Properties of Nephrolepis exaltata microfiber-reinforced polylactic acid composites manufactured by fused deposition modelling

Raw materials

The Polylactic acid (PLA) was employed as the primary matrix material in the 3D fabrication process due to its biodegradability, ease of processing and compatibility with composite reinforcement. This PLA was secured from Multichem, Chennai, India. Microfibers were separated from the Nephrolepis exaltata collected from Jodhi Traders, Chennai, India. Fibers were silane treated with the help chemicals such as ethanol, 3APTMS, Whatman filter paper, beakers etc. these are secured from Laywell Composites, Chennai, India.

Microfibers separation from Nephrolepis exaltata

Freshly collected Nephrolepis exaltata stems were manually inspected and sorted to eliminate impurities such as dust and unwanted plant residues. The selected stems were thoroughly washed under running tap water to achieve proper surface cleaning. Subsequently, the cleaned stems were cut into small segments (approximately 2-4 cm in length) to enhance retting efficiency and ensure uniform microbial activity. The cleansed stem pieces were then immersed in distilled water at a material-to-liquid ratio of 1:20 (w/v) using a non-reactive container. ¹¹ The container was loosely covered to permit limited aeration while minimizing external contamination. The retting process was conducted at ambient temperature at 30°C for a duration of 5 days. Throughout this period, the soaking medium was monitored regularly, and the water was replaced at 48h intervals to avoid excessive fermentation and maintain optimal conditions. The completion of retting was confirmed when the fibrous bundles could be easily separated from the softened plant matrix through gentle manual rubbing. The retted material was then removed and repeatedly rinsed with distilled water to remove degraded residues and non-fibrous components. The extracted microfibers were evenly spread on a clean surface and oven-dried at 75°C to eliminate residual moisture. After drying, the fibers were finely ground and stored for subsequent fabrication processes. The overall procedure for the extraction of microfibers from Nephrolepis exaltata stems is illustrated in Figure 1.OPEN IN VIEWER

Surface treatment on microfibers

An ethanol-water mixture was prepared in a defined ratio of 80:30 under constant magnetic agitation. Subsequently, 1vol. % of the selected silane coupling agent, 3-APTMS (aminopropyltrimethoxysilane) was gradually introduced into the solution. The pH was carefully adjusted and maintained within the range of 5 to facilitate the hydrolysis of the silane. The solution was continuously stirred for 30min to ensure complete conversion of silane molecules into reactive silanol groups. The pre-treated microfibers were then immersed in the prepared silane solution and maintained under ambient conditions for a duration of 2h. Mild stirring was applied throughout the process of guarantee uniform interaction between the fiber surfaces and the silane solution. During this stage, the generated silanol groups reacted with the hydroxyl groups present on the fiber surface, resulting in the formation of stable covalent Si–O–C bonds. Following the treatment, the fibers were carefully removed and gently rinsed with ethanol or distilled water to eliminate any unreacted silane residues. ¹² Excess solution was drained, and the microfibers were lightly blotted to remove surface moisture. The modified microfibers were subsequently dried in an oven at 120°C for 2 h to remove residual solvents and to promote silane condensation reactions. This curing process ensured strong adhesion of silane molecules onto the fiber surface, thereby improving interfacial bonding with polymer matrices. Finally, the treated fibers were preserved in containers under dry conditions to prevent moisture uptake and potential degradation prior to their use in composite fabrication. The overall surface modification process is illustrated in Figure 2. Figure 3 shows the Fourier transform infrared spectroscopy spectrum of 3-Aminopropyltrimethoxysilane treated Nephrolepis exaltata microfiber. A band around 3400 cm^–1 corresponds to N–H stretching, indicating the presence of amine groups. The peak near 2920 cm^–1 is attributed to C–H stretching of alkyl chains. A band around 1700 cm^–1 represents C = O stretching, while the peak near 1550 cm^-1 corresponds to N–H bending. The absorption near 1250 cm^–1 is assigned to Si–C stretching, and the strong peaks around 1050 cm^–1 and 800 cm^–1 correspond to Si–O–C and Si–O–Si stretching, respectively. These characteristic peaks confirm the successful grafting of silane onto the microfiber surface and effective surface modification.OPEN IN VIEWER

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Figure 3.

