsruhe, Germany), featuring a copper (Cu) anode X-ray tube with Cu Kα radiation (wavelength of 1.5406 Å), an applied voltage/current intensity of 40 kV/40 mA, a nickel (Ni) Kβ filter, a step size of 0.04°, a measurement time per point of 2 s, and a 2θ measurement range of 2–60°. DIFFRAC.EVA v.6.0.0.7 software (Bruker, Karlsruhe, Germany) was used for diffraction data analysis, and the ICDD PDF 2, v.2.2021 database (Newtown Square, PA, USA) was employed for phase identification.
  • − Thermal Analysis
Thermal analysis with coupled techniques (TG-DTA-DSC) was performed using a Netzsch STA 409 PC Luxx simultaneous thermal analyzer (Gerätebau GmbH, Selb, Germany). The recording conditions were as follows: temperature range of 25 °C → 600 °C, static air atmosphere, heating rate of 10 K/min, and Pt-Rh crucible. Proteus v.4.8.1 software (Netzsch-Gerätebau GmbH, Selb, Germany) was used for thermal data analysis.
  • − Density
The density was determined using Archimedes’ principle with a Mettler Toledo XS204 analytical balance (Greifensee, Switzerland) and a density kit, according to standard SR EN ISO 1183-1:2019 [35]. The measurements were performed in triplicate, in water, at 22 °C, and the arithmetic mean and standard deviation values were reported.
  • − Mechanical Tests
Vickers microhardness determination was performed using a microhardness tester, FM 700 Ahotec, Germany, with a Vickers diamond indenter. All the indentations were performed with a force of 0.05 kgf and dwell time of 10 s. Ten measurements were taken for each sample, according to standard SR EN ISO 6507-1:2023 [41]. The minimum and maximum hardness values were removed, and from the remaining eight measurements, the arithmetic mean and standard deviation values were reported.
Determination of the flexural strength by three-point bending was conducted on a Zwick Roell TR FR 005 TN (Ulm, Germany) mechanical testing machine with testXpert II v.3.7 software (Zwick Roell, Ulm, Germany), with three specimens of each type of polymer material (PP and composite), using a nominal force of 5 kN and a crosshead speed of 1 mm/min in accordance with the SR EN ISO 178:2019 standard [40].
  • − Dielectric Tests
The dielectric characteristics analyzed were volume resistivity (ρv) and surface resistivity (ρs). The tests were conducted on five disc-shaped samples of each type, in accordance with standards IEC 62631-3-1:2023 [42] and IEC 62631-3-2:2023 [43], using a Keithley 6517A electrometer (Cleveland, OH, USA), an A8009 measurement cell, a circular electrode system with a guard electrode setup. The test voltage was 100 V, and the measurement time was 1 min. The conditioning of the samples before testing was performed for 24 h at 90 °C using a Memmert UNE 500 oven (Schwabach, Germany). Before the determination, the samples were short-circuited for 2 min. The environmental conditions during testing were as follows: temperature of 22 ± 3 °C and relative humidity of 50 ± 15%. A Caloris Monit-T/UR digital device (Bucharest, Romania) was used for measuring temperature and humidity.
  • − Deterioration Tests Due to the Action of Fungi
Determining the deterioration of polymer materials due to the action of fungi was performed on square-shaped samples with sides of about 6–8 mm and thicknesses of 0.75 mm. The samples studied were placed in an incubator for a duration of 6 months, within a temperature range of 28–32 °C and a humidity level of 90–95% [44]. Method B was chosen for testing according to standard SR EN ISO 846:2019 [45]. The test medium was Czapek–Dox medium with glucose as the carbon source. The tested fungal strains were Aspergillus niger, Aspergillus flavus, Aspergillus terreus, Aspergillus amstelodami, Penicillium ochro-chloron, Penicillium funiculosum, Paecilomyces variotii, Chaetomium globosum, Myrothecium verrucaria, Scopulariopsis brevicaulis, Stachybotrys atra, and Trichoderma viride.
Images of the polymer samples after biodegradation were captured using a 4× objective on a Nikon SMZ 1000 stereomicroscope with a magnification of 50×, along with a Nikon D60x digital camera (included) with a 60× macro lens (Nikon, Tokyo, Japan).

3. Results and Discussion

3.1. Optical Microscopy

The optical microscope images showing the cross-sections of the PP/FF/WF polymer materials studied are presented in Figure 1.
In Figure 1a, several surface scratches are observed on the pure PP polymer (M1) due to the prior polishing of the analyzed sample. The M1 material appears homogeneous and contains no fillers, as it consists only of pure PP polymer.
In Figure 1b,c, natural hybrid fillers are seen embedded within the PP structure of the M2 and M3 polymer composites, exhibiting an orange-peel-like appearance. This texture is attributed to the irregular shape of the fillers. Additionally, polyhedron-shaped flax fibers are noted, as reported in the literature [12,24]. Elemental flax fibers are characterized by a polygonal cross-section with smooth-edged corners and stratified walls enclosing a central hollow space known as the lumen [12].
In Figure 1b,c, dark-colored spots are observed, representing a mixture of natural flax fibers, wood flour, and carbon black. Additionally, the overall appearance of the composites was influenced by the small amount of carbon black pigment, which gave a black coloration to both composites. Moreover, the overall appearance of the composites was influenced by the addition of the low amount of carbon black pigment, which colored both composites in black. Carbon black also reduces electrostatic charging due to its conductive properties and may act as an ultraviolet (UV) stabilizer, preventing polymer degradation under UV and visible light exposure [46]. Its even dispersion throughout the composites resulted in more stable and uniform materials.

3.2. XRD Analysis

The diffraction patterns resulting from the X-ray diffraction analysis of the PP/FF/WF polymer materials studied are presented in Figure 2.
The XRD results revealed that all the polymer materials contain crystalline phases characteristic of PP, including gamma-isotactic PP (iPP, PDF reference card no. 00-0451807) and syndiotatic PP (sPP, PDF reference card no. 00-049-2204). All methyl groups are arranged on the same side of the iPP polymer backbone, while the methyl groups alternate sides along the sPP polymer chain [47]. Due to the microstructural arrangement influenced by the position of the methyl groups along the PP polymer backbone, iPP exhibits a higher degree of crystallinity, greater mechanical strength and stiffness, and a higher melting point compared to sPP [47]. The presence of both γ-iPP and sPP crystalline phases suggests that the developed polymer materials exhibit a combination of properties derived from both isotactic and syndiotactic arrangements of methyl groups, which can enhance the mechanical and thermal properties of the PP/FF/WF materials.
The diffractograms for the M2 and M3 polymer composites displayed several differences in patterns compared to the PP polymer (M1). The highest peak intensities were exhibited in the diffraction patterns of the PP polymer (M1), while lower peak intensities were displayed in those of the M2 and M3 polymer composites due to the texturing effect introduced by the hybrid fillers (wood flour and flax fiber). This suggests a lower crystalline nature in both polymer composites upon the incorporation of hybrid fillers compared to the pure PP polymer (M1). Furthermore, the highest proportion of ordered (crystalline) phases (55.3%) was found in the PP polymer (M1), while 45.3% was found in the M2 composite, and 45.9% in the M3 composite. In contrast, the content of amorphous phases was 44.7% in the PP polymer (M1), 54.7% in the M2 composite, and 54.1% in the M3 composite.
All the PP/FF/WF polymer materials crystallize in an orthorhombic system, with crystallite sizes of 20.8 nm (M1), 19.2 nm (M2), and 23.2 nm (M3). The addition of natural hybrid fillers did not alter the structural integrity of the PP polymer matrix.
Crystallinity influences the mechanical and thermal properties of polymers. Higher crystallinity typically enhances strength, stiffness, wear resistance, and thermal stability by creating an orderly molecular structure that improves load transfer. Crystalline regions require more energy to break down, increasing the melting temperature (Tm) [48].
In this study, it was found that the PP polymer (M1) with higher crystallinity demonstrated superior mechanical properties compared to the M2 and M3 composites, as shown in Section 3.5. Similarly, thermal stability improved with greater crystallinity, as evidenced by the better heat resistance in more crystalline structures, as confirmed in Section 3.3.

