Characterization study of Holoptelea integrifolia tree bark fibres reinforced epoxy composites

Abstract This study characterised the composite plate fabricated by epoxy matrix reinforced with alkaline-treated Holoptelea integrifolia tree bark fibre. Tensile and flexural test results clearly show that the mechanical characteristics of pure resin improve in direct proportion to the fibre up to 40%. However, impact test results show that 30% fibre mass ratio composite showed higher mechanical properties. The H. integrifolia fibre composites (HIFC) specimens were also characterised by using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), Energy dispersive X-ray analysis (EDAX) and thermogravimetric analysis–differential scanning calorimetry (TGA–DSC) analysis. FESEM results show that the bonding between fibre and matrix was excellent. EDAX reveals the elemental proportion of HIFC. O–H, C– H, C–O–C, moisture content and aromatic structure are evident by FTIR spectroscopy. Thermal analysis reveals that the composites degrade rapidly when exposed above 210 °C. Graphical Abstract


Introduction
Natural fibre is a readily available and inexpensive material.Natural fibres play an important role in the industry.Their biodegradability and eco-friendliness make them a viable alternative to carbon fibres and conventional glass (Sathishkumar et al. 2022).The cellulosic fibre of the Shwetark plant have excessive cellulose percent, excessive crystalline index, a large crystallite size, better thermal firmness, relatable tensile value, and low density, making them potential polymeric reinforcements for average weight application such as packing and vehicle inner parts (Raja et al. 2021).The influence of fibre proportion on tensile, flexural, impact and water affinity characteristics of Calotropis procera fibre with epoxy was investigated.Fibre addition enhances the composites mechanical characteristics (Yoganandam et al. 2020).The crystallinity index of the Calotropis gigantea plant's stem fibres was 56.08%, and the fibres were able to endure a temperature of roughly 220 C, demonstrating that the fibres may be utilised as effective reinforcements for polymer matrices in the same way as frequently used bio-fibres (Ganeshan et al. 2018).Chemical evaluation, FT-IR analysis and XRD are used to describe cellulose fibres extracted from the Senna auriculata legume.The fibres have a cellulose content of 59.6% and a density of 0.3708 g per cubic meter.The crystalline index and crystallite size of fibres are 49.6% and 2.75 nm (NagarajaGanesha et al. 2019).Natural fibre reinforced composites can be used to replace both metallic plates and synthetic fibre reinforced composites if they are made appropriately (Anandan et al. 2022).Kumar Choudhary et al. (2021) epoxy composites reinforced with short areca catechu fibres at various proportions (0-6 wt).Silk fibre reinforced epoxy biocomposites were created by the hand layup process with varied fibre loadings and their physico-mechanical characteristics were investigated according to approved ASTM standards.The characteristics of the epoxy matrix were greatly improved with the addition of silk fibres, with the best results obtained at a silk fibre loading of 50 wt% (Darshan and Suresha 2021).The effect of alkali treatment on the lingo-cellulosic content of Teff straw was investigated.The results show that alkalitreated composites have roughly 16% greater activation energy values than untreated composites, indicating that composites are more thermally stable (Devnani and Sinha 2021).The compression mould technique is used to make composite laminates from novel agricultural wastage containing 4% alkaline-treated and untreated Lagenaria siceraria fibre added epoxy matrix with four varying fibre length and five varying fibre content (Nagappan et al. 2021).The obtained results revealed that among all the prepared composites, the Coccinia grandis fibre-reinforced bio-composite has maximum tensile, impact and bonding interface property for 40% fibre weight, whereas the composite has maximum flexural property for 50% fibre weight (Ramasamy et al. 2021).The current government-enforced environmental restrictions have prompted researchers all around the world to adopt more and more green materials, notably in composites.However, there is one significant drawback to these natural fibres: their incompatible hydrophilic tendency, which inhibits their interaction with hydrophobic matrixes (Madhu et al. 2020).An overview of natural