Fourier Transform Infrared (FTIR) spectrum of silane treated Nephrolepis exaltata cellulose microfibers showing the characteristic functional groups and successful silane grafting.

Filament making process

PLA resin pellets were initially dried in a hot air oven at 75°C for 4 h to any absorbed moisture and prevent processing defects. The dried PLA was subsequently fed into a single-screw filament extruder equipped with a precisely controlled heating system. The extrusion temperature was maintained at approximately 160°C to achieve uniform melting and proper flow of the polymer. ¹³ The molten PLA was then extruded through a calibrated die to produce continuous filaments with the desired and consistent diameter. Following extrusion, the filament was passed through a water-bath cooling system to facilitate rapid solidification. Finally, the cooled filament was collected and wound onto a spool at a controlled take-up speed to ensure dimensional stability and uniformity. Table 1 displays the concentration of reinforcement with matrix to produce filament. Figure 4 illustrate the filament production process.

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Table 1.

Composition of printed composites and its designation.

Composite designations	PLA matrix (vol. %)	Surface treated Nephrolepis exaltata stem microfiber (vol. %)
V0	100	-
VN5	95	5
VN10	50	10
VN15	85	15

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Figure 4.

Schematic representation of the composite filament production process, including material mixing, extrusion, cooling, and filament winding.

3D specimen making

The digital data were exported in STL format. First, the STL file of the sample was imported to DLP 3D printer Pratham 3.0 software to arrange printing sitting. Key printing parameters such as layer thickness, raster speed, infill density and processing temperature were systematically configured to generate the required G-code. At the pre-fabrication stage, the build platform was thoroughly cleaned and coated with an appropriate release agent to ensure easy removal of the printed specimen. The printing conditions were then established, with the nozzle temperature maintained at approximately 230°C, infill density set to 100% and retraction parameters adjusted to 2 mm respectively. During the fabrication process, the composite filament was continuously fed into the heated nozzle, where it was melted and deposited layer-by-layer following the predefined tool path. ¹⁴ This process resulted in the formation of nearly 40 successive layers. Upon completion, the printed specimen was allowed to cool gradually to ambient temperature to achieve dimensional stability and promote effective interlayer bonding. Finally, the fabricated specimen was carefully removed from the build platform and prepared for subsequent evaluation. Figure 5 displays (a) 3D printing machine (b) 3D printing mechanism (c) obtained printed specimen.OPEN IN VIEWER

Mechanical properties

Figure 6(a)–(d) illustrates the mechanical properties of the developed composites. The results revealed that silane-treated Nephrolepis exaltata stem microfiber reinforced PLA composites exhibited significantly higher tensile strength than neat PLA, confirming the effectiveness of microfiber reinforcement. The unreinforced PLA specimen (V0) showed the lowest tensile strength of 58 MPa due to the absence of reinforcing phases and its limited load-bearing capability. The incorporation of silane-treated microfibers improved interfacial adhesion and stress transfer efficiency, resulting in a tensile strength of 71 MPa for VN5, representing an increase of approximately 22% over V0. The highest tensile strength was obtained for VN10 (86 MPa), corresponding to an improvement of about 48% compared with V0 and 21% compared with VN5. This enhancement is attributed to the uniform dispersion of microfibers, effective stress distribution, and strong fiber–matrix interfacial bonding within the PLA matrix. ¹⁵ Although VN15 maintained a higher tensile strength (79 MPa) than neat PLA, a decline relative to VN10 was observed, likely due to fiber agglomeration, inadequate matrix wetting, and void formation during FDM processing, which reduce stress transfer efficiency and promote premature failure. ¹⁶ OPEN IN VIEWER