3.3. Thermal Analysis

The temperature-dependent mass change (TG), the corresponding mass change rate (DTG), and the heat flow rate (DSC) curves obtained from the thermal analysis of the PP/FF/WF polymer materials in static air are presented in Figure 3.
In Figure 3, it can be observed that the PP/FF/WF polymer materials present similar thermograms. Upon inspecting these thermograms, it is noted that during the progressive heating of a polymer material, the following processes occur:
  • Process I—Water loss occurs in the M2 and M3 polymer composite materials.
  • Process II—Melting (Tmin DSC) occurs in all the analyzed polymer materials. It is noted that the M2 and M3 composites have a melting point close to that of pure PP (M1). The melting point increased in the following order: M2 (162.7 °C) < M3 (164.4 °C) < M1 (168.3 °C). The slight differences in these melting points are attributed to the hybrid fillers used and the small amount of carbon black (CB) pigment, which lowered the melting points of the PP/FF/WF composites, as also noted by Povacz et al. for CB-pigmented PP materials [49].
  • Process III—Thermal oxidation with the formation of solid products. During thermal oxidation, polymer materials react with oxygen, leading to the formation of hydroperoxides (-OOH) as primary degradation products [50]. The initial temperature of the first oxidation process (TIN) with the formation of solid hydroperoxides indicates the stability of the materials to oxidation. The stability to thermal oxidation increases as the initial temperature (TIN) of the process rises with the formation of solid hydroperoxides. The thermal stability increased with higher TIN in the following order: M1 (215.0 °C) < M3 (234.0 °C) < M2 (234.7 °C). However, it remained similar for both polymer composites.
  • Process IV—Thermal oxidation with decomposition driven by radicals and volatile oxidation occurs in all the analyzed polymer materials above the temperature of 434 °C. This process is detected as exothermic peaks due to combustion-like reactions [50,51]. The presence of natural hybrid fillers and carbon black pigment did not significantly increase the thermal decomposition temperature of the PP component in either polymer composite.
Table 3 presents a comparative summary of some information obtained from the thermal analysis (TG, DTG, and DSC) of these polymer materials.
The DSC findings indicated that the thermal behavior of the M2 and M3 composites with hybrid fillers was slightly altered compared to that of the pure PP polymer (M1). The melting points of M2 (162.7 °C) and M3 (164.4 °C) were slightly lower than M1 (168.3 °C) due to the presence of hybrid fillers, which also influence crystallinity. The hybrid fillers possibly acted as nucleating agents, either promoting or hindering the formation of crystalline regions, depending on their interaction with the PP matrix, leading to a disrupted crystalline structure, lower melting points, and reduced thermal resistance.
Thermal stability, based on the initial oxidation temperature (TIN), increased in the following order: M1 (215.0 °C) < M3 (234.0 °C) < M2 (234.7 °C). The slightly higher thermal stability of the M2 composite compared to the M3 composite may be due to differences in filler content. The M2 composite contains 5 wt.% flax fiber and 25 wt.% wood flour, while the M3 composite has 10 wt.% flax fiber and 20 wt.% wood flour. Cellulose, a hydrophilic polysaccharide, contributes to the polarity of natural fibers. Flax fibers, with higher cellulose content (65–75%) than wood fibers (40–50%) [52,53], may weaken filler–matrix bonding in the M3 composite due to their greater polarity, reducing thermal stability.
Conversely, the higher wood flour content (25 wt.%) and lower flax fiber content (5 wt.%) in the M2 composite may have improved compatibility with the PP matrix, enhancing thermal stability. Filler–matrix interactions and dispersion are key factors in thermal stability, as better filler dispersion has been shown to increase the onset decomposition temperature [54]. Additionally, the presence of a small amount of carbon black pigment in PP polymer composites has been reported to enhance long-term heat stability [49].

3.4. Density

The density of the PP/FF/WF polymer materials, determined using Archimedes’ method with water as the liquid medium, is summarized in Table 4.
Density measurement is essential for polymer composites containing lignocellulosic flax fiber and wood flour due to the hydrophilic nature of the natural fillers [10,11].
The PP polymer (M1) has a density of 0.879 ± 0.040 g/cm3 (Table 4), which is close to its theoretical density (0.905 g/cm3), due to the hydrophobic nature of PP [11]. Polyolefins, such as polypropylene, typically exhibit subunitary densities, and reinforcing them with lignocellulosic materials generally increases their density, as these fillers have higher densities than polyolefins [11]. Additionally, lignocellulosic fillers, such as flax fiber and wood flour, tend to absorb moisture and swell when exposed to humid environments, further contributing to density increases due to their hydrophilic nature [5,6].
The long-term water absorption behavior of natural-filler-reinforced polymer composites is influenced by the type and characteristics of the natural fillers and polymer matrix, the processing system, filler distribution, and the quality of the filler–polymer matrix interface [55,56]. Dimensional changes in these polymer composites caused by water absorption are the primary reason for the reduction in mechanical properties [55].
In this study, incorporating 5 wt.% flax fiber and 25 wt.% wood flour fillers into the PP polymer matrix (M2 composite) resulted in a 7.5% increase in density. However, when using 10 wt.% flax fiber and 20 wt.% wood flour (M3 composite), the density increased by only 3.0%. This suggests that while the presence of natural hybrid fillers raises the density of PP/FF/WF polymer composites, their specific composition plays a key role. By comparing the densities of the M2 and M3 composites, a 4.1% decrease in density is observed for the M3 composite. This indicates that reducing the wood flour content while increasing the flax fiber proportion leads to lower density. Consequently, to develop PP/FF/WF polymer composites with reduced water absorption, a formulation with a higher flax fiber content and a lower wood flour proportion is more effective.
Water uptake (WU) behavior and its effect on the mechanical properties and aging of natural-filler-reinforced composite polymers are commonly reported in the literature [57,58]. However, artificial neural network (ANN) and response surface methodology (RSM) prediction methods can be used to model the WU behavior observed in experiments and optimize the immersion period and fiber content in polymer composites [57].
The increase in density observed in the M2 composite (0.945 ± 0.046 g/cm3) and M3 composite (0.906 ± 0.050 g/cm3) compared to pure PP (M1), with a density of 0.879 ± 0.040 g/cm3, is due to the incorporation of natural hybrid fillers. The slight increase in density improved the Vickers hardness of the M3 composite. However, it may be a concern for weight-sensitive automotive components, potentially affecting fuel efficiency and performance. Anyway, for non-structural parts like interior panels, trim, or dashboards, where durability and thermal stability matter more than minimal weight, these polymer composites remain viable [59]. Optimizing filler content or using compatibilizers may balance mechanical performance and density-related concerns.

