Evaluating the influence of graphene nano powder inclusion on mechanical, vibrational and water absorption behaviour of ramie/abaca hybrid composites

2.1 Materials

Ramie fibre and abaca fibre were procured from the Fibre Source in Chennai, India. Epoxy LY556 and Hardener HY951 were also obtained from the same supplier. The graphene used in this research had a purity of 99 %, an average thickness of 2.5 nm, a length of 7.5 μm, a bulk density of 0.48 g/cm³, and a surface area of 220 m²/g. The graphene nanoparticles were sourced from Shilpa Enterprises in Nagpur, Maharashtra, India.

2.2 Alkali treatment of ramie and abaca fibre

Initially, we cut the long fibres of ramie and abaca into short fibres, each measuring between 5 mm and 25 mm in length. The short fibres were then cleaned with distilled water to remove dirt and foreign particles. Next, the fibres were immersed in a 5 wt% NaOH solution for 2 h, followed by washing with running tap water for 10–15 min to remove residual NaOH from the fibre surface. Finally, the alkali-treated fibres were dried in an oven at 70 °C for 24 h (Figure 1).

Figure 1:

Alkaline treatment of ramie/abaca fibres.

A 5 wt% NaOH solution was chosen based on literature evidence showing it effectively removes hemicellulose, lignin, waxes, and other amorphous components while preserving the crystalline cellulose structure of natural fibres, which is crucial for maintaining their mechanical properties [34], [35], [36]. The 2-h treatment time was selected to ensure sufficient removal of surface impurities while minimising fibre damage. Jayabal et al. reported that alkali treatment of coir fibre in epoxy composites using a 5 % NaOH solution resulted in a 31 % increase in TS compared to lower (2 %) and higher (8 %) concentrations [36].

Alkali treatment enhances the surface chemistry of NFs by reducing lignin and waxy substances that act as barriers, thereby exposing more cellulose content. This increases surface roughness and crystallinity, improving mechanical interlocking and chemical compatibility with the epoxy matrix. The drying regime at 70 °C for 24 h was adopted to remove moisture, ensuring stable and consistent fibre properties for composite fabrication.

2.3 Synthesis of graphene-modified epoxy resin

Initially, acetone was added to varying weight percentages of graphene particles (0.5, 1.0, 1.5, and 2.0) in the beaker. We used an ultrasonication technique, with a power of 70 W, to mix the acetone and graphene at 40 kHz for 30 min. Next, we added the required quantity of epoxy to the acetone-graphene mixture. The resulting mixture was magnetically stirred at 80 °C to remove the acetone, and the precise amounts of epoxy, graphene, and acetone were carefully measured and controlled to ensure complete solvent evaporation and prevent residual solvent from affecting the curing process. It was then kept in a vacuum oven at 0.25 bar pressure for 30 min at 28 °C to eliminate air pockets and any remaining traces of solvent. The resulting graphene-epoxy-modified blend was then mixed with hardener HY951 in a 10:1 resin-to-hardener ratio (Figure 2). This process ensured strong interfacial bonding and uniform dispersion of graphene, which was affirmed using AFM.

Figure 2:

Preparation of graphene/epoxy mixture.

2.4 Ramie/abaca/epoxy HC preparation

The treated short fibres of ramie (15 wt%) and abaca (15 wt%) with various wt% of (0.5, 1.0, 1.5, and 2.0) graphene-reinforced epoxy HC were prepared using the compression moulding method. Initially, the required quantity of a graphene-modified epoxy mixture (70 wt%) was poured over the randomly oriented ramie-abaca short fibre in the steel mould, as illustrated in Figure 3. The excess resin is removed by rotating the roller over the fibre’s surface. The top and bottom plates of the mould are enclosed by a steel plate and placed over the compression moulding machine at 35 kg/cm² pressure and a temperature of 50 °C for a duration of 24 h. The hybrid composites were initially fabricated into plates with fixed dimensions of 300 × 300 × 5 mm using the compression moulding process. Afterwards, the test specimens were carefully sliced according to the relevant ASTM standard dimensions. The composition of the ramie/abaca HC is illustrated in Table 1.

Figure 3:

Preparation of ramie/abaca HC.

2.5 Density measurement

The theoretical density of the composites, based on the weight fractions of the different constituents, can be calculated using the following Eq. (1).