fibre composites utilised in ballistic applications such as bulletproof vests.It aims to collate the findings and demonstrate natural fibre composites as a viable alternative to absolute synthetic composites utilised for the same purpose (Nayak et al. 2022).The compressive behaviour of short concrete columns with square cross sections confined with SRP and SRG composites was investigated experimentally.The findings demonstrated that the general failure mode of SRP-and SRG-confined concrete columns was a separation between the core concrete and the composite and detachment of overlapping fibre faces, respectively (Jahangir, Soleymani, et al. 2022;Jahangir and Esfahani 2022).Sandwich panel with a hybrid polymer matrix composite as the skin and an Alfa fibre-based core was fabricated and assessed by using bending, tensile, and non-destructive tests.The performance of the resultant sandwich was better than that of sandwiches made from other biomaterials, including cork, but it had a higher density (Laraba et al. 2022).The purpose of this work is to give an experimental analysis into the SRG bond behaviour used with masonry substrates.Laboratory single-lap shear experiments using two different test setups were performed on a masonry prism to examine the effects of bond length and breadth, test set-up configurations, and loading rate on failure mechanism and bond strength.The findings demonstrated that all experimental specimens mostly experience Mode-II failure (Jahangir, Soleymani, et al. 2022;Jahangir and Esfahani 2022).The technical qualities of cementitious compounds, such as mechanical strength and water absorption, are directly influenced by the physical and chemical properties of the Amazon NLFs.To enhance the cementitious composite's adherence and endurance, a surface treatment may be required (de Lima et al. 2022).The fly-ash content, normal load, fibre loading, and speed control parameters were used to estimate the least specific abrasive wear rate.The SEM study of worn abrasive surfaces identified the major wear mechanisms as micro-cuts, fibre breakage, micro-plowing, fractures and wear debris (Kumar, Patel, Mer, et al. 2021;Kumar, Prasad, Patel, Kumain, et al. 2021;Kumar, Prasad, Patel, Kumar, and Yadav 2021;Kumar, Prasad, Patel, Kumar, Yadav, and Winczek 2021).Natural fibre-based composites used in this study had less favourable mechanical characteristics than those of the available synthetic fibres.However, due to the characteristics of these natural fibres, such as their affordability, environmental friendliness, and low carbon emissions, these composites are increasingly widely used in their practical applications (Prasad et al. 2021).Due to their superior physical and mechanical performance, hybrid chopped fibre reinforced with polyester resin can be utilised in place of traditional composites (Prasad et al. 2021).Hybrid composites mechanical characteristics improved when hemp and nettle fibre content was raised in polyester.In polyester hybrid composites, the hemp/nettle fibre at a greater weight percentage showed the greatest tensile, flexural, impact and hardness values (Kumar, Patel, Mer, et al. 2021;Kumar, Prasad, Patel, Kumain, et al. 2021;Kumar, Prasad, Patel, Kumar, and Yadav 2021;Kumar, Prasad, Patel, Kumar, Yadav, and Winczek 2021).

Tensile properties
The tensile strengths of the Holoptelea integrifolia fibre composites (HIFC) are shown in Figure S3.The tensile strength improved as the fibre loading grew from 20% to 40%.At lower fibre loading, the fibre's orientation may self-align to a transverse direction, resulting in a delayed increase in mechanical characteristics up to 30% fibre loading (Habibi et al. 2017).The 40% HIFC is found to have the maximum tensile strength, measuring 13 MPa.The improved cellulose micro-fibrils stretching of fibres in composites with matrix are responsible for the increase in tensile strength.However, beyond 40%, the increased fibre percentage may lead to poor adhesion, which may result reduction in the tensile property.The process of alkali treatment mostly involves surface activation, which results in the production of a rough fibre surface resulting in stronger interfacial adhesion.The load versus displacement curves of tensile-tested HIFC are shown in Figure S4(a)-(c).From Figure S4(b) and (c), it is clear that 30% and 40% HIFC have the maximum load of 0.630 KN.Also from Figure S4(c), it is clear that 40% HIFC has a maximum displacement of 2.7 mm compared with 20% and 30% HIFC.

