Manufacturing,characterization and accelerated weathering of biocomposite made of poly (β-hydroxybutyrate)-co-poly(β-hydroxyvalerate) and non-woven alfa fibers

Abstract

This article investigates the feasibility of manufacturing a biocomposite composed of a biopolymer, poly (β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV), reinforced with cellulose nonwoven Alfa fibers. To better understand the behavior of this composite, accelerated weathering tests were conducted. To simulate extreme environmental conditions, composite samples were exposed to cyclic ultraviolet (UV) light, water spray, heat, and high relative humidity. The tensile and physical properties (mass and cross-sectional dimensions) of the composites were evaluated throughout the aging process. Changes in elastic modulus, stress, strain, as well as mass, width, and thickness were recorded. The study revealed the effects of accelerated weathering on PHBV–Alfa composites. Overall, the samples exhibited a relative decrease in physical and mechanical properties, except for the elastic modulus, which showed a slight increase toward the end of the test. Furthermore, the main degradation mechanisms responsible for the observed property reductions were identified as photolysis, hydrolysis, and photooxidation of the polymer surface, along with fiber–matrix interface deterioration caused by swelling of Alfa cellulosic fibers.

Graphical Abstract

1. Introduction

Technological capabilities today are nearly limitless, yet they remain constrained by dwindling fossil fuel resources and the environmental impact of industrial greenhouse gas emissions.^1,2 Manufacturers and researchers recognize this challenge, and their goal for the coming decades is to develop energy-efficient solutions that minimize environmental impact, for example, through improved product recyclability.^3–6

Composite materials were initially developed for high-performance applications such as aeronautics, aerospace, automotive, and sporting goods. Their high mechanical strength combined with low density enables superior specific properties compared to steel. Due to their lightweight nature, these materials have attracted growing interest in the transportation sector, which seeks to reduce vehicle weight to lower fuel consumption and, consequently, greenhouse gas emissions.

Currently, glass, carbon, and Kevlar fibers represent the majority of reinforcements used with thermosetting matrices such as epoxy, vinyl ester, or polyester. However, environmental concerns are increasingly driving manufacturers toward high-performance organic matrices and natural fillers to promote renewable and recyclable resources.^4,7–13

The interest in vegetable fibers for technical applications is not new. Early tools and dwellings were constructed using natural resources. Even after the advent of iron and steel, wood remained the preferred material for naval and military construction for centuries. In recent years, interest in natural fibers has grown significantly. For example, in the automotive industry, natural fibers such as hemp and sisal are now used in various components, including bumpers, mirrors, and dashboards. ¹⁴ This initiative is supported by the European Union, which mandates that 85% of a vehicle’s mass be recyclable and 10% recoverable.

The adoption of natural fibers extends beyond marketing strategies or regulatory compliance; it offers genuine advantages over synthetic alternatives. In addition to their high specific properties, natural fibers provide benefits such as low production cost, biodegradability, and excellent thermal and acoustic insulation. However, their use faces a major challenge: rapid aging due to high sensitivity to humidity, which leads to a significant decline in mechanical properties.

The long-term impact of aging on natural fiber-reinforced composites remains uncertain. Is the strength degradation small enough to justify their use in structural applications? If not, what is the expected lifespan of these materials? What precautions should be taken during design to prevent premature failure?

To address these questions, this work examines the evolution of the mechanical behavior of a fully biobased composite—composed of a biopolymer reinforced with plant fibers—under aging cycles, aiming to predict its long-term property changes. Alfa was selected as the natural fiber due to its superior mechanical properties within its category and its abundance as a Tunisian resource cultivated in arid and semi-arid regions.^15,16 Another distinctive aspect of this study is the use of nonwoven Alfa mats as reinforcement, a structure rarely described in the literature ¹⁷ and documented in only one composite application. ¹⁸

The methodology involves subjecting the PHBV–Alfa composite to accelerated weathering, simulating potential service conditions where temperature, humidity, and UV radiation act as degradation accelerators. Material degradation will be monitored by tracking changes in mechanical and physical characteristics throughout the test.

2. Material and methods

This section describes the raw materials, the processing of nonwoven Alfa, and the fabrication of the composite, followed by a detailed account of the tests conducted.