3.5. Mechanical Tests

The mechanical properties (Vickers microhardness and flexural strength) of the PP/FF/WF polymer materials are presented in Table 5.
From the perspective of Vickers microhardness (Table 5), it is observed that a decrease in microhardness by about 6.2% results from the addition of 5 wt.% flax fiber + 25 wt.% wood flour to the polypropylene matrix in the M2 composite. However, an increase in Vickers hardness by about 16.7% is achieved when 10 wt.% flax fiber + 20 wt.% wood flour is incorporated in the M3 composite.
The variation in flexural strength relative to the PP polymer matrix is highlighted in Table 5. A decrease in flexural strength (Rm) by about 30.5% is observed when 5 wt.% flax fiber + 25 wt.% wood flour is added to the M2 composite, while an increase of about 14.1% is noticed in the M3 composite compared to the M2 composite. Nevertheless, a decrease of 20.7% is noted in comparison to the pure PP polymer (M1).
It is observed that replacing 5 wt.% of the wood flour with flax fiber in the M3 composite causes an increase in both Vickers hardness and flexural strength. The presence of 30 wt.% hybrid fillers in both composites reduced flexural strength, likely due to the incompatibility between the nonpolar PP matrix and the polar wood flour and flax fibers [28]. Poor interfacial adhesion, a common challenge in natural-fiber-reinforced polymer composites, has also been reported to reduce the mechanical properties of pure PP [10,28,60].
In this study, an improvement in both hardness and flexural strength was exhibited by the M3 composite compared to the M2 composite, despite the decrease in flexural strength relative to the pure PP polymer (M1). The increased hardness can be attributed to the higher flax fiber content and lower wood flour content in the M3 composite, which acted as effective reinforcing agents, improving resistance to indentation and surface deformation. The lower flexural strength may result from weaker bonding between the components, leading to premature failure under stress. However, when an optimal content, specific morphology, and appropriate size of natural fillers (both particles and fibers) are used, along with suitable compatibilizers in the correct proportion, an improvement in the mechanical properties of polymer composites can be achieved [61,62,63].
The failure mechanisms in M2 and M3 composites under flexural loading may be caused by fiber–matrix debonding at the interface, leading to reduced flexural strength. The hydrophilic nature of flax fibers and wood flour allows moisture absorption, weakening the fiber–matrix bond. This can result in more noticeable fiber pull-out and matrix cracking, potentially altering the structural integrity of the composites under load [1,12].
In automotive applications, the trade-off in flexural strength must align with component requirements. While reduced flexural strength may limit structural use, the increased hardness and enhanced thermal stability of the M3 composite can improve heat resistance and contribute to lightweighting, as its density increased by only 3.0% compared to the pure PP polymer (M1). PP/FF/WF polymer composites may be suitable for specific automotive applications, such as interior components or non-structural parts, where weight reduction and thermal properties are prioritized over high flexural strength. However, further research is needed to assess their suitability for specific applications.

3.6. Dielectric Tests

The dielectric properties, such as volume resistivity (ρv) and surface resistivity (ρs), of the polymer materials M1–M3 are presented in Table 6.
The dielectric test results (Table 6) indicate that the tested polymer composites are classified as electrical insulating materials, as a volume resistivity greater than 109 Ω·cm is exhibited by all PP/FF/WF composites [64,65,66].
A significant decrease in volume resistivity (ρv) of the polymer composites is observed with the introduction of natural hybrid fillers and carbon black pigment, compared to the pure PP polymer (M1). The volume resistivity increased in the following order: M3 (1.50 × 1014 Ω·m) < M2 (1.71 × 1014 Ω·m) < M1 (4.67 × 1014 Ω·m). Thus, a decrease of approximately 63.4% and 67.9% is observed for the M2 and M3 composites, respectively. This reduction suggests an increase in electrical conductivity with the addition of lignocellulosic fibers and carbon black pigment [49,67].
In contrast, surface resistivity (ρs) is observed to increase with the addition of hybrid fillers to the PP polymer matrix in the order: M1 (4.70 × 1015 Ω) < M2 (7.76 × 1015 Ω) < M3 (11.20 × 1015 Ω). An increase of approximately 65.1% is found for the M2 composite, while the increase is approximately 138.3% for the M3 composite.
The pure PP (M1) was found to exhibit the highest volume resistivity (ρv) and the lowest surface resistivity (ρs). These results are consistent with previous studies [67]. Bledzki et al. [67] reported a decrease in volume resistivity with the addition of 30 wt.% flax fiber in a PP matrix compared to the control PP polymer. This suggests an increase in electrical conductivity due to the presence of polar groups in lignocellulosic fibers, which facilitate current flow when a voltage is applied. Conversely, the surface resistivity of polymer composites was found to increase with higher flax fiber content.
In the case of the PP/FF/WF polymer composites (M2 and M3) studied in this work, a 30 wt.% loading of natural hybrid fillers was used in both composites, and their resistivity values (Table 6) were found to vary with the flax fiber and wood flour filler content.
Volume resistivity plays a key role in designing polymer materials for specific automotive applications, considering variations due to humidity and temperature. It is commonly used to assess material uniformity and detect conductive impurities that affect durability but are hard to identify by other means [65].
The changes in resistivity observed in the polymer composites can affect the design and application of automotive components. The higher surface resistivity in the M3 composite with higher flax fiber content improves resistance to charge accumulation. This is beneficial in components exposed to static electricity, like dashboards. Higher surface resistivity reduces electrostatic discharge risk, improving the longevity of sensitive automotive electronics. The resistivity changes may also affect the thermal management properties [66]. Automotive components exposed to high temperatures, like those near engines, require materials with stable electrical characteristics under varying thermal conditions. As resistivity varies with temperature, these composites provide stable electrical and thermal properties, making them suitable for interior automotive components.

3.7. Deterioration Tests Due to the Action of Fungi

Deterioration tests of polymer materials due to the action of fungi were carried out on samples with a thickness of 0.75 mm and a biodegradation period of 6 months, with analyses being performed at 45 days, 90 days, and 180 days.
The experimental results regarding the behavior of the materials under mold exposure are presented in Table 7 and Table 8.
The highest weight loss was observed in the M3 composite, reaching 6.91%, followed by the M2 composite at 4.73% and the pure PP polymer (M1) at 0.69% after 3 months (Table 8). Generally, these findings align with the degree of mold coverage for the samples exhibiting the highest and lowest weight losses (Table 7).
After 180 days (6 months) of exposure in the biodegradation environment, the weight loss of the M3 composite increased to 12.58%, while the M2 composite exhibited a weight loss of 7.58%. The PP polymer (M1) demonstrated the lowest weight loss, with values remaining within a narrow range of 0.61–0.81% after both 1.5 months and 6 months of exposure. In both polymer composites, sporodochia of Myrothecium verrucaria, known for their biodeterioration potential in textiles, such as flax fibers, were observed. The grade 5 for both polymer composites indicates full fungal coverage (Table 7).
The biodegradability tests were conducted under accelerated environmental conditions, exposing PP/FF/WF polymers to various fungal species, including Aspergillus and Penicillium species, Paecilomyces variotii, Chaetomium globosum, Myrothecium verrucaria, Scopulariopsis brevicaulis, Stachybotrys atra, and Trichoderma viride. Under normal atmospheric conditions, not all of these species affect polymer materials, nor are they kept in darkness at 90% relative humidity and 30 ± 2 °C for optimal development.
A significant weight loss of 12.58% was exhibited by the M3 composite after 6 months of exposure (Table 8), indicating higher degradation compared to the M2 composite (7.58%) and pure PP (0.81%). This increased degradation may be beneficial in reducing long-term environmental impact, especially in applications where end-of-life disposal is a concern. However, in automotive applications, excessive biodegradation may compromise the longevity and structural integrity of components. Thus, the balance between biodegradability and durability must be carefully considered based on the intended application. Further studies on protective coatings or stabilization treatments are needed to mitigate deterioration while maintaining sustainability benefits.
The susceptibility of the M2 and M3 polymer composites to fungi depends on the type of natural fillers, as well as the content, size, and morphology of flax fibers and wood flour particles within the PP polymer matrix. Moreover, it is known that fungal growth and colony spread are facilitated when the surface of natural fillers is not fully covered by the PP polymer matrix [68]. This issue can be addressed by functionalizing natural fillers to enhance their compatibility and uniform distribution within the PP matrix.
The stereomicroscope images of the polymer composite materials M2 and M3, deteriorated due to mold exposure for 6 months, are shown in Figure 4.
The durability and biodegradability of polymer materials for automotive applications can be effectively adjusted through the use of surface treatments and coatings. However, the painting and bonding of chemically inert polyolefins, such as nonpolar PP, are challenging due to their poor wettability and low surface energy. To enhance the affinity of PP matrix substrates for chemical bonding, prior surface treatments are required. Various surface treatment techniques, including mechanical methods (such as shot-peening and abrasion), chemical or chemical wet etching, and physical–chemical treatments (such as flaming and plasma treatments), can be applied to PP-based substrates [69,70].
An improvement in the mechanical properties of plasma-irradiated and surface-coated wood plastic composites (WPCs) developed through the injection molding technique, using 25 wt.% and 50 wt.% short wood fiber (WF) as a reinforcement and PP as the matrix, was reported by Ondiek et al. [70]. The surface-coated WPCs exhibited higher tensile strength compared to untreated materials. Thus, the application of appropriate surface coatings to treated PP composites can enhance material properties and durability. However, further studies on surface treatments and protective coatings applicable to the PP/FF/FF polymer materials obtained in this study are required to provide additional insights into optimizing their performance for specific automotive applications.

