1 Introduction

Biocomposite materials play an important role in environmental preservation since they substitute natural ingredients for synthetic ones. We can find them in a wide variety of products and services, including housing, oil and gas machinery, electrical and circuit board components, aviation and military goods, and automobile engine parts [1]. The industrial revolution brought about a dramatic increase in carbon emissions, the acceleration of global warming, and the production of non-biodegradable materials as a result of manufacturing operations that made use of modern technology [2]. The life cycles of several species on our planet are impacted by all these problems. Researchers responded to these issues by implementing a plethora of measures meant to lessen pollution in the natural world. Scientists have thus devised environmentally friendly engineering materials to address these issues; these materials are biodegradable, lightweight, renewable, chemically inactive, and easy to process in comparison to the majority of synthetic plastics [3].

It is still an ongoing endeavor to develop the qualities of biocomposite material. In order to obtain fibers for biocomposite materials, a great deal of research is being done on plants, both woody and non-woody. Biocomposite materials get their components from a wide range of agricultural crop residues, including but not limited to wheat, kenaf, corn, hemp cassava, jute, and others [4]. Plants provide a number of advantages over other sources, including their abundance, high quality, quantity, and physical and chemical characteristics, including the amount of cellulose in their fiber as well as their level of polymerization [5]. There is a correlation between the crystallinity, density, and porosity levels and the manufacturing process. Furthermore, even for synthetic polymers, starch derived from natural sources is a viable substitute [6].

Corn ranks high among the world's most widely grown crops. United States Department of Agriculture (USDA) data shows that global corn production for the 2023–2024 season was 1,232.57 Mt, up 5.5% from the previous year's figures [7]. It is a non-wood plant that is widely cultivated and produces a lot of residue [8]. Corn is an excellent food choice because it is high in starch and fiber [9]. Corn silk fiber is one option that can stand in for glass or carbon fiber. The byproduct of corn silk is one of the underutilized materials that could be utilized as a composite fiber. Even after being buried for a long period, corn silk residue is not easily decomposed. This proves, with evidence, that corn silk is resistant to dampness, acids, and caustic solutions. As a result, there are several engineering applications for corn silk waste, the most promising of which is composite material reinforcing. The outer layer of hair prevents the connection from being formed when corn fiber is used in composites, which can lead to the fiber pulling out. Consequently, the fiber must undergo a chemical treatment, specifically a NaOH treatment, before being used as an amplifier in the composite. This will enhance the mechanical connection between the matrix and fiber [10].

Several studies have been conducted worldwide on composite materials preparation and performance testing. For instance, Soltannia et al. conducted a study on the static and dynamic properties of nano-reinforced 3D-fiber metal laminates utilizing non-destructive techniques [11]. Soltannia et al. studied how nano-reinforcement affected the mechanical behaviour of adhesively bonded single-lap joints under static, quasi-static, and impact loading [12]. Another study examined how strain rate affects nanoparticle-reinforced polymer composites [13]. Zamani et al. conducted an experimental investigation on the impact of hybrid graphene nano-platelet/nano-silica reinforcement on the static and fatigue life of aluminum-to-GFRP bonded joints under four-point bending [14]. Javanmard Sistani investigated the Mode I and II fracture behaviour of metal-to-composite bonded interfaces using a graphene nano platelet-reinforced structural epoxy adhesive [15]. From these studies, we can learn details about the applications, processing criteria, dispersion process, epoxy reinforcement: using synthesized, natural nanoparticles, fibers, hybrid nanoparticles, preparation process, etc. of composite materials.

Multiple research have investigated the utilization of corn silk fiber. For example, Tran et al. conducted a study to investigate the mechanical characteristics of soil reinforced with corn silk fibers, a by-product of corn [16]. The strength behavior of cemented sludge reinforced with corn silk fiber, a by-product of corn, was the subject of another investigation. Therefore, a number of unconfined compression tests were run with varying concentrations of water, cement, and fiber [17]. Another study was conducted to examine the physicochemical characteristics and anti-diabetic activities of a polysaccharide derived from corn silk [18]. Composite biodegradable films have their physical and chemical qualities improved in another study by adding corn silk and starch [19]. An independent study looked at the utilization of maize silk as a sulfur scaffold and modified materials for a commercial separator in lithium-sulfur batteries, after transforming it into porous carbon [20]. Utilizing a pair of distinct procedures, Ghareib et al. isolate cellulose and zein protein from Egyptian corn silk [21]. However, according to the past no specific study was performed about utilization of cellulose of corn silk as a reinforcing filler for unsaturated polyester resin. Furthermore, the comparison between mechanical properties of two composite one is cellulose/UPR, and another is corn silk/UPR was not found in literature. To bridge this research gap, authors conducted a thorough research on this topic.