2.1. Raw materials

2.1.1. Alfa fibers

The natural fiber used as reinforcement in this work is Alfa, also known as Esparto grass or by its Latin name Stipa tenacissima. It grows in dry, hot Mediterranean regions such as Tunisia, where it covers approximately 600,000 hectares, with an annual production estimated at 42,000 tons.^19,20 The Alfa stem has a complex structure composed of numerous bundles of cellulosic fibers bonded by lignin, pectin, and hemicellulose.

To obtain cellulose fibers, the stems undergo a chemical process consisting of hydrolysis in a 3N sodium hydroxide solution at 373.15 K for 2 hours, followed by bleaching with a 40% aqueous solution of sodium hypochlorite (NaClO) for 1 hour. To produce long technical fibers, the material is then dried at 323 K for 12 hours and mechanically brushed. These extraction and bleaching parameters were selected based on previously optimized conditions established by our team, ¹⁹ which provided a good balance among fiber purity, yield, and mechanical integrity.

It is worth noting that alternative methods for cellulose fiber isolation are reported in the literature, including chemical treatments (kraft, sulfite, organosolv), mechanical methods (grinding, sonication, steam explosion), enzymatic/biological approaches, and hybrid or oxidative processes, as reviewed in recent studies.^20,21

The properties of these long Alfa fibers are reported in Table 1.

Table 1.

Tensile properties of long Alfa Fibers ¹⁹ .

Density (Kg/m3)	Strain at break (%)	Stress at break (Pa)	Young modulus (Pa)
1,4× 10-3	1,50 - 2,40	[187 – 306] × 103	[18,2 - 24,7] × 106

2.1.2. PHBV

Poly (β-hydroxybutyrate) (PHB) is a biocompatible and biodegradable polymer naturally produced by bacteria as a carbon storage mechanism. It represents a promising alternative to many non-biodegradable thermoplastic synthetic polymers.

The synthesis of poly (β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV) is achieved through copolymerization of hydroxybutyrate with hydroxyvalerate to enhance flexibility, processability, and thermal stability. The PHBV used in this study contains 3% hydroxyvalerate (HV) and was supplied by Tianan, under the trade name Enmat-Y1000P. Its mechanical properties are presented in Table 2.

Table 2.

Tensile properties of PHBV ²² .

Density (Kg/m3)	Strain at break (%)	Tensile strength (Pa)	Young modulus (Pa)
1,25× 10-3	2,5 - 4,0	[31 – 36] × 103	[2,8 - 3,5] × 106

The Y1000P compound includes an undisclosed amount of proprietary nucleating agents added by the manufacturer to improve melt processing.

2.2. Non-woven alfa processing manufacture

For this work, Alfa was used in the form of a needle-punched nonwoven produced from long Alfa fibers. To homogenize the fibers, a turbo-opening machine was employed. A vacuum system at the outlet of the opener enabled direct feeding into a bulk loader equipped with a spike belt and a detacher, which subsequently fed the fibers into a carding machine to form a veil of parallel fibers. The opener, loader, and carding machine were BONINO equipment (Italy) with a working width of 0.5 m. A topping step was performed before feeding the fibers into the needling consolidation machine.

The average diameter of Alfa fibers was approximately 0.19 × 10^-3 m, as determined from microscopic measurements on carded fibers in a previous study on Alfa nonwoven materials. ¹⁸ This value reflects the inherent variability typical of natural lignocellulosic fibers.

The tablecloth was produced by depositing eight layers of veil on a conveyor belt to obtain a sheet with randomly oriented fibers. This step was carried out using a Technoplants model (Italy). Once the tablecloth was formed, mechanical consolidation by needling was performed using a Fehrer needling machine. This bonding technique relies on physical entanglement of fibers within the sheet through the action of notched needles. Specifically, the machine is equipped with a head containing numerous GEBECON-type notched needles. During the downward movement, the notches catch the fibers, pull them through the sheet, and leave them in place during the upward movement.

The nonwovens obtained through this consolidation process are uniform, soft, and highly flexible, allowing them to easily conform to complex shapes (Figure 1). A flow chart summarizing the entire process up to the needling step is presented in Figure 2. All steps were performed at Cetelor (Centre de Test Textile Lorrain) in Épinal, France.