The ductility of the composites was strongly influenced by microfiber content. While tensile strength increased with reinforcement loading up to 10 vol.%, the addition of rigid lignocellulosic microfibers restricted the mobility of PLA molecular chains, resulting in reduced elongation at break. The improved interfacial bonding produced by silane treatment enabled more efficient stress transfer from the matrix to the fibers, leading to a stiffer and stronger composite but with lower plastic deformation capability. At higher microfiber loading (VN15), the presence of fiber clusters and microvoids further accelerated crack initiation and propagation, causing earlier failure and a more brittle fracture response. Thus, the progressive increase in reinforcement content enhanced strength and stiffness at the expense of ductility, demonstrating the typical strength–ductility trade-off observed in natural fiber-reinforced PLA composites.

Incorporating microfibers into the composites significantly improved flexural strength compared with neat PLA. The V0 composite exhibited the lowest flexural strength of 92 MPa owing to its lower stiffness and poor resistance to bending deformation. The addition of 5 vol.% treated microfiber increased the flexural strength to 108 MPa, representing an improvement of approximately 17% over V0. VN10 achieved the highest flexural strength of 126 MPa, surpassing V0 and VN5 by about 37% and 17%, respectively. The superior flexural performance can be attributed to the homogeneous distribution of microfibers, increased stiffness, and strong interfacial bonding between the fiber and matrix. ¹⁷ Although VN15 exhibited a flexural strength of 118 MPa, the slight reduction compared with VN10 may be associated with excessive fiber loading, which can introduce microvoids, matrix discontinuities, and fiber clustering that facilitate crack initiation under bending loads. ¹⁸

The incorporation of silane-treated Nephrolepis exaltata stem microfibers also enhanced impact resistance and surface hardness. The neat PLA composite (V0) exhibited the lowest Shore D hardness (76) and impact strength (2.8 kJ/m²) due to its inherent brittleness and lack of reinforcement. The impact strength increased to 4.1 kJ/m² for VN5, corresponding to an improvement of approximately 46%, owing to enhanced energy absorption and crack-bridging mechanisms provided by the treated microfibers. VN10 recorded the highest impact strength of 5.6 kJ/m², nearly double that of neat PLA, as a result of efficient stress distribution, strong fiber–matrix adhesion, and improved resistance to crack propagation. However, the impact strength decreased to 4.9 kJ/m² for VN15, likely due to microfiber agglomeration and stress concentration sites that reduced the effectiveness of impact energy dissipation. ¹⁹ In contrast, Shore D hardness increased steadily from 76 for V0 to 80, 84, and 87 for VN5, VN10, and VN15, respectively. This gradual increase can be attributed to the rigid lignocellulosic structure of the microfibers and the restriction of polymer chain mobility, which enhanced surface stiffness and resistance to localized deformation with increasing microfiber content.

Furthermore, the anisotropic mechanical behaviour inherent to FDM-fabricated composites is influenced by printing temperature and build orientation. Higher printing temperatures improve interlayer diffusion and bonding, thereby enhancing stress transfer between deposited rasters and reducing void content. Conversely, lower temperatures can result in poor interlayer adhesion and premature delamination under loading. Build orientation also plays a critical role, as specimens printed with rasters aligned parallel to the loading direction generally exhibit higher tensile and flexural strengths than those printed perpendicular to the load due to more effective load transfer along the filament paths. Therefore, the observed mechanical performance is governed not only by microfiber reinforcement and interfacial bonding but also by the processing-induced anisotropy associated with FDM manufacturing.