4. Conclusions

In this work, two polymer composite materials were obtained and characterized, with a polymer matrix made of polypropylene (PP) and natural hybrid fillers consisting of flax fiber (FF) and wood flour (WF). The weight concentrations in which the polymer materials were obtained, expressed in the form PP/FF/WF, were 100/0/0, 70/5/25, and 70/10/20. The characterization of these polymer materials revealed several key findings.
The pure PP polymer (M1) exhibited a homogeneous structure, while the M2 and M3 composites showed embedded natural hybrid fillers, creating an orange-peel-like appearance due to the irregular filler shapes. Polyhedron-shaped flax fibers with polygonal cross-sections and hollow lumens were observed in both composites. A small amount of carbon black pigment gave the PP/FF/WF composites a black coloration, improved material uniformity, and reduced electrostatic charge while providing ultraviolet stabilization.
X-ray diffraction analyses revealed that incorporating hybrid fillers reduced the crystallinity of the polymer composites compared to pure PP (M1). The highest crystalline phase content was found in M1 (55.3%), followed by M3 (45.9%) and M2 (45.3%), while the amorphous phase increased, reaching 54.7% in M2, 54.1% in M3, and 44.7% in M1 (pure PP). Despite these changes, all polymer materials crystallized within an orthorhombic system, with crystallite sizes of 20.8 nm (M1), 19.2 nm (M2), and 23.2 nm (M3). Moreover, the addition of hybrid fillers did not compromise the structural integrity of the PP, confirming its ability to incorporate reinforcements without significant alterations.
Thermal analyses identified four key processes within the thermograms of all materials: water loss, melting, thermal oxidation, and decomposition. The incorporation of hybrid fillers and carbon black pigment slightly altered the thermal behavior of the composites, lowering the melting points of M2 (162.7 °C) and M3 (164.4 °C) compared to pure PP (M1) at 168.3 °C, indicating disrupted crystallinity and reduced thermal resistance. Thermal stability, based on the initial oxidation temperature (TIN), increased in the order of M1 (215.0 °C) < M3 (234.0 °C) < M2 (234.7 °C), with M2 exhibiting the highest stability, closely followed by M3. This may be due to the presence of a small amount of carbon black pigment and a higher wood flour content in M2, which enhanced compatibility with the PP matrix, while the greater flax fiber content of M3 weakened filler–matrix bonding.
The density of the polymer materials increased in the following order: M1 (0.879 ± 0.040 g/cm3) < M3 (0.906 ± 0.050 g/cm3) < M2 (0.945 ± 0.046 g/cm3). The M2 composite showed a 7.5% increase in density, while the M3 composite exhibited a smaller increase of 3.0% compared to pure PP (M1). This increase in density, relative to pure PP, is attributed to the incorporation of hydrophilic natural hybrid fillers, which have a higher density than PP and tend to absorb moisture, unlike the hydrophobic PP matrix.
The mechanical properties of the polymer materials were also affected by the hybrid fillers. Mean Vickers hardness increased in the order of M2 (6.47 HV) < M1 (6.90 HV) < M3 (7.55 HV), while mean flexural strength followed the order of M2 (78.52 N/mm2) < M3 (89.60 N/mm2) < M1 (112.97 N/mm2). Higher flax fiber content in the M3 composite improved hardness but weakened interfacial bonding, reducing flexural strength.
The dielectric properties confirmed that all M1–M3 materials function as electrical insulators, with volume resistivity exceeding 109 Ω·cm. The volume resistivity increased in the order: M3 (1.50 × 1014 Ω·m) < M2 (1.71 × 1014 Ω·m) < M1 (4.67 × 1014 Ω·m). Natural fillers and carbon black reduced volume resistivity and increased electrical conductivity in the composites, indicating potential use in static electricity-exposed parts. Surface resistivity increased in the order of M1 (4.70 × 1015 Ω) < M2 (7.76 × 1015 Ω) < M3 (11.20 × 1015 Ω), with M2 and M3 showing increases of about 65.1% and 138.3%, respectively.
The biodegradability tests were conducted under accelerated environmental conditions, exposing PP/FF/WF polymers to various fungal species. After six months of exposure, the degradation rate (weight loss) followed the order of M3 (12.58%) > M2 (7.58%) > M1 (0.81%). The full fungal coverage (grade 5) on the M2 and M3 composites indicates the biodegradability of natural-fiber-reinforced polymer composites, highlighting their potential for environmentally friendly applications.
Despite the trade-off in flexural strength, the M3 polymer composite, containing 10 wt.% flax fiber and 20 wt.% wood flour, exhibited improved lightweight properties, high thermal stability, enhanced hardness, good resistance to surface charge accumulation, and biodegradability. These properties make it a promising candidate for automotive applications, including interior ornaments, dashboard panels, door panels, bumpers, front-end modules, and other non-structural components [5,59,71].
However, further research is needed to optimize these composites through PP or filler functionalization, tribological testing [72], UV resistance evaluation [21], and surface treatments or coatings [69,70], and to assess their performance in specific automotive components. Additionally, machine learning (ML) techniques can be utilized to support the design and optimization of polymer composite properties [73,74,75].

Author Contributions

Conceptualization, A.R.C.; methodology and investigation of PP and fillers, A.R.C. and D.C.; methodology and investigation—density, I.I.; methodology and investigation—Vickers hardness tests, D.P.; methodology and investigation—XRD analysis, B.G.S.; methodology and investigation—optical microscopy, D.P. and V.E.M.; methodology and investigation—dielectric tests, A.R.C.; resources, A.R.C.; data curation, A.R.C., M.V.L., I.I., D.P., B.G.S., V.E.M. and D.C.; writing—original draft preparation, A.R.C., M.V.L. and R.C.C.; writing—review and editing, A.R.C., M.V.L. and R.C.C.; visualization and supervision, A.R.C., M.V.L. and R.C.C.; project administration, A.R.C.; funding acquisition, A.R.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Romanian Ministry of Research, Innovation and Digitalization, project number PN23140201-42N/2023.

Data Availability Statement

The data presented in this study are available on reasonable request from the main author (A.R.C.) and corresponding author (M.V.L.).

Acknowledgments

We acknowledge the support of our colleagues from INCDIE ICPE-CA Bucharest, Romania, Petru Budrugeac for conducting thermal analysis, Nicoleta Burunția for performing antifungal tests, and Dorina Vlad for providing technical support in conducting three-point bending tests.