Figure 1.

Needled non-woven Alfa.

Figure 2.

Nonwoven manufacturing process up to the needling step.

2.3. Composite processing manufacture

PHBV pellets were extruded into tapes with a thickness of 1.0 × 10^-3 m and a width of 2.5 × 10^-2 m using a Brabender PlastiCorder extruder equipped with a single screw of 1.9 × 10^-2 m diameter and an L/D ratio of 25. The screw angular speed was set at 2.62 rad·s^-1. The temperature profile for extrusion was set as follows: 423 K in Zone 1 (hopper), 433 K in Zones 2 and 3, and 423 K in Zone 4 (die). The feed section used an oval die, and air cooling was applied to the extruder. The extrusion process was stable, with no processing issues observed, and produced uniform tapes suitable for lamination.

To fabricate natural fiber-reinforced composites, laminates were prepared by alternately stacking the PHBV films and non-woven Alfa fibers. Care was taken to ensure uniform fiber distribution and intimate contact between the polymer and fibers to promote good interfacial adhesion. The stacked laminates were then hot-pressed using a PHI 276 × 10³ N hot press under controlled pressure and temperature conditions, resulting in well-consolidated composite panels with minimal void content. Specifically, the composites were pressed at 207 kPa for 4–6 minutes, while both the top and bottom heating platens were maintained at 180 °C. After compression molding, the composites were cooled at room temperature under a 15-kg weight for 24 hours. This procedure ensured reproducible thickness, density, and surface quality, suitable for subsequent mechanical testing. ²³

2.4. Morphological testing

To carry out a morphological characterization of the different samples, the composite (PHBV-Alfa) cross-sections were observed using scanning electron microscopy (SEM).

Before each analysis, the samples were sputtered with a thin gold layer using a Fine Coat JFC-1100E ion sputtering device to ensure clear imaging of the fiber–matrix interface. The morphology of the different samples was characterized using a thermoscientific SM-5400 SEM (JEOL, Japan), operating at an accelerating voltage of 15 × 10³ V.

2.5. Accelerated weathering method

To assess the degradation of the PHBV–Alfa composite material over time, an alteration procedure was applied in accordance with ASTM standards G151 and G155.^24,25 Accelerated aging was carried out using a Xenon Weather-Ometer Atlas Ci3000 equipped with a Xenon Arc light source. This system generates a UV spectrum closely matching natural sunlight under maximum radiation conditions.

The alteration procedure consisted of a cycle of UV irradiation at a wavelength of 3.40 × 10^-7 m with an irradiance of 0.85 W·m^-2 for 9.0 × 10³ s under dry conditions, followed by a 1.8 × 10³ s cycle with direct water spraying and zero relative humidity. This illuminated phase under Xenon Arc radiation was subsequently followed by a dark condensation phase conducted at 90% relative humidity for 8.64 × 10⁴s. The details of this alteration procedure are summarized in Table 3.

Table 3.

Accelerated weathering test details.

Phase	Light Source	Irradiance at 340 nm (W/m2)	Chamber temperature (K)	Relative humidity (%)	Duration (s)	Water Spray
1	Xenon -Arc	0.85	333	60	9× 10 3	Off
2	Xenon -Arc	0.85	298	0	1.8× 10 3	On
3	-	0	318	90	8.649× 10 3	Off

The PHBV–nonwoven Alfa composite samples were subjected to 92 cycles of the alteration procedure described above, for a total duration of 2,500 hours. Tensile properties, mass, and transverse dimensions (thickness and width) were measured at eight intervals during the test: the first at 750 hours, followed by seven additional measurements at 250-hour intervals.

2.6. Mechanical and physical characterization

Changes in the mechanical properties of the Alfa–PHBV composite exposed to accelerated weathering were determined through tensile tests. These tests were performed using an MTS 854 tabletop hydraulic actuator. The applied load was measured with a 14.7 × 10³ N load cell, while tensile strain was recorded immediately using an extensometer. Specimens were tested without end tabs, in accordance with ASTM D638. ²⁶ Each mechanical analysis was repeated three times per interval, and the average values with standard deviations were reported.

