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Abstract

In the context of the development of sustainable materials with reduced environmental impact, this study investigates the mechanical performance of biocomposites reinforced with Posidonia oceanica, a marine plant abundantly present on the Mediterranean coast. Two matrices were used: conventional polypropylene (PP) and a biodegradable polypropylene-based hybrid resin (PP-Bio). The composites were reinforced with different mass fractions (20%, 30%, and 40%) of Posidonia fibers or leaves and were manufactured by a thermocompression molding process. The mechanical behavior was evaluated by tensile and three-point bending tests, complemented by digital image correlation (DIC) to analyze the strain distribution and Poisson’s ratio. The results showed that Posidonia fibers significantly improved the tensile and bending properties of both matrices, with optimal reinforcement observed at 30 wt% fibers. The maximum tensile strength reached 31.77 MPa for PP composites and 33.86 MPa for PP-Bio, while the flexural strength increased up to 65 MPa for fibre-reinforced PP-Bio composites. The Poisson’s ratio decreased from 0.37 to 0.23 as the fibre content increased, confirming enhanced stiffness and reduced transverse deformation. In contrast, sheet-based reinforcements failed to provide substantial mechanical improvements due to poor matrix adhesion and poor fiber morphology. Scanning electron microscopy (SEM) revealed structural defects, including porosities and poor fiber dispersion, especially at higher fiber contents. These findings highlight the potential of Posidonia oceanica as a valuable natural reinforcement for environmentally friendly composites, particularly for lightweight interior automotive applications, while emphasizing the need for improved fiber processing and treatments to maximize performance.

Keywords

Posidonia oceanica fibre reinforced composite PP PP bio mechanical properties

Introduction

In response to the need to limit the environmental impact of materials used in modern industries, particularly in the construction, automotive, and aerospace sectors, the development of bio-based composite materials has increased. Developed from bio-based biopolymers such as polylactide (PLA) and polyhydroxyalkanoates (PHA) and/or reinforced with natural fibres, these materials help to reduce dependence on fossil resources and improve the carbon footprint of finished products,^1,2 in particular, PLA-based composites, enriched with plant oils or natural fibres, have shown strong potential in sustainable applications such as agriculture and biomaterials.³ In this context, traditional plant fibres such as flax, hemp, and jute have been widely studied for their ability to reinforce polymer matrices while maintaining satisfactory mechanical performance. However, beyond terrestrial fibres, certain unexploited marine resources are also of considerable interest for the development of ecological composites.⁴ One of these is the Posidonia plant, an underwater plant endemic to the Mediterranean, known scientifically as Posidonia Oceanica, and abundant and renewable biomass in the Mediterranean region and especially in Tunisian coastline. Many dead leaves and rhizomes wash up on beaches, generating high clean-up costs for local authorities and environmental problems.⁵ Using this resource as a filler or reinforcement in composites represents an interesting economic and ecological opportunity.

Indeed, the fibrous structure of Posidonia oceanica is characterized by a high cellulose content (around 30–35%), high intrinsic porosity, and a lignin and hemicellulose composition comparable to that of other plant fibres used for reinforcement.⁶ These physico-chemical properties give Posidonia oceanica fibres favorable specific performances for applications requiring lightness and moderate stiffness.

In recent years, numerous studies have focused on natural fiber-reinforced thermoplastic composites using lignocellulosic reinforcements such as flax, jute, hemp and kenaf. These works have mainly investigated the effect of fiber treatment, compatibilizers, and processing parameters on the mechanical, thermal, and rheological behavior of biocomposites.^7–11 For instance, Lotfi et al.⁷ reviewed the manufacturing routes and machinability of natural fiber thermoplastic composites, while Zaman et al.⁸ reported that acetylation and surface modification techniques considerably improve interfacial adhesion and thermal stability. Agrawal et al.⁹ demonstrated the influence of fiber loading on the rheological and mechanical behavior of polypropylene/grape-skin composites, and Bourgogne et al.¹⁰ emphasized the critical role of fiber curvature and orientation in determining the stiffness and failure behavior of natural fiber mats. Furthermore, Mofokeng et al.¹¹ compared the performance of natural fiber composites based on polypropylene and polylactic acid matrices, confirming that the choice of polymer matrix significantly affects the balance between strength and sustainability.