Flexural properties
The flexural strength increased with respect to fibre loading as shown in Figure S5.With increasing fibre content flexural characteristics follow the same pattern as tensile values.The highest flexural strength is observed in fibre loading of 40%.The flexural strength of the HIFC with 40% fibre loading is 20 MPa, which is 4% greater than 30% HIFC and 10% higher than 20% HIFC.The highest value is attained for 40% HIFC because of maximal fibre occupancy that enhances flexural strength.Lower fibre loading results in a 20% reduction in flexural characteristics.The presence of components such as lignin, wax, and moisture content may cause the creation and propagation of additional fracture channels, resulting in material breaking (Jumaidin et al. 2020).The rise in flexural modulus up to 40% HIFC was seen in this study, indicating that fibre addition was advantageous in increasing flexural modulus.Further addition of fibre will cause the composite more brittle due to the inadequate bonding between the matrix and entire region of fibre.The load versus displacement curves of flexural tested HIFC are shown in Figure S6(a)-(c).From Figure S6(b) and (c), it is clear that 30% and 40% HIFC have the maximum load of 0.65 and 0.70 KN.Also from Figure S6(c), it is clear that 40% HIFC has the maximum displacement of 7.3 mm compared with 20% and 30% HIFC.

Impact strength
When comparing the impact strength of the 30% HIFC with 20% and 40% HIFC, it is clear that the 30% HIFC has the highest impact strength of 35 kJ/m 2 as shown in Figure S7.Crack propagation is prevented by homogeneous dispersion and good interfacial adhesion; high energy is easily absorbed in the 30% HIFC, resulting in enhanced impact strength.Several reasons can induce failure during impact stress, including matrix fracture, fibre pull-out, fibre breakage, and fibre/matrix de-bonding.Incorporating fibres, on the other hand, increased the impact strength of the pure plate.The presence of void might result in residual stress, which could lead to fracture development (Jumaidin et al. 2017).The impact strength of the composite, on the other hand, is reduced to 13.66 kJ/m 2 when more fibre is added (40% HIFC).The bonding is inadequate because the matrix does not cover the entire region of the fibre, making the composite more brittle (Kumar et al. 2019).Fibre size, fibre spacing and fibre percentage all have an influence on impact strength.Because HIFC contains short fibres, impact energy absorption may be reduced slightly (Jumaidin et al. 2020).

Field emission scanning electron microscopy-energy dispersive X-ray analysis evaluation
For assessing interfacial adhesion, FESEM characterisation of the specimen's was performed.Figure S8(a)-(c) shows the microstructure of 20%, 30% and 40% HIFC specimens.Small holes and bubbles can be seen in Figure S 8(a), which might be caused by predominant elements such as cellulose and hemicellulose in HIFC when the cure is taking place (Zegaoui et al. 2018).Due to the lower matrix content in the 40% HIFC, the matrix holes and pores are visible in Figure S8(b).Figure S8(c) shows the randomly organised fibres in the matrix, which are predominantly in a transverse and longitudinal pattern.The fibres exterior surface is smooth possessing fibrils, as seen in Figure S8(b).Thus from the results, it is evident that the bonding between fibre and matrix was excellent.Furthermore, the matrix penetration into the fibre prevents fibre breakage and promotes effective bonding.
An energy-dispersive spectrometer was used to characterize the surface of the HIFC.This is mainly done to identify if any additional element is present in HIFC. Figure S9 clearly shows the elemental proportion of HIFC as C 85.59%, O 14.41% and O/C ratio as 0.1683.Table S1 shows the elemental accumulation in the HIFC with line type, apparent concentration, K ratio, weight % and atomic %.It is clearly proven that the HIFC is free from other foreign particle accumulation.