To assess changes in mass, thickness, and width of the composite, samples were gently wiped to remove surface moisture before measurements were taken.

3. Results and discussion

3.1. Tensile properties of unweathered composites

In order to choose the optimal number of plies, thereby providing the composite with the highest fiber mass fraction and best mechanical properties, three composites were developed, as depicted in Figure 3.

• C1: 1 ply non-woven Alfa and 2 plies PHBV, with a thickness of (2.12 ± 0.047) × 10^-3 m

• C2: 2 plies non-woven Alfa and 3 plies PHBV, with a thickness of (3.46 ± 0.14) × 10^-3 m

• C3: 3 plies non-woven Alfa and 4 plies PHBV, with a thickness of (3.92 ± 0.12) × 10^-3 m

Figure 3.

Composites C1, C2, C3 of PHBV-alfa.

The fiber weight fractions of the produced composites, calculated from the initial masses of fiber and matrix prior to processing, ranged from 17% to 20%. To obtain volume fractions, the fiber weight fractions were converted using the known densities of PHBV and Alfa fibers, which are 1.25 × 10³ kg·m^-3 and 1.40 × 10³ kg·m^-3, respectively. For comparison of the three composites, tensile test specimens were cut according to standard ASTM D638. The specimens were tested without end tabs, exhibiting valid failure modes, as illustrated in Figure 4.

Figure 4.

Typical tensile failure mode for composites.

The average constants of elastic and ultimate strength and their deformation at break are provided in Figure 4 for the PHBV-Alfa composites. Average composites volume fractions were 15, 18.2 and 17.3 % for the C1, C2 and C3, respectively.

The difference in the composite volume fractions is mainly due to PHBV waste which is concentrated during molding on the edges of the plates and which increases according to the number of plies, however, volume fraction variation is accounted for in the mechanical analysis of the three composites. A total of five specimens were tested in each composite (Figure 5).

Figure 5.

Tensile properties of composites of PHBV-Alfa: a) Young modulus, b) tensile strength and c) tensile strain.

The comparison of the composites’ Young’s modulus shows a close correspondence among the three samples. However, the results for tensile strength and strain at break reveal clear differences, with C2 (2ply of nonwoven Alfa fiber) showing the highest values.

3.2. Modeling of tensile properties of PHBV-Alfa composite

In order to investigate the difference between the experimental and the theoretical tensile properties of PHBV-Alfa composite, a method of evaluating the tensile modulus of elasticity of natural fiber reinforcement composite has been applied. According to Summerscales research, ⁷ the rule of mixture (ROM) used to predict the elastic modulus of composite in isotropic materials taking into account the porosity effect is:

$E_c=\left(k_{{\eta }_d{\eta }_l{\eta }_0}{V}_f{E}_f+V_m{E}_m\right){\left(1-V_p\right)}^{nE}$

(1)

where: E_c, E_f, E_m are elastic modulus of composite, fiber and matrix, respectively, V_f, V_m, V_p, are volume fraction of fibers, matrix and voids, respectively (assuming V_f+V_m+V_p=1), k is the fiber area correction factor, ${\mathrm{\eta }}_{\mathrm{d}}$ is the fiber diameter distribution factor, ${\mathrm{\eta }}_{\mathrm{l}}$ is the fiber length distribution factor, ${\mathrm{\eta }}_0$ is the fiber orientation distribution factor and nE is the porosity efficiency exponent (PEE).

${\mathrm{\eta }}_{\mathrm{l}}=1-\frac{\mathit{tan}\hspace{0.17em}h\frac{\beta L}{2}}{\frac{\beta L}{2}},\frac{\beta L}{2}=2\frac{L}{D}\hspace{0.17em}\sqrt{\frac{G_m}{E_f\hspace{0.17em}\mathit{ln}\frac{k}{V_f}\hspace{0.17em}}}$

(2)

${\mathrm{\eta }}_0=\sum {V_f}_i\hspace{0.17em}{\cos }^4\hspace{0.17em}{\theta }_i$

(3)

${\mathrm{\eta }}_{\mathrm{d}}=1-\frac{m.\hspace{0.17em}dm}{{\text{Ef}}_0}$

(4)

where Ef₀ is the fiber modulus obtained by back-extrapolation of the line to a notional fiber diameter of zero, m is the (negative) slope of the line, and dm is the fiber diameter.