However, most of these studies are limited to terrestrial fibers, whereas marine biomass resources, such as Posidonia oceanica, remain underexplored despite their abundance and ecological interest. The present work introduces an innovative approach by utilizing Posidonia ocanica fibers and leaves-marine by products largely unexploited in material science as reinforcements in both conventional polypropylene (PP) and biodegradable PP-based matrices (PP-Bio). Furthermore, this study uniquely combines digital image correlation (DIC) for full-field strain mapping with scanning electron microscopy (SEM) analysis to correlate mechanical response and interfacial morphology. This integrated methodology allows a deeper understanding of the structure property relationships in marine plant-based thermoplastic composites and contributes new insight into the valorization of coastal biomass for sustainable engineering applications.

The first studies devoted to incorporating Posidonia oceanica into biobased or conventional polymer matrices highlighted its potential for improving certain mechanical properties, in particular impact resistance and flexural rigidity.¹² However, owing to their hydrophilic nature, these fibres often require surface treatment or matrix modification to optimize their compatibility and improve interfacial cohesion.¹³

In addition to mechanical performance, the thermal stability of natural fibre-reinforced composites is an essential parameter for predicting their field of application.^14,15 Thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy were carried out on composites based on Posidonia oceanica, which revealed a moderate improvement in thermal stability compared with pure polymers, notably due to the thermal contribution of the lignocellulosic constituents.¹⁶ In addition, the glass transition temperature (Tg) of matrices can be influenced by the incorporation of fibres, thereby modifying the overall viscoelastic properties of the material.¹⁷

A further notable advantage of Posidonia oceanica composite material is its acoustic properties. The porous and fibrous structure of the raw material promotes efficient absorption of sound waves, offering promising applications in the fields of sound insulation and acoustic performance.¹⁸ Recent studies have shown that incorporating natural fibres into composite matrices can improve sound absorption performance, particularly in the medium and high frequency ranges, where industrial needs are particularly crucial.^18,19

However, despite the growing interest in the Posidonia oceanica bio-composite material, the available scientific and technical information is still limited when it comes to a comparative and rigorous evaluation of its mechanical, thermal, and acoustic performance in bio sourced matrices obtained by the thermocompression moulding process.

To this end, this study aims to develop, characterize, and optimize bio-based composites reinforced with Posidonia fibres and leaves. The work focuses on: (i) The evaluation of manufacturing parameters (type and rate of reinforcement), (ii) The mechanical and morphological characterization of composites and (iii) the comparison between biodegradable (PP bio) and thermoplastic (PP) matrices to determine their relevance according to the intended applications.

With this research, we intend to contribute to the transition to more sustainable materials and the intelligent exploitation of renewable marine resources.

Materials and methods

Posidonia leaves

Posidonia leaves are harvested directly from the coasts of Skanes (Monastir-Tunisia) in March 2024. Figure 1 shows raw Posidonia leaves, which are rinsed several times with water to remove impurities and sand residue. They are then dried in an oven at a temperature of 30°C for 48 hours and then stored until its used. Posidonia leaves are 20 to 80 cm long and 8 to 11 mm wide, with a density of 0.97 g/cm³ in accordance with the NFT 20-053 standard.

Figure 1.

Biomass waste from Posidonia oceanica leaves.

Posidonia fibres

Sea balls are brown balls with a fibrous texture. These balls result from the decomposition of the leaves of the Posidonia. When the leaf is decomposed, the fibres become entangled according to the movement of the water and accumulate on the beaches, under the action of the waves. As with posidonia leaves, posidonia balls are also harvested from the coasts of Skanes (Monastir-Tunisia) in March 2024. Thereafter the mechanical extraction of the Posidonia fibres is carried out using a crusher. The balls are incorporated between two brushes, one rotating and the other fixed, where the fibres are torn off. This operation allows the separation and destruction of the Posidonia balls and the recovery of the short fibres without affecting their properties. After crushing, the latter retain their constituent elements: cellulose, hemicellulose and lignin.

After crushing the fibres, the process is carried out by sieving them using a vibratory sieve with very small meshes, which allows all the sand residue to be separated from the balls. Once the Posidonia fibres are ready, they are rinsed with water several times to remove all impurities and salt. They are then dried in an oven at 30°C for 48 hours and finally stored (Figure 2).

Figure 2.

(a) Sea balls, (b) Extracted Posidonia fibres.