Fourier transform infrared spectroscopy (FTIR)
Composite samples' FTIR spectra were obtained at room temperature.FTIR spectra were obtained to study the interactions between treated fibres and matrix material.The band at 3786 and 3220 cm À1 in the spectra of the composites was due to O-H stretching in the specimens as shown in Figure S10(a)-(c).All samples have a sharp band at 2344 and 2181 cm À1 , indicating the presence of C-H stretching.The sharp bands at 1980-1817 cm À1 reveal the quantity of moisture content consumed by noncrystalline cellulose in composites.The aromatic structure of lignin displaying different cellulosic characteristics with alkali-treated composites is revealed by the peak band at 1500 and 1400 cm À1 .The sharp band between 800 and 440 cm À1 indicates the presence of a C-O-C stretching formed by polysaccharides.

Thermal analysis (DSC-TGA)
The thermal stability behavior of 20%, 30% and 40% HIFC is shown in Figure S11(a)-(c).When the temperature reached 100 C, all of the HIFC materials tested showed the same behaviour in terms of thermal deterioration, which corresponded to a minor weight loss.The primary deterioration in HIFC occurred between 150 and 350 C, with thermal degradation of hemicelluloses.Due to cellulose breakdown and epoxy matrix was degraded, the sharp peak at 370-380 C demonstrates a weight loss of 50%.These findings are similar to those of earlier investigations on natural fibre composites.Composites with a weight proportion higher than 20% fibre have a stronger thermal characteristic compared to virgin resin, possibly because of cellulose component.Thus fibre addition improves thermal properties, as previously documented (Tajvidi and Takemura 2009;Sahari et al. 2013;Zadeh et al. 2017).It is also worth noting that composites degrade rapidly when exposed to temperatures above 210 C.

Materials
Holoptelea integrifolia is species that belongs to the family Ulmaceae.The length of the extracted raw H. integrifolia fibres ranges between 5 and 20 mm and its diameter was found to be around 15-40 lm (Balasubramanian et al. 2022).The botanist at CSMDRIA (CCRAS), Arumbakkam, Chennai, examined the stem bark of Holoptelia integrifolia and reported it using Flora of the Presidency of Madras (voucher specimen no.L/245) (Saraswathy et al. 2008).H. integrifolia tree barks were peeled off from the stem and dried in sunlight for a period of 1 week to remove moisture content present in it.After proper drying, it is then immersed in fresh water for a period of seven days for enhancing the manual retting process.The fibres are then carefully removed from the soaked bark and cleaned with plenty of water to eliminate any dirt or undesired contaminants accumulated on its surface.They are then dried in the sunshine to eliminate any moisture content (Yoganandam et al. 2020).Sodium hydroxide (NaOH) pellets, epoxy resin along with hardener and silicon wax purchased from Thirumal Nadar and sons, Madurai, Tamilnadu state, India, was employed in this work for fibre treatment and fabrication of necessary composites.

Fibre treatment
Collected fibre was treated with 5% NaOH solution (5 g of NaOH per 1 L of distilled water) for a period of 1 h in an open air container.The fibres were then carefully removed from the open air container and washed with ordinary water to remove any excess NaOH solution from the fibre surface.After proper washing, the fibres are dried in sunlight for removing the moisture content present in it completely.Fibres were cut into a length of about 3 mm for utilizing the same as reinforcement in Epoxy matrix for the fabrication of composites.Surface changes of natural fibre also play a significant role in enhancing the physical and mechanical characteristics of composite materials.Hydrogen bonds in the fibre are disturbed by alkaline treatment (NaOH), which increases the surface roughness of the fibre.The alkaline treatment removes the predetermined amount of lignin, wax, inorganic salt, and oil from the fibre's outer surface, revealing the short-length crystallites, and depolymerizing the cellulose component (Kumar, Patel, Mer, et al. 2021;Kumar, Prasad, Patel, Kumain, et al. 2021;Kumar, Prasad, Patel, Kumar, and Yadav 2021;Kumar, Prasad, Patel, Kumar, Yadav, and Winczek 2021).