Based on the linear equation of several authors and taking into account the diameter of the Alfa fiber, we obtain values of ${\mathrm{\eta }}_{\mathrm{d}}$ ranging from 0.997 to 0.999.

For the fiber area correction, according to the modified Halpin-Tsai model, ²⁷ k can take three values k=0.785 for cylindrical, k=0.907 for hexagonal shaped fibers and k=0.820 for random shape fibers. For our case we considered k=0.785.

The alignment parameter ${\mathrm{\eta }}_0$ according to Krenchel gives us a minimum value of 0.984. ²⁸

For ${\mathrm{\eta }}_{\mathrm{l}}$ $,$ taking the average values of the diameter and the length of the Alfa fiber, 1.0 × 10^-4 m and 3.50 × 10^-1 m respectively, ²³ we obtain: ${\mathrm{\eta }}_{\mathrm{l}}=0.989$ .

For the modeling, the following assumption was made:

If all the voids in the composite have been removed during manufacturing, we can suppose that it is free of porosity, and the volume composition of the mixture becomes:

$Vf+Vm=1$

(5)

The properties constants for the alfa and those of PHBV resin are reported in Tables 1 and 2 respectively.

The predicted young modulus by the ROM model (Eq.1) are compared with the experimental values presented in Table 4.

Table 4.

Experimental results compared with rule of mixtures (ROM) results.

​	Experimental young modulus (Pa)	Theoretical young modulus (Pa)
C1	(4.85±0.14) × 109	4.79 × 109
C2	(4.51±0.72) × 109	4.9 × 109
C3	(4.61±0.62) × 109	4.68 × 109

The comparison of numerical and experimental results shows that the predicted Young modulus of PHBV-Alfa composites are very close to those obtained experimentally.

However, the tensile strength could not be accurately estimated using the Rule of Mixtures (ROM), as it is highly dependent on the composite porosity, which is neglected in this model. Therefore, a comparison between the theoretical and experimental strengths would be unreliable. Indeed, using the ROM (Eq. 6) for strength modeling while neglecting porosity resulted in theoretical tensile strengths of 5.66 × 10⁷ Pa, 5.88 × 10⁷ Pa, and 5.77 × 10⁷ Pa for C1, C2, and C3, respectively. In contrast, the experimental tensile strengths, as shoxwn in Figure 4, were (3.02 ± 0.157) × 10⁷ Pa, (3.58 ± 0.30) × 10⁷ Pa, and (2.675 ± 0.12) × 10⁷ Pa, respectively.

${\mathrm{\sigma }}_c=\left(k_{{\eta }_d{\eta }_1{\eta }_0}{V}_f{\mathrm{\sigma }}_f+V_m{\mathrm{\sigma }}_m\right){\left(1-V_p\right)}^{n\sigma }$

(6)

3.3. Morphological characterization

To examine microscopic differences among the three composites that explain the variations observed in tensile properties, SEM images were taken at different magnifications and are shown in Figures 6–8.

Figure 6.

SEM of composite C1 across the thickness: (A) 1mm, (B) 100µm, (C) 40 µm.

Figure 7.

SEM of composite C2 across the thickness: (A) 1mm, (B) 100µm, (C) 40 µm.

Figure 8.

SEM of composite C3 across the thickness: (A) 1mm, (B) 300µm, (C) 50 µm.

At a global scale, images 5a, 6a, and 7a clearly reveal the laminate structure of the composites—alternating PHBV/Alfa layers and PHBV layers—as well as the good quality of fiber impregnation by the matrix and the overall homogeneity of composite density, confirming good compatibility between the two materials.

However, micrographs 5b and 5c, and 7b and 7c illustrate differences among the three composites, where voids caused by fiber pull-out can be observed in composites C1 and C3. These void areas negatively affect the mechanical properties of the reinforced composites. In contrast, for composite C2, micrographs 6b and 6c show almost no gaps between fibers and matrix, which explains the superior performance observed in its characteristics.