The fibres extracted from Posidonia sea balls are short fibres. Microscopic observations were carried out to characterise their geometry. The results show that the average fibre length is 8.36 mm (measured over 100 samples, with values ranging from 2.4 to 11.5 mm) and the average diameter is 0.13 mm. Several measurements were performed and revealed that the apparent density of the raw Posidonia fibres is 1.49 g/cm³ in accordance with the NFT 20-053 standard.

Hybrid resin 103

Hybrid 103© is a biodegradable matrix that belongs to the Cereplast Hybrid Resins family. It is a blend of starch and high-quality conventional bio polypropylene (PP-BIO). It is available in the form of white granules. Table 1 lists the mechanical characteristics of the Hybrid 103 matrix.

Table 1.

Properties of hybrid resin 103.

	Hybrid resin 103
Density (g/cm³)	1.05
Melt temperature (°C)	190
Mechanical stress at break (MPa)	17.4
Young’s modulus in tension (MPa)	1.07
Elongation at break (%)	10
Young’s modulus in bending (MPa)	758

Polypropylene (PP)

It is a polymer that has a very versatile semi-crystalline structure. In our study, we used a homopolymer that comes in the form of white granules. The PP has good mechanical properties such as impact resistance. These characteristics are grouped in Table 2.

Table 2.

Characteristics of homopolymer polypropylene.

	PP homopolymer
Density (g/cm³)	0.9–0.91
Glass transition temperature (°C)	≈5
Melt temperature (°C)	160–170
Mechanical stress at break (MPa)	32–38
Elongation at break (%)	100–600
Young’s modulus in tension (GPa)	1.2–1.7

Composite processing

In this study, we focus on the development of novel bio-composites materials by varying the raw material parameters as shown in Table 3.

Table 3.

Novel bio-Composite material formulations.

Matrix	Reinforcement		wt.%
PP-BIO	Fibres	Leaves	0%
			20%
			30%
			40%
			60%
PP	Fibres	Leaves	0%
			20%
			30%
			40%
			60%

The process used to fabricate the bio-composites is based on thermocompression, carried out using a JOOS hydraulic press. This machine is equipped with two plates, one fixed and one movable, each measuring 600 x 600 mm. The plates can be heated to a maximum temperature of 250°C.

The specimens are manufactured layer by layer. First, matrix films are prepared by compressing the PP-Bio and PP granules (Figure 3(a)) between tow parchments papers to avoid sticking with two press plates at a temperature of T = 190°C and a pressure of P = 230 kN with a holding time of 10 minutes to obtain raw films as shown by (Figure 3(b)).The processing parameters are determined following preliminary tests.

Figure 3.

Steps of preparing PP and PP-BIO matrix sheets: (a) PP and PP-BIO granules; (b) Compressed matrix films; (c) Cutted films.

The films are then cut to the shape of the specimens, and the amount of reinforcement required for each fraction is weighed. The superimposed components are then compressed under the conditions described above.

Figure 4 illustrates the thermo-moulding device with a Joos laboratory press (Figure 4(a)) and composite mould which is compressed between two heating plates (Figure 4(b)). The composite lamination technique for each composite is shown in Figure 4(c). Before assembly, the mould is treated with a release agent to facilitate final composite sheet demoulding. The fibres are thus protected on both sides by the PP-Bio and PP sheets.

Figure 4.

Composite preparation setup (a) Composite thermo-moulding device, (b) Mould compressed by heating plates and (c) Mould for laminate composite, (d) Mould for tensile specimens.

Tensile test

Tensile specimens are prepared following ASTM D638 type I standards, with dimensions of 165*13*3.2 mm³ (cf. Figure 5(a) and (b)). Tensile tests on PP, PP-BIO, PP-composites specimens and are carried out on a LLOYD EZ 20 machine (Figure 7(a)). The specimen is clamped between the machine jaws and subjected to a loading speed of 10 mm/min.

Figure 5.

Bio-composite tensile specimens: (a) PP-BIO matrix-fibres; (b) PP-BIO-Leaves; (c) PP matrix-fibres; (d) PP-Leaves.

Three-point bending test

To characterize Bio-composites under three-point bending loading, specimens with dimensions of 127*12.7*3.2 mm³ (cf. Figure 6(c)) are cutted from prepared plate (Cf. Figure 6(a) and (b)) following ASTM D790. The three-point bending test is performed on the same tensile testing machine at a speed of 10 mm/min (cf. Figure 7(b)).

Figure 6.