This work aims to extract cellulose from corn silk fiber and utilize it as a reinforcing filler for UPR and also to compare the mechanical properties of composite from corn silk and UPR. The study analyzes cellulose using a Fourier Transform Infrared Spectrometer (FTIR) and evaluates the mechanical properties of the UPR/cellulose biocomposite using flexural and impact tests. This research breaks new ground by investigating a novel composite blend of UPR and corn silk, a combination previously unexplored in the literature. Notably, our findings reveal that the composite loaded with 12% fiber exhibits superior performance, representing a significant advancement in the field.

2 Materials and methods

2.1 Materials

Corn silk (CS) is depicted in below, where Figure-1(a) represents raw corn silk and Fig. 1b represents dry corn silk. The corn silk was sourced from a local corn company named Pure Crops Limited, Dhaka, Bangladesh. To ensure purity, it underwent a thorough washing with distilled water to eliminate impurities. Subsequently, the corn silk was dried in an oven at 85 °C for a duration of 20 h. The dried corn silk was then processed into short fibers through grinding. Materials utilized in the study included UPR obtained from Merck in Germany, and various chemicals such as sodium hydroxide (50% conc. NaOH), Sodium hypochlorite (12% conc. NaOCl), hydrogen peroxide (40% conc. H2O2), and hydrochloric acid (10% conc. HCl). These chemicals were procured from Marina Trading Company, Hatkhola in Dhaka, Bangladesh and supplied by Pidilite Industries Ltd in Andheri, Mumbai, India. The selection of high-quality materials contributes to the reliability and reproducibility of the experimental procedures. Figure 2 shows different compositions of final composite prepared such as (a) No fiber-loaded with UPR, (b) 12% corn silk-loaded composite, and (c) 12% cellulose-loaded composite.

Fig.1
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a Raw corn silk. b Dry corn silk

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Fig.2
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a No fiber-loaded with UPR, b 12% corn silk-loaded composite, and c 12% cellulose-loaded composite

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2.2 Corn silk treatment

For 2.5 h, ground CS was boiled and stirred in 0.5M NaOH in a beaker at 150°C, where liquor ratio was 1:25. The excess NaOH was then removed from the fiber by filtering and repeatedly washing it with deionized water. The treated CS was then crushed into flour and dried in an oven for 20 h at 85°C.

3 Cellulose preparation from corn silk

Lipid and contaminants were eliminated from ground corn silk by treating it with 0.5M HCl at a liquid ratio of 1:30 for 3.5 hours at 80°C. Subsequently, alkali boiling was done for 2.5 hours at 150°C with 0.5M NaOH (a liquor to water ratio of 1:20). After that, the alkali-treated CS was bleached for 2.5 hours at room temperature using 1% wt of NaOCl (alcohol to water ratio of 1:20). Then, 4% wt of NaOH (alcohol ratio 1:20) was used to achieve alkali extraction for one hour at 75°C. Lastly, the product was rinsed with distilled water, following each treatment to eliminate any leftover solvents. The treated CS was bleached once more using 7 g/l H2O2 (alcohol ratio: 1:15) at 80°C for 3.5 hours for three times. The concluding cellulose product underwent a drying process at 60°C overnight to achieve its final state. Post-drying, the cellulose was further processed by grinding and subsequently screened through a 60-mesh sieve for particle size uniformity. To elucidate the functional groups present in both alkali-treated corn silk (CS) and the derived cellulose, Fourier Transform Infrared (FTIR) Spectrometry was employed for characterization. This analytical technique allowed for the identification and analysis of the unique molecular vibrations associated with different functional groups, providing valuable insights into the chemical composition of the alkali-treated CS and cellulose.

4 Composite preparation and mechanical test

The compounding process involved combining Unsaturated Polyester Resin (UPR) as the matrix polymer with different proportions of alkali-treated CS or cellulose filler. The filler content ranged from 0 to 16%, with increments of 2%. The polymer composites were prepared using the hand lay-up method at 30 °C temperature. Subsequently, test specimens were created by cutting the compounded materials using a cutter machine. Figure 3 illustrates the flowchart of the overall composite preparation. The evaluation of flexural properties was conducted in accordance with ASTM D790, utilizing a Testo-metric Universal Testing Machine [22]. Additionally, impact properties were analyzed following ASTM D256 standards [23], employing a Compagnia Europea Apparecchi unscientific Torino (CEAST) Impact tester. This systematic approach allowed for the examination of the composite materials' flexural and impact characteristics, providing valuable insights into their mechanical properties.