With increasing microfiber reinforcement, the macroscopic fracture morphology of the tensile-tested FDM-printed PLA composites exhibited distinct failure characteristics. Figure 7(a) (V0) shows extensive filament debonding accompanied by a large ductile deformation zone, indicating weak cohesion between adjacent deposited rasters and limited resistance to tensile crack propagation in the neat PLA specimen. The irregular fracture appearance suggests unstable failure initiated by easy separation of the printed filaments under tensile loading. In Figure 7(b) (VN5), the incorporation of 5 vol.% silane-treated Nephrolepis exaltata microfiber resulted in a more compact fracture morphology, although localized intralayer cracking was still observed. This behaviour indicates improved stress transfer within the PLA matrix, while the presence of cracks may be attributed to local stress concentrations and non-uniform fibre distribution. Figure 7(c) (VN10) exhibits a comparatively uniform fracture surface with fewer visible defects, demonstrating improved microfiber dispersion and stronger fibre–matrix interfacial adhesion. The more homogeneous fracture pattern suggests controlled crack propagation and efficient stress distribution throughout the composite, which is consistent with the superior tensile properties observed for VN10. Overall, the incorporation of silane-treated Nephrolepis exaltata microfiber enhanced the structural integrity of the FDM-printed PLA composites, with the 10 vol.%reinforcement level providing the most balanced fracture behaviour and mechanical performance.OPEN IN VIEWER

The tensile fracture surfaces reveal the dominant failure mechanisms governing the mechanical response of the microfiber-reinforced PLA composites. Figure 8(a) shows a fractured microfiber with a relatively rough fracture end, indicating effective stress transfer from the PLA matrix to the reinforcement prior to failure. The observed fiber fracture suggests strong interfacial adhesion and efficient load-bearing capability. Figure 8(b) exhibits interfacial debonding, where partial separation between the microfiber and matrix can be observed. This phenomenon occurs when local stresses exceed the interfacial bond strength, initiating crack propagation along the fiber–matrix interface. However, the limited debonding area indicates that the silane treatment improved compatibility between the microfiber and PLA matrix. Figure 8(c) demonstrates fiber pull-out, characterized by extracted fibers and voids left within the matrix. Fiber pull-out absorbs fracture energy and contributes to enhanced toughness, although excessive pull-out may indicate localized weakening of the interfacial region. Figure 8(d) shows fiber breakage accompanied by surface damage and microcracks. The broken fiber morphology indicates that the applied tensile load was effectively transferred to the reinforcement, resulting in fiber failure rather than premature interfacial separation. The coexistence of fiber fracture, limited debonding, and controlled pull-out confirms strong fiber–matrix interaction and supports the improved tensile performance of the microfiber-reinforced PLA composites.OPEN IN VIEWER

Wear properties

Figure 9(a) and (b) wear rate and coefficient of friction behaviour of the composite. Microfiber addition significantly improved the wear behaviour of silane-treated Nephrolepis exaltata stem PLA composites, as shown by the decrease in specific wear rate and coefficient of friction. The absence of reinforcement and the comparatively soft nature of PLA caused substantial material removal during sliding, leading to the tidy PLA composite V0 exhibiting the greatest specific wear rate of 0.059 mm³/Nm and coefficient of friction of 0.48. The VN5 composite had a lower wear rate and friction coefficient of 0.043 mm³/Nm and 0.41, respectively, when compared to V0. This suggests that the enhanced load-bearing capacity and decreased direct surface damage were caused by the incorporation of 5 vol.% silane-treated microfiber. With a particular wear rate of 0.031 mm³/Nm and a coefficient of friction of 0.34, VN10 showed the best tribological performance among all the composites. Strong fiber-matrix interfacial bonding, efficient stress distribution, and the creation of a solid protective barrier during sliding, which reduced material loss and surface friction, may explain why VN10 has better wear resistance. ²⁰ Nevertheless, for VN15, there was a small rise to 0.038 mm³/Nm for the specific wear rate and 0.38 for the coefficient of friction. Because of microfiber aggregation, poor matrix continuity, and fibre pull-out during sliding, which caused localised surface flaws and enhanced frictional interaction, the wear performance may decrease at higher microfiber loading. Optimal reinforcement efficiency and tribological stability were both supplied by VN10, according to the results.OPEN IN VIEWER