Bio composite bending specimens preparation: (a) PP-BIO matrix-fibres plates; (b) PP-BIO- fibres plates; (c) 3 point bending specimen.

Figure 7.

Tests settings: (a) Tensile setup; (b) 3 points bending setup; (c) DIC setup.

Digital image correlation (DIC) equipment

The strain measurement is carried out using the DIC method. Figure 7(c) resume the setup used during the tensile and three-bending tests which consists of: (1) ImpirexICL-B16 camera and a 20 W LED light source for 2D image recording, (ii) EPIX XCAP std V3.8 image acquisition software, and (3) strain analysis and measurement using GOM Correlate 2019 software. Specimen-analyzed surfaces are sprayed with black-matted paint followed by a white mist of paint with a fine stochastic pattern.

Results and discussions

Matrix properties

Tensile tests are carried out on PP and BIO-PP specimens. To ensure reproducibility, each test was repeated five times. The average results of the mechanical properties are summarized in Table 4.

Table 4.

Matrix properties.

Matrix	PP-BIO	PP
Stress (MPa)	20.87 ± 0.51	21.91 ± 0.32
Young’s modulus (GPa)	1.06 ± 0.27	1.34 ± 0.25

Effect of fibre addition on the tensile mechanical properties of composites

The addition of Posidonia fibres to a polymer matrix significantly influences the composite’s tensile properties. Tensile tests, carried out at a load rate of 10 mm/min with varying reinforcement ratios, revealed that the incorporation of these fibres improve the matrix’s mechanical behaviour. Figure 8 illustrates the effect of fibre addition on the composite’s tensile properties for two types of matrices.

Figure 8.

Evolution of tensile strength: Effect of fibres addition.

The results specify that the tensile strengths of all composites are superior to those of pure resins. This strength increases up to an optimal fibre content of 30% by weight; beyond this, a decrease is observed with increasing fibre content. Thus, Posidonia fibres act effectively as reinforcement up to a weight fraction of 30%.

The reinforcing effect of the fibres increases as the quantity increases. However, depending on their arrangement, there is a maximum mass fraction that allows complete incorporation into the matrix. Beyond this limit, the matrix cannot fill all the voids, leading to a reduction in stress due to the resulting pores.

Better adhesion between fibres and the matrix, combined with the absence of porosities, promotes increased stress. These observations are supported by several studies. For example, Khiari et al.²⁰ prepared and characterized a composite material based on Posidonia oceanica and low-density polyethylene (LDPE). Their results showed that the addition of Posidonia improves the thermal and mechanical properties of the composite, with optimal performance achieved at a certain fibre content.

For example, one study found that adding miscanthus fibres to polypropylene (PP) increases the Young’s modulus of the matrix by 107% and the tensile strength by 10.51%. These improvements are attributed to good fibre dispersion and effective adhesion between the fibres and the matrix.²¹

These results are confirmed by several studies, for example, Kiran et al²² who studied the evolution of the tensile strength of Jute/PLA composites as a function of processing parameters, they found that the best tensile strength was given by a temperature of 170°C and a pressure of 5 MPa with a time of 5 minutes.

Fibres can act as reinforcements up to a mass fraction of 30%. Nitish et al²³ showed that the effect of fibre reinforcement improves by increasing the fibre content. These results confirm that the addition of natural fibres, such as Posidonia, can improve the mechanical properties of composites up to a certain threshold, beyond which negative effects may appear due to poor dispersion or saturation of the matrix.

Increasing the reinforcement rate with Posidonia fibres, directly influences the stiffness and tensile modulus of PP and PP-Bio. As shown in Figure 9, the tensile modulus increases with the fibre ratio. This behaviour is primarily explained by the higher intrinsic stiffness of the Posidonia fibres compared to the polymer matrix, which enables them to bear a greater portion of the applied load. When stress is applied, well-bonded fibres restrict the deformation of the surrounding matrix, resulting in an overall higher composite stiffness. Moreover, the increase in the number of fibre-matrix interfaces enhances stress efficiency, provided that fibre dispersion and adhesion are sufficient. Consequently, for all tested fibre mass contents, the stiffness of the composites systematically exceeds that of the neat matrix.

Figure 9:

Evolution of tensile modulus: Effect of fibre addition.