Conflicts of Interest

Author Doina Constantinescu was employed by MONOFIL SRL. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Suriani, M.J.; Ilyas, R.A.; Zuhri, M.Y.M.; Khalina, A.; Sultan, M.T.H.; Sapuan, S.M.; Ruzaidi, C.M.; Wan, F.N.; Zulkifli, F.; Harussani, M.M.; et al. Critical Review of Natural Fiber Reinforced Hybrid Composites: Processing, Properties, Applications and Cost. Polymers 2021, 13, 3514. [Google Scholar] [CrossRef]
  2. Seydibeyoğlu, M.Ö.; Dogru, A.; Wang, J.; Rencheck, M.; Han, Y.; Wang, L.; Seydibeyoğlu, E.A.; Zhao, X.; Ong, K.; Shatkin, J.A.; et al. Review on Hybrid Reinforced Polymer Matrix Composites with Nanocellulose, Nanomaterials, and Other Fibers. Polymers 2023, 15, 984. [Google Scholar] [CrossRef] [PubMed]
  3. Maurya, A.K.; Manik, G. Advances towards Development of Industrially Relevant Short Natural Fiber Reinforced and Hybridized Polypropylene Composites for Various Industrial Applications: A Review. J. Polym. Res. 2023, 30, 47. [Google Scholar] [CrossRef]
  4. Prem Kumar, R.; Muthukrishnan, M.; Felix Sahayaraj, A. Effect of Hybridization on Natural Fiber Reinforced Polymer Composite Materials—A Review. Polym. Compos. 2023, 44, 4459–4479. [Google Scholar] [CrossRef]
  5. Ismail, S.O.; Akpan, E.; Dhakal, H.N. Review on Natural Plant Fibres and Their Hybrid Composites for Structural Applications: Recent Trends and Future Perspectives. Compos. Part C Open Access 2022, 9, 100322. [Google Scholar] [CrossRef]
  6. Neto, J.; Queiroz, H.; Aguiar, R.; Lima, R.; Cavalcanti, D.; Doina Banea, M. A Review of Recent Advances in Hybrid Natural Fiber Reinforced Polymer Composites. J. Renew. Mater. 2022, 10, 561–589. [Google Scholar] [CrossRef]
  7. Caramitu, A.R.; Ciobanu, R.C.; Ion, I.; Marin, M.; Lungulescu, E.-M.; Marinescu, V.; Aflori, M.; Bors, A.M. Composites from Recycled Polypropylene and Carboxymethylcellulose with Potential Uses in the Interior Design of Vehicles. Polymers 2024, 16, 2188. [Google Scholar] [CrossRef]
  8. Patel, R.V.; Yadav, A.; Winczek, J. Physical, Mechanical, and Thermal Properties of Natural Fiber-Reinforced Epoxy Composites for Construction and Automotive Applications. Appl. Sci. 2023, 13, 5126. [Google Scholar] [CrossRef]
  9. Ibrahim, M.; Faruruwa, M.D.; Mohammed, Y.; Garba, M. Effect of Hybrid Filler Loading on Some Physicomechanical Properties of Waste Polypropylene. Dutse J. Pure Appl. Sci. 2025, 10, 166–175. [Google Scholar] [CrossRef]
  10. Elfaleh, I.; Abbassi, F.; Habibi, M.; Ahmad, F.; Guedri, M.; Nasri, M.; Garnier, C. A Comprehensive Review of Natural Fibers and Their Composites: An Eco-Friendly Alternative to Conventional Materials. Results Eng. 2023, 19, 101271. [Google Scholar] [CrossRef]
  11. Çavuş, V. Selected Properties of Mahogany Wood Flour Filled Polypropylene Composites: The Effect of Maleic Anhydride-Grafted Polypropylene (MAPP). BioResources 2020, 15, 2227–2236. [Google Scholar] [CrossRef]
  12. Parodo, G.; Sorrentino, L.; Turchetta, S.; Moffa, G. Manufacturing of Sustainable Composite Materials: The Challenge of Flax Fiber and Polypropylene. Materials 2024, 17, 4768. [Google Scholar] [CrossRef] [PubMed]
  13. Thakur, V.K.; Thakur, M.K. Processing and Characterization of Natural Cellulose Fibers/Thermoset Polymer Composites. Carbohydr. Polym. 2014, 109, 102–117. [Google Scholar] [CrossRef] [PubMed]
  14. Sharma, A.; Venkatesh, R.; Chaturvedi, R.; Kumar, R.; Vivekananda, P.K.K.; Mohanavel, V.; Soudagar, M.E.M.; Al Obaid, S.; Salmen, S.H. Polypropylene Matrix Embedded with Curaua Fiber through Hot Compression Processing: Characteristics Study. J. Polym. Res. 2024, 31, 260. [Google Scholar] [CrossRef]
  15. Rahman, M.M.; Maniruzzaman, M.; Yeasmin, M.S. A State-of-the-Art Review Focusing on the Significant Techniques for Naturally Available Fibers as Reinforcement in Sustainable Bio-Composites: Extraction, Processing, Purification, Modification, as Well as Characterization Study. Results Eng. 2023, 20, 101511. [Google Scholar] [CrossRef]
  16. Aravindh, M.; Sathish, S.; Ranga Raj, R.; Karthick, A.; Mohanavel, V.; Patil, P.P.; Muhibbullah, M.; Osman, S.M. A Review on the Effect of Various Chemical Treatments on the Mechanical Properties of Renewable Fiber-Reinforced Composites. Adv. Mater. Sci. Eng. 2022, 2022, 2009691. [Google Scholar] [CrossRef]
  17. Lu, J.Z.; Wu, Q.; McNabb, H.S. Chemical Coupling in Wood Fiber and Polymer Composites: A Review of Coupling Agents and Treatments. Wood Fiber Sci. 2000, 32, 88–104. [Google Scholar]
  18. Sobczak, L.; Brüggemann, O.; Putz, R.F. Polyolefin Composites with Natural Fibers and Wood-modification of the Fiber/Filler–Matrix Interaction. J. Appl. Polym. Sci. 2013, 127, 1–17. [Google Scholar] [CrossRef]
  19. Jenny, M.P.; Kabecha, W.W. Advantages and Challenges of Implementing Lightweight Materials in Automobiles: A Review. Int. J. Sci. Res. Publ. 2023, 13, 91–98. [Google Scholar] [CrossRef]
  20. Stojanović, B.Ž.; Milojević, S.T. Characterization, Manufacturing and Application of Metal Matrix Composites. In Advances in Materials Science Research; Wythers, M.C., Ed.; Nova Science Publishers: New York, NY, USA, 2017; Volume 30, pp. 83–113. [Google Scholar]
  21. Hossain, M.T.; Shahid, M.A.; Mahmud, N.; Habib, A.; Rana, M.M.; Khan, S.A.; Hossain, M.D. Research and Application of Polypropylene: A Review. Discov. Nano 2024, 19, 2. [Google Scholar] [CrossRef]
  22. Le Duigou, A.; Davies, P.; Baley, C. Environmental Impact Analysis of the Production of Flax Fibres to Be Used as Composite Material Reinforcement. J. Biobased Mater. Bioenergy 2011, 5, 153–165. [Google Scholar] [CrossRef]
  23. Goyal, A.; Sharma, V.; Upadhyay, N.; Gill, S.; Sihag, M. Flax and Flaxseed Oil: An Ancient Medicine & Modern Functional Food. J. Food Sci. Technol. 2014, 51, 1633–1653. [Google Scholar] [CrossRef] [PubMed]
  24. Romhány, G.; Karger-Kocsis, J.; Czigány, T. Tensile Fracture and Failure Behavior of Technical Flax Fibers. J. Appl. Polym. Sci. 2003, 90, 3638–3645. [Google Scholar] [CrossRef]
  25. Charlet, K.; Beakou, A. Interfaces within Flax Fibre Bundle: Experimental Characterization and Numerical Modelling. J. Compos. Mater. 2014, 48, 3263–3269. [Google Scholar] [CrossRef]