Fig. 3
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Flowchart of composite preparation

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5 Results and discussion

Cellulose derived from corn silk (CS) underwent successful preparation and subsequent characterization, confirming the presence of functional groups. The characterization was performed using Fourier Transform Infrared Spectroscopy (FTIR), and the obtained results were compared with sodium hydroxide (NaOH) treated CS. Following this, biocomposites were developed by incorporating UPR as the matrix material, with varying proportions of fillers including UPR/treated CS and UPR/cellulose. The filler content ranged from 0 to 16% in increments of 2%. The impact of the treatment method and filler content on the mechanical properties of the biocomposites was thoroughly investigated. The mechanical properties assessed included flexural strength, flexural modulus, and impact strength. The results of these analyzes are visually represented below, providing a comprehensive overview of how the treatment process and filler content influence the mechanical characteristics of the studied biocomposites.

5.1 FTIR spectra of treated corn silk (CS) and cellulose

Figure 4a and b present the FTIR spectra of NaOH-treated CS and cellulose isolated from CS, respectively. Both spectra show the presence of C = O stretching and O–H related to hemicelluloses and lignin, observed within the ranges of 1650—1760 cm–1 and 3200—3500 cm-1, respectively. However, in the cellulose spectra, the peaks associated with these functional groups are smaller compared to those in the treated CS spectra. This suggests that the alkali extraction and repeated bleaching processes have resulted in the removal of a greater amount of hemicellulose and lignin from the cellulose structure.

Fig. 4
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a FTIR spectrum of the corn silk. b FTIR spectrum of the cellulose

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5.2 Flexural Characteristics of UPR/cellulose and UPR/Treated CS biocomposites

Flexural strength and flexural modulus of UPR biocomposites filled with treated CS were compared with those filled with cellulose at different filler loadings (2%, 4%, 6%, 8%, 10%, 12%, 14%, and 16%) as shown in Fig. 5 and 6, respectively. The findings revealed that UPR/cellulose biocomposites exhibited slightly lower flexural strength and modulus than UPR/treated CS biocomposites. This discrepancy can be attributed to the varying chemical compositions, with treated CS having higher amounts of hemicellulose and lignin compared to cellulose. Lignin in treated CS contributed to rigidity and strength, while both hemicellulose and lignin acted as adhesive agents, improving interfacial adhesion with the polymer matrix [24]. With an increase in filler loading, both UPR/treated CS and UPR/cellulose biocomposites demonstrated gradual improvements in flexural strength and modulus. This enhancement was linked to the integration of a rigid filler into the flexible UPR matrix [25]. When compared to neat UPR, UPR biocomposites with a 12% filler content showed significant enhancements. For UPR/treated CS biocomposites, there was a 13.12% increase in flexural strength and a 34.90% increase in flexural modulus. Similarly, UPR/cellulose biocomposites exhibited an 11.25% increase in flexural strength and a 36.47% increase in flexural modulus compared to neat UPR. These results underscore the potential of both treated CS and cellulose as effective reinforcing fillers in UPR biocomposites.

Fig. 5
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Variation of Flexural strength of neat UPR, UPR/NaOH-treated CS and UPR/cellulose biocomposites

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Fig. 6
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Variation of Flexural modulus of neat UPR, UPR/NaOH-treated CS and UPR/cellulose biocomposites

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5.3 Impact characteristics of UPR/cellulose and UPR/treated CS biocomposites