For example, recent work has shown that the addition of miscanthus fibres to polypropylene (PP) results in a 107% increase in Young’s modulus, highlighting the significant impact of fibres on the material’s stiffness.²¹ Another study showed that the introduction of corn, alfa and kenan fibres significantly improves the stiffness of polymer matrix composites, due to better load distribution and the barrier effect against crack propagation.^1,3,24

However, the positive effect of adding fibres is limited by the maximum mass fraction that can be incorporated into the matrix. Indeed, beyond a certain threshold (generally around 30% by mass), stiffness begins to decrease due to pore formation and less homogeneous fibre dispersion. Poor adhesion between the fibre and the matrix can also lead to reduced load transfer, thus limiting the effectiveness of the reinforcement.

In addition, composite stiffness is influenced by other factors, including the nature of the matrix, fibre orientation, and processing parameters. For example, one study found that using an alkali treatment on the fibres improved fibre/matrix adhesion and further increased the tensile modulus of the composite.²⁵ According to a study conducted by Ismail et al.²⁶ which showed that during tensile loading, separate voids were partially created which blocked the stress propagation between the fibres and the matrix, when the fibre ratio increases, the degree of blocking increases, and consequently the stiffness increases.

In conclusion, the incorporation of natural fibres, such as Posidonia, improves the stiffness and tensile modulus of composites to an optimal level. These improvements are mainly due to the intrinsic stiffness of the fibres, good load transfer between the fibres and the matrix, as well as the minimization of structural defects such as pores. To maximize these benefits, it is essential to optimize manufacturing conditions and ensure chemical compatibility between the fibres and the matrix.

Mechanical behaviour of the composite reinforced with Posidonia leaves

In this study, we analyse the mechanical behaviour of the biodegradable composite reinforced with Posidonia leaves. The histogram in Figure 10 illustrates the evolution of the tensile stress of the composite as a function of the reinforcement mass ratio. Unlike fibre-reinforced composites, the results show that the addition of Posidonia leaves does not improve the tensile strength of the material. All obtained stress values are lower than those of the pure polypropylene (PP) matrix, indicating a negative effect of the reinforcement in this form.

Figure 10.

Evolution of tensile strength: Effect of leaves addition.

Asri et al.²⁷ reported that this decrease in mechanical strength can be attributed to insufficient fibre-matrix interaction and heterogeneous dispersion of sheets in the polymer matrix. Indeed, weak adhesion between the reinforcement and the matrix results in limited load transfer and, consequently, degradation of the overall mechanical properties of the composite.

However, an exception is observed for the biodegradable PP matrix (PP Bio) with a reinforcement rate of 30%, where a slight improvement in tensile stress is noticed. This increase suggests that, under certain conditions, a better sheet-matrix interaction can be achieved, thus promoting a more efficient load transfer. However, for reinforcement rates lower or higher than 30%, no notable improvement is observed, which highlights the complexity of the behaviour of these composites.

A thorough analysis reveals that the mechanical behaviour of composites reinforced with Posidonia leaves differs significantly from that of composites reinforced with fibres. Several factors may explain this difference, including the morphology and surface of the leaves, which have a structure less favorable to anchoring in the matrix compared to conventional fibres. Unlike long and continuous fibres, which offer good stress transfer, Posidonia leaves have a more random distribution and can induce stress concentration zones, making the material more brittle.²⁸

Furthermore, the comparison between PP and PP Bio matrices in terms of elastic modulus shows a distinct trend (Figure 11). The addition of leaves in standard polypropylene leads to a decrease in the tensile modulus, while for PP Bio, a slight reinforcement is observed up to a level of 30% reinforcements. This improvement could be attributed to a better impregnation of the leaves into the matrix at elevated temperatures. However, above 190°C, thermal degradation of the leaves components, especially hemicelluloses, may occur, reducing the mechanical stability of the composite.²⁹

Figure 11:

Evolution of tensile modulus: Effect of leaves addition.

In conclusion, although the incorporation of Posidonia leaves into a polymer matrix can influence the mechanical properties of the composite, their effectiveness as reinforcement is limited by their morphology and their interaction with the matrix. A better understanding of surface treatments and processing parameters could help optimize these composites for specific applications.

Tensile tests conducted on composites reinforced with fibres and Posidonia leaves revealed that the addition of these reinforcements positively modified the behaviour of hybrid resin 103. For both types of reinforcement (fibres and leaves), the maximum tensile stress value was obtained at a mass fraction of 30%.