Where ρ _ct = theoretical density of the composite, ρ _fa = density of abaca fibre, ρ _fr = density of ramie fibre, ρ _m  = density of epoxy matrix, w _fa, w _fr, and w _m  = weight fractions of abaca fibre, ramie fibre, and matrix, respectively. The experimental density of the composites was determined using the water immersion method based on Archimedes’ principle, in accordance with ASTM D 570–98 (Table 1).

3.1 Tensile, flexural and impact testing

Ramie/abaca HC were tested to determine their tensile and flexural properties. Tensile properties were assessed by using an Instron 8801 universal testing machine (UTM), and flexural properties were evaluated using the Tinius Olsen UTM, as shown in Figure 4(a and b). For evaluating TS, a 2 mm/min strain rate and a 50 kN load cell were used. The tensile specimens were made with dimensions of 250 mm, 25 mm, and 5 mm, as per the ASTM D 3039 standard. The specimens utilised in the flexural test had the following dimensions: 127 mm by 25 mm by 5 mm (ASTM D790), a strain rate of 2 mm/min, and a 10 kN load cell. For each experiment, three samples were chosen to assess the composite’s tensile and flexural strengths. Impact test was carried out according to the ASTM D256. For that, the samples were fixed vertically, and a 5 kg pendulum with a 150° folding angle and 2.5 m/s velocity was used for impact loading.

Figure 4:

Mechanical testing of ramie/abaca fibre-reinforced HC (a) tensile analysis, (b) flexural test.

3.2 Free vibration analysis

For carrying out free vibrational analysis, the ramie/abaca HC is stimulated using the impulse hammer method (Figure 5). The samples were excited using a Dytran impulse hammer, and a lightweight sensor was utilised to record the vibration response. The DEWE four-channel data-collecting system provides the force input and vibration response, which vary over time, to the DEWE 7 modal analysis software. The software then transforms the time-domain data into a frequency-domain signal using Fast Fourier Transformation (FFT). The frequency response function is produced by evaluating the reactions to impulse excitations at different selected locations. The NF, modal damping ratio, and free vibration mode for a particular mode are then immediately provided by the software. The modal damping of a given mode is determined using the circle fit method, which involves a specific relationship (Equation (2)).

Figure 5:

Free vibrational analysis of graphene-reinforced ramie/abaca HC.

The angular frequency at resonance is represented as ω 0 , where ω 1 & ω 2 denote angular frequencies α ₁ & α ₂ are the angles between the angular frequencies.

3.3 Micro-hardness

Using a Shore D hardness tester (Model: MGW Precision Tools), the hardness of the developed ramie/abaca HC was assessed following ASTM D2240-15 testing guidelines. This test measures the hardness of rubbers, elastomers, polymers, and plastics. Ten different areas were evaluated for hardness, and the average value was determined. A material is considered highly resilient if its digital value exceeds 60. Values under 60 suggest that the material is not very resilient.

3.4 Water absorption and contact angle analysis

Ramie/abaca HC samples with dimensions of 64 mm × 12.7 mm × 5 mm were used for the water absorption test, as per ASTM D570–98 guidelines. The samples were placed in a desiccator and dried for one day at 60 °C before being allowed to cool to ambient temperature. The initial weight of the specimen was then measured (W _i ). After that, the samples were immersed in deionised water at ambient temperature. The composites were removed from the water every one day, wiped with a cotton cloth to remove water from the composite surface, weighed again (W _f ), and then returned to the water. For 15 days, this method was repeated. The following Eq. (3) was utilised to determine the amount of water absorbed by the specimens. µ.

The contact angle analysis was conducted using a goniometer, specifically the Kyowa contact angle meter (DME-211 series). The water droplet size (4 μL) and capture time (40 s) remained unchanged for all composites, following the release of a droplet onto a 2 cm × 2 cm sample area. For each configuration, three samples were assessed, and the average contact angle values along with their corresponding standard deviations were recorded.

3.5 Microstructure analysis

FE-SEM, Carl Zeiss- Sigma 300, was used to analyse the fractured HC samples, which had dimensions of 10 mm × 10 mm × 5 mm, at an acceleration voltage of 20 kV. Gold coating was applied to the samples using a sputter coating process to improve conductivity. AFM, Model: Nanosurf AFM was used to analyse the dispersion of graphene particles in the prepared composite.