  26. Mattrand, C.; Béakou, A.; Charlet, K. Numerical Modeling of the Flax Fiber Morphology Variability. Compos. Part Appl. Sci. Manuf. 2014, 63, 10–20. [Google Scholar] [CrossRef]
  27. Andrzejewski, J.; Barczewski, M.; Czarnecka-Komorowska, D.; Rydzkowski, T.; Gawdzińska, K.; Thakur, V.K. Manufacturing and Characterization of Sustainable and Recyclable Wood-Polypropylene Biocomposites: Multiprocessing-Properties-Structure Relationships. Ind. Crops Prod. 2024, 207, 117710. [Google Scholar] [CrossRef]
  28. Bakar, M.B.A.; Ishak, Z.A.M.; Taib, R.M.; Rozman, H.D.; Jani, S.M. Flammability and Mechanical Properties of Wood Flour-filled Polypropylene Composites. J. Appl. Polym. Sci. 2010, 116, 2714–2722. [Google Scholar] [CrossRef]
  29. Caramitu, A.R.; Avadanei, L.; Mitrea, S.; Sbarcea, G.; Budrugeac, P. New Composites Based on Polypropylene Reinforced with Biodegradable Fibres for Automotive Applications. Mater. Plast. 2011, 48, 255–258. [Google Scholar]
  30. Caramitu, A.R.; Banciu, C.; Mitrea, S.; Avadanei, L.; Nicolae, V. New Composites Based on Polypropylene Reinforced with Biodegradable Fibres for Automotive Applications. II. Mater. Plast. 2012, 49, 198–200. [Google Scholar]
  31. Zhang, J.; Wang, H.; Ou, R.; Wang, Q. The Properties of Flax Fiber Reinforced Wood Flour/High Density Polyethylene Composites. J. For. Res. 2018, 29, 533–540. [Google Scholar] [CrossRef]
  32. Pant, M.; Palsule, S. Comparative Performance of Flax Fibers and Ramie Fibers Reinforced Functionalized Polypropylene Composites. Fibers Polym. 2025. [Google Scholar] [CrossRef]
  33. Miled, B.; Kammoun, S.; Belyamani, I.; Cauret, L. Manufacturing, Characterization, and Macromechanical Modeling of Short Flax/Hemp Fiber-Hybrid Reinforced Polypropylene. Forces Mech. 2024, 15, 100269. [Google Scholar] [CrossRef]
  34. SR EN ISO 1133-1:2022; Plastics—Determination of the Melt Mass-Flow Rate (MFR) and Melt Volume-Flow Rate (MVR) of Thermoplastics. Part 1: Standard Method. International Organization for Standardization (ISO): Geneva, Switzerland, 2022.
  35. SR EN ISO 1183-1:2019; Plastics—Methods for Determining the Density of Non-Cellular Plastics. Part 1: Immersion Method, Liquid Pycnometer Method and Titration Method. International Organization for Standardization (ISO): Geneva, Switzerland, 2019.
  36. SR EN ISO 306:2023; Plastics—Thermoplastic Materials. Determination of Vicat Softening Temperature (VST). International Organization for Standardization (ISO): Geneva, Switzerland, 2023.
  37. SR EN ISO 527-1:2020; Plastics. Determination of Tensile Properties Part 1: General Principles. International Organization for Standardization (ISO): Geneva, Switzerland, 2020.
  38. SR EN ISO 527-2:2012; Plastics. Determination of Tensile Properties Part 2: Test Conditions for Moulding and Extrusion Plastics. International Organization for Standardization (ISO): Geneva, Switzerland, 2012.
  39. SR EN ISO 14125:2000/AC:2003; Fibre-Reinforced Plastic Composites. Determination of Flexural Properties. International Organization for Standardization (ISO): Geneva, Switzerland, 2003.
  40. SR EN ISO 178:2019; Plastics. Determination of Flexural Properties. International Organization for Standardization (ISO): Geneva, Switzerland, 2019.
  41. SR EN ISO 6507-1:2023; Metallic Materials—Vickers Hardness Test. Part 1: Test Method. International Organization for Standardization (ISO): Geneva, Switzerland, 2023.
  42. IEC 62631-3-1:2023; Dielectric and Resistive Properties of Solid Insulating Materials. Part 3-1: Determination of Resistive Properties (DC Methods)—Volume Resistance and Volume Resistivity—General Method. International Electrotechnical Commission (IEC): Geneva, Switzerland, 2023.
  43. IEC 62631-3-2:2023; Dielectric and Resistive Properties of Solid Insulating Materials. Part 3-2: Determination of Resistive Properties (DC Methods)—Surface Resistance and Surface Resistivity. International Electrotechnical Commission (IEC): Geneva, Switzerland, 2023.
  44. SR EN 60068-2-10:2006/A1:2019; Environmental Testing. Part 2–10: Tests—Test J and Guidance: Mould Growth. International Electrotechnical Commission (IEC): Geneva, Switzerland, 2019.
  45. SR EN ISO 846:2019; Plastics—Evaluation of the Action of Microorganisms. International Organization for Standardization (ISO): Geneva, Switzerland, 2019.
  46. Spahr, M.E.; Rothon, R. Carbon Black as a Polymer Filler. In Polymers and Polymeric Composites: A Reference Series; Palsule, S., Ed.; Springer: Berlin/Heidelberg, Germany, 2016; pp. 1–31. [Google Scholar] [CrossRef]
  47. Shirvanimoghaddam, K.; Balaji, K.V.; Yadav, R.; Zabihi, O.; Ahmadi, M.; Adetunji, P.; Naebe, M. Balancing the Toughness and Strength in Polypropylene Composites. Compos. Part B Eng. 2021, 223, 109121. [Google Scholar] [CrossRef]
  48. Murmu, U.K.; Adhikari, J.; Naskar, A.; Dey, D.; Roy, A.; Ghosh, A.; Ghosh, M. Mechanical Properties of Crystalline and Semicrystalline Polymer Systems. In Encyclopedia of Materials: Plastics and Polymers; Elsevier: Amsterdam, The Netherlands, 2022; pp. 917–927. [Google Scholar] [CrossRef]
  49. Povacz, M.; Wallner, G.M.; Lang, R.W. Black-Pigmented Polypropylene Materials for Solar Thermal Absorbers—Effect of Carbon Black Concentration on Morphology and Performance Properties. Sol. Energy 2014, 110, 420–426. [Google Scholar] [CrossRef]
  50. Gijsman, P. Review on the Thermo-Oxidative Degradation of Polymers during Processing and in Service. E-Polymers 2008, 8, 065. [Google Scholar] [CrossRef]
  51. Mentes, D.; Nagy, G.; Szabó, T.J.; Hornyák-Mester, E.; Fiser, B.; Viskolcz, B.; Póliska, C. Combustion Behaviour of Plastic Waste—A Case Study of PP, HDPE, PET, and Mixed PES-EL. J. Clean. Prod. 2023, 402, 136850. [Google Scholar] [CrossRef]
  52. Zommere, G.; Vilumsone, A.; Kalnina, D.; Solizenko, R.; Stramkale, V. Comparative Analysis of Fiber Structure and Cellulose Contents in Flax and Hemp Fibres. Mater. Sci. Text. Cloth. Technol. 2013, 8, 96–104. [Google Scholar] [CrossRef]