According to the results of our study, for processing conditions set at a temperature of 190°C and a pressure of 230 kN, the modulus of elasticity of the Posidonia fibre-reinforced composite ranges from 1.06 to 1.7 GPa for the standard PP matrix and from 1.34 to 1.46 GPa for the Bio PP matrix. The maximum tensile stress reaches values between 21.91 and 31.77 MPa for the standard PP matrix, and between 20.83 and 33.86 MPa for the Bio PP matrix.

For composites reinforced with Posidonia leaves, the modulus of elasticity ranges from 1.06 to 1.33 GPa for the Bio PP matrix and from 1.34 to 1.7 GPa for the standard PP matrix. The maximum tensile stress increased slightly in the case of the Bio PP matrix, but remains overall lower than that of the fibre-reinforced composites.

These results allow us to conclude that composites reinforced with Posidonia fibres have superior mechanical properties compared to composites reinforced with leaves, with an optimization observed for a reinforcement rate of 30%. Indeed, Posidonia fibres offer better adhesion with the matrix and a more homogeneous distribution, which results in higher tensile strength and modulus of elasticity. On the other hand, Posidonia leaves, although they show an improvement in mechanical properties for a rate of 30%, remain less efficient than fibres due to their morphology and their less favorable contact surface with the matrix.

Mechanical characterization in bending

Three-point bending tests, performed at a loading rate of 10 mm/min and repeated five times to ensure reproducibility, were conducted on composites reinforced with Posidonia fibres. The study examined the influence of the reinforcement ratio, ranging from 20% to 40%, on the mechanical behavior of PP resin and hybrid resin 103.

The results demonstrated an improvement in the mechanical properties of the resins with the addition of Posidonia fibres (Figure 12). In particular, the flexural strengths of the composites were found to be higher than those of the resins alone. This strength tended to increase with the fibre mass fraction, reaching a maximum of 30%. Beyond this ratio, a decrease in mechanical performance was observed.

Figure 12.

Evolution of mechanical bending properties as a function of fibre content.

Thus, Posidonia fibres effectively act as reinforcement up to an optimal concentration of 30% by mass.

The addition of Posidonia fibres improves flexural mechanical properties, including strength and flexural modulus. Maximum strength is achieved for a composite containing 30% fibres by mass, suggesting better fibre distribution within the matrix and reduced fibre breakage. This indicates that the adhesion between the fibre and matrix allows for efficient transfer of the applied load, thereby enhancing the composite’s performance.

Load transfer between the matrix and fibres depends not only on the intrinsic properties of the components but is also influenced by geometric parameters and the arrangement of the fibres within the matrix.

Furthermore, Allègue et al.³⁰ studied cementitious composites reinforced with Posidonia fibres and observed an improvement in flexural strength up to a fibre content of 15%, followed by a decrease beyond this threshold. These results are consistent with those of other researchers. Takagi et al.³¹ studied the effect of pressure on green composites. They observed an increase in flexural strength and modulus with increased pressure. Pressure leads to better cohesion between the fibres and the matrix. Rassmann et al.³² made the same observation: increased pressure leads to a reduction in porosity and therefore an increase in flexural strength.

Figure 13 illustrates the evolution of the flexural modulus as a function of pressure for each reinforcement ratio. Tests have shown that the flexural strength of composites reinforced with Posidonia fibres is higher than that of PP. This strength increases progressively with the fibre content, reaching a maximum at 40% by mass. This result is explained by better fibre dispersion in the matrix and a reduction in their breakage, thus promoting load transfer between the fibre and the matrix.

Figure 13.

Evolution of flexural modulus as a function of fibre content.

However, beyond 40%, the flexural strength decreases. This decrease is attributed to difficulties in homogenizing the mixture, leading to the formation of fibre clumps and an increase in porosity, which impairs fibre-matrix cohesion.

The influence of pressure on the structure of composites has also been studied. Higher pressure improves cohesion between fibres and the matrix by reducing porosities, leading to an increase in flexural strength and modulus. Rassmann et al.³² confirmed this trend by showing that increased pressure decreases porosity and improves mechanical strength. Similarly, Wakeman et al.³³ observed that maintaining pressure reduces the void content in the material, with an increase in pressure from 10 to 40 bar decreasing porosity by approximately 1%.

Comparing PP and PP bio composites reinforced with Posidonia fibres, the flexural stress of PP composites is slightly lower than that of hybrid resin composites up to a fibre content of 30%. The same observation applies to the flexural modulus. The optimal mass fraction of fibres is 30% for biodegradable composites and 40% for those with a thermoplastic matrix.