The worn surface morphology provides evidence for the tribological behaviour observed in Figure 10(a) and (b). The neat PLA composite (V0) exhibited numerous deep and continuous grooves, indicating severe abrasive wear and extensive material removal during sliding. These features are consistent with the highest specific wear rate (0.059 mm³/Nm) and coefficient of friction (0.48), demonstrating the limited wear resistance of the unreinforced PLA matrix. The VN5 composite displayed comparatively shallower grooves along with scattered wear debris, suggesting that the incorporation of 5 vol.% silane-treated Nephrolepis exaltata microfiber improved the load-bearing capability of the composite and reduced direct surface damage. Consequently, the wear rate and coefficient of friction decreased to 0.043 mm³/Nm and 0.41, respectively. Among all composites, VN10 showed the smoothest worn surface with the presence of a compact protective film and minimal surface deterioration. The formation of this transfer layer reduced direct contact between the sliding surfaces, thereby lowering friction and material loss. Strong fiber–matrix adhesion and effective stress transfer contributed to the superior wear resistance, resulting in the lowest wear rate (0.031 mm³/Nm) and coefficient of friction (0.34). In contrast, the VN15 composite exhibited signs of fiber pull out, localized delamination, and fragmented wear debris. Excessive microfiber loading likely promoted fiber agglomeration and reduced matrix continuity, creating stress concentration sites during sliding. These defects increased surface damage and frictional interaction, leading to a slight increase in wear rate (0.038 mm³/Nm) and coefficient of friction (0.38) compared with VN10. Overall, the wear morphology confirms that VN10 achieved the optimum balance between reinforcement efficiency and tribological stability.OPEN IN VIEWER

Thermogravimetric and differential scanning calorimetric analysis

Figure 11 presents the (a) TGA and (b) DSC behaviour of the FDM-printed PLA composites reinforced with silane-treated Nephrolepis exaltata stem microfibers. The thermal degradation behaviour observed from TGA is further supported by the thermal transition characteristics obtained from DSC analysis. The TGA curves exhibited three distinct degradation stages. During the initial stage (30–200°C), all composites showed negligible weight loss, which was associated with the removal of absorbed moisture and low-molecular-weight volatile compounds. The silane-treated microfiber-reinforced composites exhibited lower mass loss than neat PLA, indicating reduced moisture affinity due to surface modification. The intermediate degradation stage (200–350°C) corresponds to the onset of thermal decomposition involving cleavage of weaker molecular bonds and degradation of amorphous regions. ²¹ The neat PLA composite (V0) exhibited the lowest onset degradation temperature (Ti = 302°C), whereas VN5 and VN10 showed higher Ti values of 311°C and 324°C, respectively. The delayed degradation onset demonstrates improved thermal resistance arising from enhanced fibre–matrix adhesion and restricted molecular mobility of the PLA chains. In the final degradation stage (350–500°C), rapid decomposition of the PLA backbone and thermally stable cellulose-rich constituents occurred. The final degradation temperatures increased from 382°C for V0 to 395°C and 408°C for VN5 and VN10, respectively. The superior thermal stability of VN10 is attributed to uniform microfiber dispersion, enhanced interfacial bonding through silane treatment, and the formation of a thermally resistant char layer that delayed heat transfer and volatile release. Although VN15 exhibited slightly lower thermal stability (Ti = 318°C and Tf = 401°C) than VN10, its thermal resistance remained higher than that of V0 and VN5.OPEN IN VIEWER