  53. Seddiqi, H.; Oliaei, E.; Honarkar, H.; Jin, J.; Geonzon, L.C.; Bacabac, R.G.; Klein-Nulend, J. Cellulose and Its Derivatives: Towards Biomedical Applications. Cellulose 2021, 28, 1893–1931. [Google Scholar] [CrossRef]
  54. Leontiadis, K.; Achilias, D.S.; Tsivintzelis, I. Effect of the Filler Modification on the Thermal and Mechanical Properties of Composite Polypropylene/Wollastonite Drawn Fibers. Polymers 2023, 15, 2986. [Google Scholar] [CrossRef]
  55. Tajvidi, M.; Takemura, A. Recycled Natural Fiber Polypropylene Composites: Water Absorption/Desorption Kinetics and Dimensional Stability. J. Polym. Environ. 2010, 18, 500–509. [Google Scholar] [CrossRef]
  56. Zhang, M.; Jia, L.; Li, M.; Yoo, C.G.; Peng, H.; Arvelli, S.; Zhao, J. Challenges and Perspectives in Lignin-Derived Polyurethane Foam Synthesis. Adv. Sustain. Syst. 2025, 2401054. [Google Scholar] [CrossRef]
  57. Ladaci, N.; Saadia, A.; Belaadi, A.; Boumaaza, M.; Chai, B.X.; Abdullah, M.M.S.; Al-Khawlani, A.; Ghernaout, D. ANN and RSM Prediction of Water Uptake of Recycled HDPE Biocomposite Reinforced with Treated Palm Waste W. filifera. J. Nat. Fibers 2024, 21, 2356697. [Google Scholar] [CrossRef]
  58. Mohammed, M.; Jawad, A.J.M.; Mohammed, A.M.; Oleiwi, J.K.; Adam, T.; Osman, A.F.; Dahham, O.S.; Betar, B.O.; Gopinath, S.C.B.; Jaafar, M. Challenges and Advancement in Water Absorption of Natural Fiber-Reinforced Polymer Composites. Polym. Test. 2023, 124, 108083. [Google Scholar] [CrossRef]
  59. Milojevic, S.; Pešić, R.; Lukić, J.; Taranović, D.; Skrucany, T.; Stojanović, B. Vehicles Optimization Regarding to Requirements of Recycling Example: Bus Dashboard. IOP Conf. Ser. Mater. Sci. Eng. 2019, 659, 012051. [Google Scholar] [CrossRef]
  60. Nuñez, A.J.; Sturm, P.C.; Kenny, J.M.; Aranguren, M.I.; Marcovich, N.E.; Reboredo, M.M. Mechanical Characterization of Polypropylene–Wood Flour Composites. J. Appl. Polym. Sci. 2003, 88, 1420–1428. [Google Scholar] [CrossRef]
  61. Stark, N.; Rowlands, R. Effects of Wood Fiber Characteristics on Mechanical Properties of Wood/Polypropylene Composites. Wood Fiber Sci. J. Soc. Wood Sci. Technol. 2003, 35, 167–174. [Google Scholar]
  62. Gozdecki, C.; Wilczynski, A.; Kociszewski, M.; Tomaszewska, J.; Zajchowski, S. Mechanical Properties of Wood-Polypropylene Composites with Industrial Wood Particles of Different Sizes. Wood Fiber Sci. 2012, 44, 14–21. [Google Scholar]
  63. Arbelaiz, A.; Fernández, B.; Cantero, G.; Llano-Ponte, R.; Valea, A.; Mondragon, I. Mechanical Properties of Flax Fibre/Polypropylene Composites. Influence of Fibre/Matrix Modification and Glass Fibre Hybridization. Compos. Part Appl. Sci. Manuf. 2005, 36, 1637–1644. [Google Scholar] [CrossRef]
  64. Moser, A.; Feuchter, M. Mechanical Properties of Composites Used in High-Voltage Applications. Polymers 2016, 8, 260. [Google Scholar] [CrossRef]
  65. Haque, S.M.; Ardila-Rey, J.A.; Umar, Y.; Mas’ud, A.A.; Muhammad-Sukki, F.; Jume, B.H.; Rahman, H.; Bani, N.A. Application and Suitability of Polymeric Materials as Insulators in Electrical Equipment. Energies 2021, 14, 2758. [Google Scholar] [CrossRef]
  66. Chen, Q.; Yang, K.; Feng, Y.; Liang, L.; Chi, M.; Zhang, Z.; Chen, X. Recent Advances in Thermal-Conductive Insulating Polymer Composites with Various Fillers. Compos. Part Appl. Sci. Manuf. 2024, 178, 107998. [Google Scholar] [CrossRef]
  67. Bledzki, A.K.; Lucka, M.; Al Mamun, A.; Michalski, J. Biological and Electrical Resistance of Acetylated Flax Fibre Reinforced Polypropylene Composites. BioResources 2008, 4, 111–125. [Google Scholar] [CrossRef]
  68. Włodarczyk-Fligier, A.; Polok-Rubiniec, M. Studies of Resistance of PP/Natural Filler Polymer Composites to Decomposition Caused by Fungi. Materials 2021, 14, 1368. [Google Scholar] [CrossRef] [PubMed]
  69. Bernardi, C.; Toury, B.; Chefdeville, E.; Galipaud, J.; Salvia, M.; Contraires, E.; Virelizier, F.; Ourahmoune, R.; Surowiec, B.; Benayoun, S. Chemical Surface Treatment Mechanisms Involved in a Flamed Polypropylene/Polyurethane Bonded Interface: Toward a Treatment by Nitrogen Plasma. Appl. Surf. Sci. 2022, 593, 153475. [Google Scholar] [CrossRef]
  70. Ondiek, W.; Kondo, M.; Adachi, M.; Macadre, A.; Goda, K. Effect of Surface Coating and Plasma Treatment on Mechanical Properties of Wood Plastic Composites. J. Compos. Sci. 2023, 7, 296. [Google Scholar] [CrossRef]
  71. Azman, M.A.; Asyraf, M.R.M.; Khalina, A.; Petrů, M.; Ruzaidi, C.M.; Sapuan, S.M.; Wan Nik, W.B.; Ishak, M.R.; Ilyas, R.A.; Suriani, M.J. Natural Fiber Reinforced Composite Material for Product Design: A Short Review. Polymers 2021, 13, 1917. [Google Scholar] [CrossRef]
  72. Ray, S.K.; Banerjee, A.; Bhangi, B.K.; Pyne, D.; Dutta, B. Tribological Analysis—General Test Standards. In Tribology of Polymers, Polymer Composites, and Polymer Nanocomposites; Elsevier: Amsterdam, The Netherlands, 2023; pp. 17–50. [Google Scholar] [CrossRef]
  73. Maniraj, J.; Arockiasamy, F.S.; Kumar, C.R.; Kumar, D.A.; Jenish, I.; Suyambulingam, I.; Rangappa, S.M.; Siengchin, S. Machine Learning Techniques for the Design and Optimization of Polymer Composites: A Review. E3S Web Conf. 2023, 428, 02013. [Google Scholar] [CrossRef]
  74. Champa-Bujaico, E.; Díez-Pascual, A.M.; Lomas Redondo, A.; Garcia-Diaz, P. Optimization of Mechanical Properties of Multiscale Hybrid Polymer Nanocomposites: A Combination of Experimental and Machine Learning Techniques. Compos. Part B Eng. 2024, 269, 111099. [Google Scholar] [CrossRef]
  75. Sharma, A.; Mukhopadhyay, T.; Rangappa, S.M.; Siengchin, S.; Kushvaha, V. Advances in Computational Intelligence of Polymer Composite Materials: Machine Learning Assisted Modeling, Analysis and Design. Arch. Comput. Methods Eng. 2022, 29, 3341–3385. [Google Scholar] [CrossRef]
Processes 13 01078 g001
Figure 1. Optical microscope images (magnification 200×, 500×, respectively) showing the cross-sections of the polymer materials: (a) M1, (b) M2, and (c) M3.
Figure 1. Optical microscope images (magnification 200×, 500×, respectively) showing the cross-sections of the polymer materials: (a) M1, (b) M2, and (c) M3.
Processes 13 01078 g001
Processes 13 01078 g002
Figure 2. Diffraction patterns of the polymer materials: (a) M1, (b) M2, and (c) M3.