Composite fabrication
A wooden mould with specifications of (250 Â 15 Â 3 mm) was created for making epoxy composite.The produced wooden mould was then covered with a layer of silicon wax, which enhances the easy removal of the epoxy composite from the mould.Utilizing a manual lay-up approach with disorderly distributed fibres, composite samples are made with various weight proportions of H. integrifolia fibres ranging from 20% to 40% at increments of 10% mass in an epoxy matrix.By weight, the resin and hardener are combined in 3:1 proportion.A roller was utilised to disperse the matrix around the mould's edges.After that, the mould was held at room temperature for 24 h under a normal compressive force assisted by fasteners to achieve consistent thickness and eliminate remaining resin mixture to ensure 100% curing of composite.Test samples of the required size were cut out of the composite with the assistance of an automatic switchboard cutting machine and a mild steel blade of 1 mm thickness.

Mechanical characterisation
As per ASTM D638 criteria, the tensile test on HIFC samples was performed using a Tensile Testing Machine setup and its details are shown in Figure S1.With 1.5 mm/min crosshead speed on specimens measuring 165 Â 13 Â 3 mm and 50 mm gauge length (Prasad et al 2022).Three samples are evaluated in each case.HIFC samples are flexural tested using the same machine with crosshead speed of 1.5 mm/min on specimens measuring 100 Â 13 Â 3 mm in accordance with ASTM D790 standards.Three samples of each kind are evaluated, with the average value recorded.To find the impact strength of the composites, unnotched HIFC specimens are prepared as per ASTM D256 standards.The specimens measure 65.5 Â 13 Â 3 mm.The test is carried out using an impact testing machine setup and its details are shown in Figure S2.

Microstructural analysis
The HIFC was morphologically characterised using a FESEM (CARL ZEISS model Gemini 300).The specimens were cleaned, gold-coated and inserted into a specimen holder in the FESEM.The morphological features of the FESEM images, such as surface morphology and interfacial adhesion, were then acquired at the microscale.The composite constituent examination is carried out by an energy dispersive spectrometer (EDS) analyzer coupled with CARL ZEISS model Gemini 300 with 20,000Â magnification, at 10.0 kV accelerating voltage and at 10.1 mm working distance.This analysis establishes the elements that are existing in the composites.

Fourier transform infrared spectroscopy
Chemical bonding in composite samples was investigated using Bruker Fourier transform infrared spectroscopy (FTIR) to look at the possible chemical linkages in treated fibre composite samples.
3.2.6.Thermal analysis (DSC-TGA) Thermal analysis TGA and DSC were used to investigate the specimen's thermal characters ranging 3-5 mg (TA Instrument, Model-STD Q600).The tests were carried out around 20-700 C at 20 C/min pace.DSC was carried out with the same equipment TA Instrument, Model-STD Q600.

Conclusions
The following conclusion was reached after analyzing tensile, flexural, and impact testing, as well as characterisation investigations.
i.The mechanical characteristics of HIFC are greatly influenced by random orientation of the fibres.The 40% HIFC have higher tensile strength (13 MPa) and flexural strength (20 MPa), whereas the 30% HIFC have higher impact strength (35 kJ/m 2 ).ii.FESEM results show that the bonding between fibre and matrix was excellent and this is because of alkaline treatment.iii.EDAX reveals that the elemental proportion of HIFC is C 85.59%, O 14.41% and free from other foreign particle accumulation.iv.FTIR spectroscopy confirms the stretching's of O-H, C-H, C-O-C, moisture content and aromatic structure present in the composites.v. Thermal analysis reveals that the primary deterioration of the composite was between 150 and 350 C. Weight loss of 50% of the composite was observed between 370 and 380 C. Thus HIFC degrade rapidly when exposed above 210 C.

Disclosure statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.