Regarding maximum mechanical performance, the 103-hybrid resin-based composite achieves a strength of 65.00 MPa at a mass fraction of 30%, while the Posidonia fibre-reinforced PP composite achieves 63.56 MPa at 40%. These values, although very similar, remain consistently higher than those of the pure matrices.

Finally, the difference in optimal mass fractions (30% for the hybrid resin and 40% for the PP) is related to the properties of the polymer matrix. The reinforcing effect of the fibres depends on their arrangement and the matrix’s ability to completely encase them. Beyond a certain volume fraction, the matrix is no longer able to fill all the voids, leading to increased porosity and a decrease in mechanical performance.

DIC test results

Tensile tests and three-point bending tests were performed on bio-composite specimens made of bio-based polypropylene (PP Bio) reinforced with 20%, 30%, and 40% fibre content, respectively designated ET1, ET2, and ET3 for tensile tests, and E1, E2, and E3 for flexural tests. The tests were monitored using a non-contact Digital Image Correlation (DIC) measurement technique.

For the tensile tests, the DIC is used to determine the Poisson’s ratio for each type of material. To do this, two virtual extensometers are created using the GOM-Correlate 2019© software. The first extensometer is positioned along the y axis (main loading axis) in the middle of the region of interest (ROI) with a length of 35 mm and the second extensometer is positioned along the x axis, orthogonal to y with a length of 9 mm. The Poisson’s ratio is then calculated based on the equation: ν = -ε_x/ε_y.

Figure 14(a) depicts the initial state of major deformation with reference to the ROI zone, which covers almost the entire useful surface of all the tensile specimens. Figure 14(b)–(d) represent the strain levels for which the Poisson’s ratios will be calculated. The values of the Poisson’s ratio obtained from these deformation levels ν_ET1 = 0.376, ν_ET2 = 0.29 and ν_ET3 = 0.23. This variation is essentially due to the variation in the fibre rate in each type of material.

Figure 14.

Calculation of the poisson’s ratio: (a) Initial state for all specimens; (b) ET1 at ε_xy = 0.443%; (c) ET2 fibre at ε_xy = 0.492%; (b) ET3 at ε_xy = 0.464%.

For the three-point bending tests, the DIC technique was used to identify the maximum deformation areas, particularly around the loading point and the lateral supports. Figure 15 shows the location of the region of interest (ROI), which is almost the same for all three types of specimens. This figure also shows that the value of the strain at stage 0 is equal to nil.

Figure 15.

Location of the region of interest (ROI) for all three point bending tests.

Figure 16 shows the evolution of this maximum strain for four strain levels. The DIC strain mas revealed heterogeneous strain distributions within the composites flexures tests Maps with local strain concentrations observed whenever the level of strain increases. The first level, at 2% strain, shows a quasi-elastic behavior of the material with a weak strain distribution. Above this value, the distribution begins to become concentrated in the mid-section of the specimens up to a level of 6%, where a concentration of tensile stresses begins to be observed in the lower faces of specimens E1 and E2 in the mid-section. For the E3 specimen and for the same strain rate, a dual strain rate concentration was observed.

Figure 16.

Evolution of this maximum strain.

This is mainly explained by the random fiber distribution at this location. Subsequently, the onset of cracking was observed by the accumulation of strain fields for each specimen. The initiation of cracking is observed at 8.409%, 10.496% and 9.909% for E1, E2 and E3 respectively. Subsequently and at fracture, a level of 10, 13.460% and 16% strain was recorded for the three specimens E1, E2 and E3 respectively. This fracture was observed at the end of each test in the middle for specimens E1 and E2 and slightly to the right of the central loading point for specimen E3, as shown in Figure 17(a)–(c) respectively. From these figures, we can see that the main damage modes are matrix fracture, fibre fracture and fibre-matrix decohesion. These damage modes will be explained further in the next section, based on SEM observation.

Figure 17.

Observation on the specimen’s failure: (a) E1 specimen; (b) E2 specimen; (c) E3 specimen.

Scanning electron microscopy (SEM)

In this study, we used SEM to examine the fibre-matrix fracture interfaces of composites tested in tension. For this purpose, two samples, containing 30% and 60% fibres, respectively, were analysed at a temperature of 190°C and a pressure of 230 kN. Macroscopic defects such as cracks, porosities, and cavities were observed in the 30% fibre sample (Figure 18), indicating poor fibre distribution in the matrix due to the composite’s heterogeneity and the use of short fibres.