Consequently, for the weathering test, only the optimal composite—composite C2—was selected.

3.4. Physical properties

Changes in the physical properties—mass, thickness, width, and cross-sectional area—of PHBV–Alfa composites during the weathering procedure described in Section 2.4 are shown in Figure 9. The values reported on the curves represent the average of three tested samples. Error bars in the graphs indicate the standard deviation for each set of three samples.

Figure 9.

Change in physical properties of PHBV-Alfa composite during weathering procedure compared to unweathered composite: a) change in mass, b) change in thickness, c) change in width and d) change in cross area.

The PHBV–Alfa composites exposed to simulated meteorological conditions (UV radiation, heat, water spray, and relative humidity) exhibited an initial increase in mass. This was primarily due to moisture absorption by the hydrophilic Alfa fibers and, secondarily, to increased surface energy and decreased water contact angle of PHBV following UV exposure. Indeed, UV exposure is known to improve polymer wettability. ⁹

Contact between a hydrophilic material and water molecules leads to surface absorption, followed by diffusion into the bulk material, in accordance with diffusion laws. According to various authors, ²⁹ water diffusion in bio-based composites with an organic matrix occurs through successive mechanisms. First, water molecules penetrate by capillarity between polymer chains and at the fiber/matrix interface, often resulting from poor adhesion during composite processing. Water molecules then form hydrogen bonds with hydroxyl groups in cellulose and hemicellulose. Subsequently, diffusion occurs at the interface and into the matrix. At this stage, certain fiber components may undergo hydrolysis, causing decohesion at the fiber/matrix interface and contributing to the reduction of functional properties.

Composite swelling, resulting from increased mass, led to a 2% increase in thickness compared to only 0.35% in width. This suggests that radial expansion of fibers primarily occurs along the composite thickness, where resistance is provided only by the polymer, while expansion along the width is constrained by both fibers and polymer.

After 750 hours of exposure, PHBV–Alfa samples began to lose weight progressively until the end of the test. This weight loss is attributed to surface layer degradation caused by hydrolysis, photolysis, and photooxidation of the polymer. ⁹ The thickness of the oxidized layer in aged samples is known to decrease with increasing temperature.

Additionally, weight reduction is linked to cracking and biodegradation of the biocomposite, resulting from mismatched thermal expansion coefficients between Alfa fibers and PHBV polymer. Such incompatibility induces additional stresses, as biopolymer expansion coefficients tend to increase with temperature.

Although polymer erosion due to photooxidation contributes to composite degradation, the primary initiator remains the deterioration of fiber/matrix interfacial bonds and the moisture content of Alfa fibers. This observation aligns with previous research on aging of natural fiber-reinforced composites, which suggests that weight loss is mainly driven by fiber–polymer interactions.^30–32

3.5. Tensile properties

The variations in the tensile modulus, the maximum stress, as well as the strain of the PHBV-Alfa composites during the alteration procedure already described are shown in Figure 10. The strength, strain and modulus values given, with their standard deviation, present the means of three individual samples.

Figure 10.

Evolution of tensile properties of PHBV-Alfa composite compared to unweathered composite during weathering procedure: a) E/E°, b) σ/σ₀ and c) ε/ε₀.

The aging cycle of PHBV–Alfa composites resulted in a progressive decrease in tensile strength and elongation at break as exposure time increased, whereas stiffness exhibited only minor variations compared to the unweathered material. The reduction in mechanical properties is primarily attributed to cyclic swelling of the fibers upon moisture absorption, which deteriorates the interfacial bonding between fibers and polymer.^33,34 Swelling generates internal stresses on the PHBV matrix, potentially exceeding its tensile strength and causing microcracking, while residual stresses from composite heterogeneity contribute to ply failure.

UV exposure additionally induces localized matrix degradation, leading to surface erosion and cracking, which further reduces ultimate tensile strength. ³⁵ The observed decrease in elongation is likely a consequence of photo-induced crystallization: chain segments resulting from photo-oxidative cleavage gain sufficient mobility at elevated temperatures to reorganize into crystalline domains.