The DSC thermogram further corroborated the TGA results. The glass transition temperature (Tg) increased from 58.2°C for V0 to 60.1°C, 62.8°C, and 61.5°C for VN5, VN10, and VN15, respectively. The increase in Tg indicates restricted segmental motion of PLA molecular chains due to stronger fibre–matrix interactions. Similarly, the cold crystallization temperature (Tcc) shifted from 111.4°C in V0 to 114.2°C, 117.6°C, and 115.8°C for VN5, VN10, and VN15, respectively, suggesting that the silane-treated microfibers acted as effective heterogeneous nucleating agents and promoted crystallization. The melting temperature (Tm) exhibited only a marginal increase from 167.8°C for V0 to 168.5°C, 169.3°C, and 168.9°C for VN5, VN10, and VN15, respectively, indicating that the fundamental crystal structure of PLA remained largely unchanged after reinforcement. Furthermore, the degree of crystallinity increased significantly from 14.6% for V0 to 18.9%, 24.7%, and 21.3% for VN5, VN10, and VN15, respectively, demonstrating improved crystalline ordering resulting from the nucleating effect of the treated microfibers. The slight reduction in crystallinity observed for VN15 may be attributed to fibre agglomeration, which hindered crystal growth at higher reinforcement loading.

Overall, the combined TGA and DSC analyses demonstrate that the incorporation of silane-treated Nephrolepis exaltata microfibers enhanced both the thermal stability and crystallization behaviour of PLA composites. ²² Among all formulations, VN10 exhibited the highest thermal stability, highest glass transition temperature, and greatest crystallinity, confirming that 10 vol. % microfiber loading provided the optimum reinforcement level for improving the thermal performance of the FDM-printed PLA composites.

Water absorption properties

Figure 12 water absorption behaviour of the composite. The hydrophilic properties of the lignocellulosic reinforcement caused the water absorption behaviour of the PLA composites treated with silane to improve with increasing microfiber content, namely from the stems of Nephrolepis exaltata. The absence of hydroxyl groups in PLA and its relatively low moisture affinity allowed the clean PLA composite V0 to have the lowest water absorption of 0.42%. Incorporating 5 vol.% treated microfiber into VN5 raised the water absorption to 0.71% compared to V0, suggesting the presence of more hydrophilic sites that may absorb moisture. The higher microfiber content and expanded interfacial areas that allowed water to diffuse into the composite structure may explain why the VN10 composite increased up to 1.08%. With a water absorption rate of 1.54%, VN15 outperformed all of the other composites and was around 267% more effective than plain PLA. It is possible that the increased presence of cellulose-rich components, microfiber agglomeration, and micro void creation during FDM processing improved moisture penetration and retention inside the composite, which would explain the considerable increase with higher microfiber loading.^23,24 Despite the fact that silane treatment improved fiber-matrix compatibility and reduced surface hydroxyl activity, which in turn lowered the level of moisture uptake, the intrinsic hydrophilic character of the natural reinforcement caused the overall water absorption to increase with microfiber content.OPEN IN VIEWER

Correlation of structure–property relationships in FDM-printed PLA composites

The results obtained from mechanical, thermal, and morphological analyses exhibited strong mutual correlation. The tensile properties increased progressively from V0 to VN10, which was attributed to the improved fibre–matrix interfacial adhesion resulting from silane treatment. This explanation was supported by the fracture morphology observations, where V0 exhibited filament debonding and extensive ductile deformation, whereas VN10 displayed a more uniform fracture surface with fewer defects, indicating efficient stress transfer and controlled crack propagation. The thermal analyses further corroborated these findings. TGA revealed an increase in both initial and final degradation temperatures from V0 to VN10, while DSC demonstrated higher glass transition temperature and crystallinity for VN10. The enhanced thermal stability and crystallinity indicated restricted polymer chain mobility and stronger interfacial interactions, which contributed directly to the improved load-bearing capability. However, at higher microfiber loading (VN15), slight reductions in tensile strength, crystallinity, and thermal stability were observed, which were associated with fibre agglomeration and the formation of localized stress concentration sites. Therefore, the mechanical, thermal, and fracture analyses collectively confirmed that 10 vol.% silane-treated Nephrolepis exaltata microfiber provided the optimum reinforcement level for achieving balanced structural integrity, thermal resistance, and mechanical performance in the FDM-printed PLA composites.