Figure 18.

Defects observed for the sample at 30% fibres (a) Magnification 100 μm, (b) Magnification 500 μm.

At higher magnification (Figure 18(b)), a detachment between the Posidonia fibres and the matrix is visible, with fibres surrounded by voids and pores corresponding to the locations of the fibres torn off during rupture. This observation reveals a weak fibre-matrix adhesion, leading to embrittlement of the material. This phenomenon could be linked to the hydrophilic nature of the fibres and the formation of air bubbles during processing at high temperatures.

Figure 19, corresponding to the sample containing 60% fibres, reveals a microstructure with porosities and voids. These defects are generally attributed to air entrapment during processing, particularly during the impregnation of the resin by the fibres.

Figure 19.

Defects observed for the sample at 60% fibres (a) Magnification 100 μm, (b) Magnification 500 μm.

Two types of pores can form: macrospores, which appear between the fibre strands, and micropores, located within the strands themselves. Resin flow, influenced by its viscosity, plays a key role in this phenomenon. High viscosity can prevent proper impregnation and promote the formation of macrospores, while lower viscosity promotes capillary flow, thus increasing the risk of micropores.

Parameters influencing this impregnation step include pressure, reinforcement permeability, resin viscosity, and processing temperature. For the samples tested in bending, two samples corresponding to 30% and 40% fibres at T = 190°C and P = 230 KN were chosen (Figure 20).

Figure 20.

Defects observed for the sample at 30% and 40% fibres (a, c) Magnification 100 μm, (b) Magnification 500 μm.

The analysis of Figure 20 highlights several structural defects. First, poor homogenization of the mixture leads to the formation of voids and pores in the internal structure, a rough and heterogeneous surface, as well as the presence of micro voids and cavities. These defects are linked to insufficient dispersion of the fibres in the bio-PP matrix. Furthermore, processing parameters, particularly pressure and temperature, play a crucial role. The applied pressure and vacuum must be optimized according to the rheological properties of the materials to limit pore formation. Too high a temperature promotes the appearance of porosities. Kobayashi et al. [40] studied the evolution of the porosity rate in a PLA/Hemp composite and showed that it increased with the processing temperature. This phenomenon is attributed to the thermal degradation of cellulose, which releases gases responsible for the formation of voids within the material.

Conclusion

In this work, we developed and studied two types of composites: a biodegradable matrix composite reinforced with Posidonia fibres or leaves, and a polypropylene (PP)-based composite reinforced with the same reinforcements. The main objective was to evaluate the impact of adding Posidonia fibres and leaves on the mechanical properties of the composites, as well as the influence of processing parameters, particularly temperature and pressure.

The results showed that the incorporation of Posidonia fibres significantly improves the mechanical properties of the composites, particularly in tension and bending. Strengths and moduli increase up to an optimal fibre content of between 30% and 40% by mass, beyond which a decrease is observed. In contrast, the addition of Posidonia leaves did not provide any significant improvement in mechanical properties compared to fibres. Morphological analysis revealed the presence of defects such as porosities, cavities, and poor fibre dispersion, which can be attributed to the lack of reinforcement treatment as well as processing conditions. These observations highlight the need to optimize these parameters to improve fibre-matrix adhesion and limit defect formation.

This experimental study is part of an approach to valorising Tunisia’s abundant natural fibres, with the prospect of integrating these composites into industrial applications, particularly in the automotive sector for interior elements such as door panels, trunk linings, and roof linings.

In future work, we plan to perform treatments on Posidonia fibres and leaves to improve their compatibility with the matrix and optimize the composites’ performance. Furthermore, exploring fibre/leaf blends could offer new opportunities for strengthening these materials and expanding their application areas.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Lamis Allègue

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Investigation of the mechanical properties of posidonia-reinforced thermoplastic composites for eco-friendly applications

Abstract

Keywords

Introduction

Materials and methods

Posidonia leaves

Posidonia fibres

Hybrid resin 103

Polypropylene (PP)

Composite processing

Tensile test

Three-point bending test

Digital image correlation (DIC) equipment

Results and discussions

Matrix properties

Effect of fibre addition on the tensile mechanical properties of composites

Mechanical behaviour of the composite reinforced with Posidonia leaves

Mechanical characterization in bending

DIC test results

Scanning electron microscopy (SEM)

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References