Despite these degradative effects, composite stiffness showed only minor variations. This behavior can be attributed to the concurrent increase in PHBV crystallinity, resulting from chain rearrangements in the crystalline phase during accelerated weathering cycles, which offsets the weakening effect of chain scission. ³⁶ Since the tensile modulus of polymers is known to be proportional to their degree of crystallinity,^37,38 UV exposure can lead to a slight increase in modulus. Additionally, surface oxidation may promote crosslinking within the composite core, further contributing to stiffness enhancement. ³⁹

Regarding the fibers, UV irradiation generates free radicals that degrade lignin, slightly weakening Alfa fibers and partially offsetting the stiffness increase of the composite.

Overall, after the complete aging procedure, PHBV–Alfa composites exhibited a reduction of approximately 59% in tensile strength and 62% in elongation at break. The elastic modulus initially decreased by about 25%, remained relatively constant during intermediate cycles, and slightly increased in the final weathering stages, ultimately exceeding its initial value (E₀) by roughly 20%.

4. Conclusions and perspectives

This study demonstrates the feasibility of using Alfa fibers in a nonwoven form to produce an original biobased composite consisting of PHBV polymer reinforced with natural Alfa fibers. The effects of UV light, heat, water spray, and relative humidity on the physical and mechanical behavior of the PHBV–Alfa material were investigated.

Accelerated weathering caused a significant decline in several composite properties over prolonged exposure. Initially, sample weight increased due to moisture absorption by Alfa fibers and enhanced wettability of the PHBV matrix following UV exposure. After 1,250 hours, weight decreased sharply due to cracking and biodegradation. Fiber swelling further weakened the material by deteriorating the fiber–matrix interface.

The observed anisotropic swelling of the composites may be attributed to the intrinsic directional behavior of Alfa fibers, which swell more in the transverse direction than along their axis, as well as to local preferential orientations within the nonwoven structure. Heterogeneous adhesion at the fiber–matrix interface and non-uniform water diffusion further contribute to uneven dimensional changes. These factors collectively explain the anisotropic swelling behavior observed, consistent with previous reports on natural-fiber-reinforced polymers.

In addition to physical changes, the composites exhibited a marked reduction in mechanical performance, with tensile strength and elongation at break decreasing by 59% and 62%, respectively, while the elastic modulus initially decreased by 25% but ultimately increased to about 20% above its initial value. This decline is mainly attributed to microcracking in the matrix, polymer crosslinking reducing flexibility, and ply failure caused by residual stresses associated with material heterogeneity. The degradation of mechanical properties may also result from hydrogen bonding between water molecules and cellulose fibers, leading to interfacial decohesion. Higher fiber content amplifies this phenomenon, accelerating property loss under weathering. Importantly, mechanical deterioration results from the combined action of PHBV’s hydrolytic sensitivity and loss of adhesion at the fiber–matrix interface.

These findings align with current literature emphasizing the susceptibility of bio-based materials to environmental aging. Studies on nonwoven Alfa fiber composites remain limited, with most focusing on PMMA-based composites for prosthetic applications, such as Mankai et al. (2021) ⁴⁰ and Vermeulen et al. (2008). ¹⁸ These works demonstrate promising mechanical performance, supporting the potential of Alfa fibers as reinforcement. Our study extends this knowledge by using PHBV as a biopolymer matrix, offering a biodegradable alternative while maintaining competitive mechanical behavior. Consequently, the developed PHBV–Alfa composite may be suitable for indoor applications or short-term outdoor use under controlled conditions. For long-term or structural applications, strategies such as fiber surface treatment, compatibilizers, or hybridization with more hydrophobic components should be considered. Additionally, multiscale modeling could improve predictions of service life under various climatic conditions.

Although biological factors such as mold or bacterial growth were not specifically investigated, PHBV–nonwoven Alfa composite samples stored under standard laboratory conditions for several years showed no visible signs of microbial growth, suggesting stability under controlled indoor conditions. Similarly, no noticeable color changes, such as yellowing, were observed, indicating that the composite’s aesthetic appearance remains preserved over time.

Future work will include additional structural analyses, such as DSC, FTIR, and XRD, to further investigate the thermal, chemical, and crystalline properties of PHBV–Alfa composites, providing deeper insight into their structure–property relationships and long-term performance.