4.1 Tensile behaviour of ramie/abaca HC

Figure 6(a–d) depicts the tensile behaviour of ramie/abaca HC prepared with distinct wt% of graphene. The tensile analysis (Table 2) shows that the addition of graphene improves the TS of composites compared to neat composites.

Figure 6:

Tensile behaviour of ramie/abaca HC (a) load versus displacement, (b) stress versus strain, (c) tensile strength versus graphene wt%, (d) tensile modulus versus graphene wt%.

The tensile properties increased with up to 1.5 wt% graphene inclusion; however, further additions resulted in lower tensile properties compared to the presence of 1.0 and 1.5 wt% graphene. The sample reinforced with 1.5 wt% graphene exhibited a TS of 22 MPa and a modulus of 3,116.897 MPa, which are 175 % and 93.77 % higher than those of neat composites. This indicates that adding graphene to epoxy enhances its ability to bear more load and provides higher resistance to tensile loading. This improvement is a result of the nanoscale reinforcement effect of well-dispersed graphene, which strengthens interfacial adhesion and stress transfer between the matrix and NFs. Meanwhile, the inclusion of 2.0 wt% graphene demonstrated lower TS (17 MPa) and modulus (2,835.162 MPa) compared to samples prepared with 1.5 wt% graphene. This decline is primarily caused by the agglomeration of excess graphene at elevated filler contents, which acts as stress intensification sites and disrupts the matrix’s continuity, thereby weakening the composite. The reason for the improvement in TS is the even distribution of graphene in the epoxy matrix, which facilitates better bonding between NFs, as confirmed by AFM (Figure 7).

Figure 7:

Uniform dispersion of graphene 1.5 wt% in epoxy matrix.

Moreover, examining tensile-tested samples using FESEM has revealed that the inclusion of graphene influences the microstructure of the ramie/abaca HC. The damage to the matrix and fibres, along with poor bonding between them, is evident in Figure 8(a), which is the reason for the lower load-carrying capacity of the neat composites. However, with a 1.5 wt% loading of graphene, the presence of nanoparticles helps carry more load owing to better adhesion between the matrix and NFs, as shown in Figure 8(b). The samples with higher graphene content failed at a lower load compared to those with 1.5 wt% loading, due to the accumulation (Figure (8c)) of nanoparticles in the matrix, which decreased the load-carrying ability and caused early failure from matrix and fibre damage, as shown in Figure 8(d).

Figure 8:

FESEM image of ramie/abaca HC (a) neat composite, (b) 1.5 wt%, (c) 2.0 wt%, (d) 2.0 wt%.

The TS of ramie/abaca HC was improved by 62.5 %, 137.5 %, 175 %, and 112.5 %, respectively, compared to unmodified composites. A similar kind of observation was also noted by researchers; for instance, Hosseini et al. found that incorporating 2 wt% graphene to glass fibre/NFs has enhanced the TS by 37 % [37]. Dhilipkumar et al. also stated that adding graphene has improved the TS of kenaf/pineapple fibre composite by 133.75 %. The present study confirmed that the incorporation of graphene significantly enhances the tensile strength of HC, highlighting its potential as an effective reinforcement in weight-sensitive structural applications [38].

4.2 Flexural properties of ramie/abaca HC

The flexural behaviour of ramie/abaca HC infused with graphene nanopowder is illustrated in Figure 9(a–c). A transverse bending load was applied to assess the failure loads of the HC. The flexural results indicated that the addition of graphene improved the flexural strength of the composites compared to neat composites (Table 3). Findings show that ramie/abaca HC prepared with 1.5 wt% graphene enhanced the flexural strength of the composites compared to neat composites. This improvement can be ascribed to the more efficient distribution of stress and the crack-bridging ability of the well-dispersed graphene, which reinforces the epoxy matrix and strengthens the fibre-matrix surface. The inclusion of graphene in the matrix reduced fibre pull-out and matrix damage, enabling it to support higher loads than other composites. Meanwhile, the sample containing 2.0 wt% graphene exhibited a lower flexural strength of 61.47 MPa compared to the samples with 1.5 wt% graphene (70.64 MPa) due to the slight accumulation of graphene in the epoxy matrix. Such agglomeration acts as micro-defects that initiate premature cracking under flexural loads, thereby lowering the overall strength.

Figure 9:

Flexural behaviour of ramie/abaca HC (a) load versus displacement, (b) stress versus strain, (c) flexural strength versus graphene wt%.