Figure 7 illustrates the notched Izod impact strength of UPR biocomposites filled with treated corn silk (CS) in comparison to cellulose-filled UPR biocomposites across various filler loadings. As the loading of either treated CS or cellulose increased, there was an observed enhancement in the stiffness of the interfacial area. However, this increase in filler loading also led to a weakening of the interaction between the bio-filler and the UPR matrix. Consequently, this trend contributed to a decline in the impact strength of the biocomposites. Nevertheless, it is noteworthy that the impact strength of UPR/cellulose biocomposites surpassed that of UPR/treated CS biocomposites across all filler loadings. Additionally, the findings indicated that as the cellulose loading reached 12%, the impact strength of the UPR/cellulose biocomposite approached a level comparable to that of neat UPR. This observation underscores the potential of cellulose as a reinforcing agent in UPR biocomposites, highlighting its ability to enhance impact strength and, at a certain loading percentage, even match the performance of the neat UPR matrix. This phenomenon can be attributed to the substantial cellulose content, where at 12%, the cellulose is sufficiently high, and the inherent similarities in the nature of both cellulose and UPR. At this specific loading percentage, the cellulose exhibits a more uniform distribution throughout the UPR matrix compared with other cellulose loadings, and all treated CS loadings. To elaborate, the extraction of cellulose enhances the exposure of hydrophilic cellulose on the filler surface while simultaneously reducing particle size. This dual effect contributes to an enhanced compatibility between the natural filler and the UPR matrix, promoting increased surface area adhesion. As a consequence, the improved compatibility and increased surface area adhesion facilitate more effective energy dispersion and adsorption from applied force when compared with the UPR biocomposites containing treated CS. This phenomenon is elucidated in reference [7], highlighting how the unique combination of factors at the 12% cellulose loading leads to superior mechanical performance in terms of energy dispersion and adsorption compared to other configurations.

Fig. 7
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Variation of Izod Impact Strength of neat UPR, UPR/cellulose and UPR/NaOH-treated CS biocomposites

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6 Failure mode

Upon subjecting the composites to flexure loading, the failures are observed and depicted in Fig. 8. In Fig. 8a, the absence of any reinforcement (fiber) in UPR led to failure occurring in two locations, highlighting the vulnerability of UPR. However, when fibers (Corn silk and cellulose) were added in the next cases, the failure condition was improved and appears to be similar in both Figs. 8b and 8c.

Fig. 8
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a only UPR. b UPR + Corn silk. c UPR + Cellulose

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6.1 Scanning electron microscope (SEM) analysis

Figure-9 indicates the morphology of the composites with 12% CS and Cellulose filler.

The SEM analysis was conducted with the primary objective of assessing the void fraction within the composite material. Figure 9a visually presents the outcomes of the SEM analysis, specifically focusing on the composite with the inclusion of CS filler. The image illustrates the fractures within the composite, depicting a dense morphology. This denseness is a consequence of the tensile forces exerted on the composite during the tensile strength test. The filler appears to adequately occupy and fill the entire surface of the matrix.

Fig. 9
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FE-SEM of a 12% CS/UPR composites and b 12% cellulose/UPR composites

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In Fig. 9b, the SEM analysis results for the composite with a 12% composition are presented. The image specifically focuses on the fractured section, revealing a composite surface where cellulose functions solely as a matrix strengthener. The SEM image indicates that the cellulose filler did not adequately occupy all available spaces within the matrix, leading to regions where the matrix remains unfilled.

7 Conclusions

The effective extraction of cellulose from corn silk (CS) was prepared, and it was confirmed by the reduction of the ketone (C = O) and hydroxyl (-OH) groups in FTIR spectra. The study revealed that the increase in flexural properties, including modulus and strength, in UPR/cellulose biocomposites at higher cellulose loadings is comparable to that observed in UPR/treated CS biocomposites. Despite the reduction in Izod impact strength resulting from the inclusion of fillers such as treated CS or cellulose, particularly due to the introduction of rigid fillers into the ductile UPR, it was observed that a cellulose content of 12% managed to maintain the Izod impact strength of UPR biocomposites at a level similar to that of pure UPR. At a 12% filler composition, it is evident that the filler adequately covers the entire surface of the matrix. However, the distribution of the fillers within the matrix is not uniform at this composition. These findings suggest that a straightforwardly prepared cellulose from CS can serve as a viable alternative in biopolymer composites, enhancing strength and stiffness while preserving the ductile characteristics of UPR. The properties exhibited by12% fiber-loaded corn silk-reinforced composites make them well-suited for applications, such as decorative items, food trays, interior panels, and more. Consequently, these composites have the capacity to supplant conventionally used materials in such applications, enhancing the overall product quality.

8 Limitations and future research direction

Every research has some limitations, this research is not out of them. This study only deals with flexural strength, flexural modulus, and impact strength. Other mechanical properties like tensile strength, thermal study, etc. could be covered in further studies. Authors used only UPR as matrix for both reinforcement. Further research may use other polymers as matrix and compare their results with this study.