Figure 2. Diffraction patterns of the polymer materials: (a) M1, (b) M2, and (c) M3.
Processes 13 01078 g002
Processes 13 01078 g003aProcesses 13 01078 g003b
Figure 3. TG, DTG, and DSC curves of the polymer materials: (a) M1, (b) M2, and (c) M3.
Figure 3. TG, DTG, and DSC curves of the polymer materials: (a) M1, (b) M2, and (c) M3.
Processes 13 01078 g003aProcesses 13 01078 g003b
Processes 13 01078 g004
Figure 4. Stereomicroscope images (magnification 50×) of polymer composites deteriorated due to mold exposure for 6 months: (a) M2 composite showing sporodochia of Myrothecium verrucaria, along with cracks, and (b) M3 composite showing Myrothecium verrucaria and Chaetomium globosum, along with cracks.
Figure 4. Stereomicroscope images (magnification 50×) of polymer composites deteriorated due to mold exposure for 6 months: (a) M2 composite showing sporodochia of Myrothecium verrucaria, along with cracks, and (b) M3 composite showing Myrothecium verrucaria and Chaetomium globosum, along with cracks.
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Table 1. Main properties of the PP J700 TEHNOLEN polymer.
Table 1. Main properties of the PP J700 TEHNOLEN polymer.
PropertyValueUnitDetermination Method
Melt flow index (230 °C, 2.16 kg)9.96g/10 minSR EN ISO 1133-1:2022 B [34]
Density (23 °C)0.905g/cm3SR EN ISO 1183-1:2019 [35]
Vicat softening temperature—load 50 N163°CSR EN ISO 306:2023 [36]
Tensile flow strength39.5MPaSR EN ISO 527-1:2020 [37]
SR EN ISO 527-2:2012 [38]
Tensile breaking strength25.6MPa
Tensile elongation at break10.52%
Tensile modulus of elasticity1923.73MPa
Maximum flexural stress53MPaSR EN ISO 14125:2000/AC:2003 [39]
Flexural modulus1782.3MPaSR EN ISO 178:2019 [40]
Table 2. Codification and composition of the prepared polymer materials based on PP and fillers.
Table 2. Codification and composition of the prepared polymer materials based on PP and fillers.
Material TypeSample
Code		PP/Flax Fiber/Wood Flour
(wt.%)
PPM1100/0/0
PP + 5 wt.% flax fiber + 25 wt.% wood flour M270/5/25
PP + 10 wt.% flax fiber + 20 wt.% wood flour M370/10/20
Table 3. Results obtained from the thermal analysis (TG, DTG, and DSC) of the M1–M3 polymer materials.
Table 3. Results obtained from the thermal analysis (TG, DTG, and DSC) of the M1–M3 polymer materials.
Sample
Code		Process I
Water Loss		Process II
Melting		Process III
Oxidation		Process III
Thermal Oxidation		Process IV
Thermal Oxidation		Δm
Total
(%)
Tmin DSC
(°C)		TDTG
(°C)		Δm
(%)		Tmin DSC
(°C)		TIN
(°C)		Tmax DSC
(°C)		TDTG
(°C)		Δm
(%)		Tmax DSC
(°C)		TDTG
(°C)		Δm
(%)
M146.8--168.3215.0257.0
285.2		249.096.66434.1
455.9
495.7		491.13.34100.00
M2--2.79162.7234.7373.0
445.8
452.6		358.581.96452.6445.815.72100.47
M3--2.83164.4234.0377.7358.188.32458.1-8.4199.55
Table 4. Density of the M1–M3 polymer materials.
Table 4. Density of the M1–M3 polymer materials.
Sample CodeDensity
(g/cm3)
M10.879 ± 0.040
M20.945 ± 0.046
M30.906 ± 0.050
Table 5. Mechanical properties of the M1–M3 polymer materials.
Table 5. Mechanical properties of the M1–M3 polymer materials.
Sample CodeVickers Hardness
HV 0.05		Flexural Strength
Rm (N/mm2)
M16.90 ± 0.11112.97 ± 1.58
M26.47 ± 0.2978.52 ± 1.28
M37.55 ± 0.5989.60 ± 1.52
Table 6. Dielectric properties of the M1–M3 polymer materials.
Table 6. Dielectric properties of the M1–M3 polymer materials.
Sample
Code		Volume Resistivity,
ρv (Ω·m)		Measurement Uncertainty
for ρv (Ω·m)		Surface Resistivity,
ρs (Ω)		Measurement Uncertainty
for ρs (Ω)
M14.67 × 10141.41 × 10144.70 × 10150.98 × 1015
M21.71 × 10140.77 × 10147.76 × 10154.26 × 1015
M31.50 × 10140.49 × 101411.20 × 10155.76 × 1015
Table 7. Results obtained for the M1–M3 polymer materials subjected to deterioration under mold exposure according to [45].
Table 7. Results obtained for the M1–M3 polymer materials subjected to deterioration under mold exposure according to [45].
Sample
Code		Grades: 0–5 (According to Method B of [45])Observations
45 Days90 Days180 Days
M101, 01, 01–1, 01–11, 1–2, 1–2, 21, 1, 1, 1–2Myrothecium verrucaria, Trichoderma viride, Aspergillus flavus, Paecilomyces variotii
M23, 3–4, 4, 4–53, 3, 3–4, 3–44–5, 4–5, 5, 5Sporodochia of Myrothecium verrucaria, Trichoderma viride, Paecilomyces variotii, and Chaetomium globosum, along with cracks
M31–2, 2–3, 3, 3–43–4, 3–4, 4, 4–55, 5, 5, 5Sporodochia of Myrothecium verrucaria,
and Chaetomium globosum, along with cracks
Table 8. Weight loss of the M1–M3 polymer materials subjected to deterioration under mold exposure according to [45].
Table 8. Weight loss of the M1–M3 polymer materials subjected to deterioration under mold exposure according to [45].
Sample
Code		Weight Loss (%)
45 Days90 Days180 Days
M10.610.690.81
M22.894.737.58
M34.826.9112.58
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Caramitu, A.R.; Lungu, M.V.; Ciobanu, R.C.; Ion, I.; Pătroi, D.; Sbârcea, B.G.; Marinescu, V.E.; Constantinescu, D. Biodegradable Polymer Composites Based on Polypropylene and Hybrid Fillers for Applications in the Automotive Industry. Processes 2025, 13, 1078. https://doi.org/10.3390/pr13041078

AMA Style

Caramitu AR, Lungu MV, Ciobanu RC, Ion I, Pătroi D, Sbârcea BG, Marinescu VE, Constantinescu D. Biodegradable Polymer Composites Based on Polypropylene and Hybrid Fillers for Applications in the Automotive Industry. Processes. 2025; 13(4):1078. https://doi.org/10.3390/pr13041078

Chicago/Turabian Style

Caramitu, Alina Ruxandra, Magdalena Valentina Lungu, Romeo Cristian Ciobanu, Ioana Ion, Delia Pătroi, Beatrice Gabriela Sbârcea, Virgil Emanuel Marinescu, and Doina Constantinescu. 2025. "Biodegradable Polymer Composites Based on Polypropylene and Hybrid Fillers for Applications in the Automotive Industry" Processes 13, no. 4: 1078. https://doi.org/10.3390/pr13041078

APA Style

Caramitu, A. R., Lungu, M. V., Ciobanu, R. C., Ion, I., Pătroi, D., Sbârcea, B. G., Marinescu, V. E., & Constantinescu, D. (2025). Biodegradable Polymer Composites Based on Polypropylene and Hybrid Fillers for Applications in the Automotive Industry. Processes, 13(4), 1078. https://doi.org/10.3390/pr13041078

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