The examination of tensile test samples using FESEM has revealed that the incorporation of graphene has influenced the microstructure of the ramie/abaca HC. As shown in Figure 10(a), adding 1.5 wt% graphene has aided in better bonding between fibre and matrix, with resin-rich regions providing more resistance to transverse loading. The well-bonded interface facilitates effective load transfer, delays crack propagation, and enhances energy absorption during flexural deformation. Meanwhile, increasing the concentration leads to lower resistance to transverse loading because matrix accumulation reduces interfacial adhesion, which in turn prompts significant matrix and fibre damage, as evident from Figure 10(b–d).

Figure 10:

FESEM image of ramie/abaca HC (a) 1.5 wt%, (b) 2.0 wt%, (c) 2.0 wt%, (d) 2.0 wt%.

The flexural strength of ramie/abaca HC was improved by 47.83 %, 73.23 %, 139.29 %, and 108.23 %, respectively, compared to unmodified composites. Researchers also noted a similar observation; for instance, Sasikumar et al. also reported that adding graphene to caesar weed fibre and roselle fibre HC has enhanced flexural properties owing to better bonding between the fibre and matrix [39]. Ashok et al. fabricated and inspected the luffa/Kevlar fibre composite reinforced with graphene. Results have shown that the accumulation of graphene improved the flexural strength by 25.2 % compared to neat composites [40].

4.3 Impact properties of ramie/abaca HC

Figure 11 illustrates the impact behaviour of HC under impact loading according to ASTM D 256 standards. The addition of graphene significantly enhanced the energy-absorbing capacity of the composites compared to the pure HC (RA0.0), which had an impact strength of 2.41 J. When 0.5 wt% graphene was added (RA0.5), the impact strength increased to 3.58 J, a 48.54 % rise. Increasing the graphene content to 1.0 wt% (RA1.0) resulted in an impact strength of 4.69 J, a 94.60 % improvement over the neat composite. The highest increase was observed at 1.5 wt% graphene (RA1.5), where the impact strength reached 5.94 J, a 146.47 % increase. This notable enhancement is due to better interfacial bonding between the ramie/abaca fibres and the epoxy matrix, facilitated by well-dispersed GNPs, which improved stress transfer and energy dissipation during impact loading. However, at 2.0 wt% graphene (RA2.0), the impact strength decreased to 3.13 J, showing only a 29.87 % increase compared to the neat composite. This decline is caused by graphene agglomeration at higher concentrations, which weakens interfacial bonding and promotes crack growth under impact. These findings emphasise that an optimal graphene content (1.5 wt%) is crucial for maximising impact performance, while excessive loading results in lower impact resistance.

Figure 11:

Impact behaviour of ramie/abaca HC.

4.4 Free vibrational behaviour of ramie/abaca HC

The free vibration properties of areca/ramie HC with varying graphene content were evaluated under fixed-free boundary conditions using an impulse hammer technique. The results demonstrated that the existence of graphene significantly influenced both the NF and the damping behaviour of the composites for the three mode shapes (Figure 12(a–c)).

Figure 12:

Free vibrational behaviour of ramie/abaca HC (a) mode 1, (b) mode 2, (c) mode 3.

The unmodified sample (RA0.0) without graphene exhibited NF of 45.775 Hz, 270.94 Hz, and 784.5 Hz for modes 1, 2, and 3, respectively, with damping factors of 0.021945, 0.021887, and 0.0036083. With the introduction of 0.5 wt% graphene (RA0.5), there was a slight reduction in NF, while damping factors increased to 0.11134, 0.035496, and 0.015821, respectively, possibly due to changes in interfacial bonding. As the graphene content increased to 1.0 wt% (RA1.0), the NF improved to 47.952 Hz, 280.78 Hz, and 768.59 Hz, with moderately lower damping values, indicating that the presence of graphene enhanced the stiffness and reduced energy dissipation to some extent. The most significant improvement was observed at 1.5 wt% graphene (RA1.5), where the composite achieved the highest NF of 64.39 Hz, 328.41 Hz, and 859.6 Hz, along with the lowest damping values of 0.012628, 0.0099287, and 0.00076811. This indicates that the presence of graphene at this concentration greatly improved the fibre–matrix interaction and stiffness of the composite. However, a further increase in graphene content to 2.0 wt% (RA2.0) did not yield additional benefits; the NF dropped back to values similar to the 1.0 wt% sample, and damping increased again. This may be ascribed to the potential agglomeration of graphene at higher concentrations, which could reduce interfacial efficiency. In summary, 1.5 wt% graphene was found to be the most effective concentration for increasing the vibrational enactment of areca/ramie HC.

4.5 Shore D hardness

The Shore D hardness of ramie/abaca HC was assessed with varying graphene content ranging from 0 to 2.0 wt%, and the results are shown in Figure 13. The neat composite without graphene exhibited an average hardness value of 76, which increased progressively with the inclusion of graphene. At 0.5 wt% graphene, the hardness improved to 78.8, and further increased to 80.8 at 1.0 wt%. The highest Shore D hardness was observed for the composite with 1.5 wt% graphene, reaching an average value of 82.2, as shown in Figure 13. This significant enhancement can be attributed to the presence of graphene, which reinforces the polymer matrix and restricts localised deformation, thereby increasing surface resistance to indentation. However, at 2.0 wt% graphene, a slight reduction in hardness was noted (79.2), possibly due to graphene agglomeration, which may have led to stress concentration points and uneven distribution, thereby affecting the material’s surface integrity. Overall, the results suggest that incorporating graphene up to 1.5 wt% significantly enhances the surface hardness of ramie/abaca HC, while higher concentrations may lead to a reduction in hardness due to potential graphene agglomeration and compromised filler dispersion.

Figure 13:

Shore D hardness of ramie/abaca HC.

4.6 Wettability analysis of ramie/abaca HC

Figure 14 depicts the water absorption performance of ramie/abaca fibre-reinforced epoxy composites with varying graphene content (0–2.0 wt%). NFs like ramie and abaca have hydroxyl-rich cellulose structures that easily form hydrogen bonds with water molecules, making them naturally hydrophilic. When used as reinforcements in polymer composites, their ability to absorb moisture can limit the long-term performance of these materials. However, hybridisation and the use of nanofillers, such as graphene, can help mitigate this behaviour by improving the fibre-matrix interface and reducing water diffusion pathways.

Figure 14:

Water absorption behaviour of ramie/abaca HC.

Across all tested samples, the composites demonstrated a typical two-stage water absorption pattern: an initial rapid intake followed by gradual stabilisation as saturation approached. The neat ramie/abaca composite without graphene had the highest water uptake, reaching 14.1 %, owing to the NFs high moisture affinity. The addition of 0.5 wt% graphene reduced water absorption to 13.0 %, indicating improved interfacial adhesion that limited capillary voids and micro-channels in the matrix. Further graphene loading to 1.0 wt% and 1.5 wt% resulted in reduced uptake values of 11.7 % and 10.8 %, respectively, which were the lowest recorded in the study. These reductions indicate that the presence of graphene improves the matrix’s compactness and effectively acts as a water barrier. However, a slight increase in water absorption was witnessed in the composite containing 2.0 wt% graphene, which had a water uptake of 12.3 %. This marginal increase can be attributed to graphene’s tendency to agglomerate at higher concentrations, resulting in microstructural defects and loosely bonded regions that allow moisture to penetrate.

Water contact angle (WCA) measurements supported these findings (Figure 15(a–e)). The ramie/abaca composite had a WCA of 67.2°, indicating its hydrophilic nature. The addition of graphene increased the contact angle in all modified composites, indicating a lower surface affinity for water. Composites with 1.0 wt% and 1.5 wt% graphene had the highest WCAs of 79.3° and 80.7°, respectively. Composites with 1.5 wt% and 2.0 wt% graphene showed improved resistance to wetting in contrast to the unmodified sample. The enhancement in WCA is attributed to graphene’s ability to reduce the composite’s surface energy and partially shield the fibres from direct contact with moisture. Ganapathy et al. (2024) also found that adding 2.0 wt% graphene has reduced the moisture absorption behaviour of HC made from flax fibre and aerial root banyan fibre [27]. In summary, adding graphene to ramie/abaca HC effectively reduces water absorption and improves surface hydrophobicity, with optimal performance observed at 1.0–1.5 wt% graphene. Beyond this point, the benefits begin to wane due to nanoparticle agglomeration.

Figure 15:

Water contact analysis of ramie/abaca HC (a) neat composites, (b) 0.5 wt%. (c) 1.0 wt% (d) 1.5 wt% (e) 2.0 wt%.