Reforming composite materials: Nano chitosan from Crassostrea madrasensis oyster shell waste as a green toughener for unsaturated polyester resin

Abstract

This work demonstrates how shell waste can be upcycled into advanced polymer composites, thereby contributing to sustainable materials research. In this study, waste oyster shells were employed as a sustainable source for the preparation of nano chitosan, which was successfully used as a reinforcing agent in unsaturated polyester resin composites. We prepared chitin from Crassostrea madrasensis oyster shell wastes, and nano chitosan was obtained by deacetylating the synthesized chitin with sodium hydroxide under autoclave conditions. Characterization techniques such as FTIR, XRD, SEM, and TEM clearly indicate the formation of chitin and chitosan, and TEM observations evidently specify the formation of nano chitosan flakes. We developed a composite material by incorporating unsaturated polyester resin with nano chitosan. Mechanical properties of unsaturated polyester resin are found to be enhanced by the addition of nano chitosan. Utilising this biopolymer derivative to improve the properties of unsaturated polyester resin (UPR) composites presents an innovative avenue for environmentally sustainable product development. These nano chitosan–UPR composites could potentially find applications in environmentally friendly coatings, structural materials, and bio-based polymer reinforcement.

Keywords

unsaturated polyester resin oyster shell waste nano chitosan mechanical properties

Introduction

The concept of converting waste into valuable products has gained significant interest due to the demand for sustainable alternatives. Nano chitosan, derived from Crassostrea madrasensis oyster shell wastes, signifies a considerable development in current materials research focused on enhancing environmental sustainability. This biopolymer derivative enhances the mechanical properties of composites derived from unsaturated polyester resin. It addresses the critical issue of waste management.

Chitin and chitosan are biodegradable, non-toxic materials mostly obtained from the shells of crustaceans (shrimp, crab, krill), squid bones, insect exoskeletons, and fungal cell walls. It is primarily produced from seafood waste as a byproduct of the fishing industry. Even though many studies have explored their production, a large amount of this waste is still unused, and most methods have not yet been developed for large-scale industrial use. Huge amounts of oyster shell waste produced from the seafood industry cause significant environmental challenges. The production of chitosan, a valuable biomaterial from the oyster shell waste, helps to reduce environmental pollution. The Crassostrea madrasensis oyster, found along coastal regions, serves as a source of nutrition, and its shell waste comprises chitin. This study is the first to report the production of chitosan from Crassostrea madrasensis oyster shell waste, highlighting its novelty and potential for environmental sustainability.

Chitin and chitosan find applications in a range of industries. Their distinctive characteristics, along with biocompatibility and biodegradability, make them valuable resources in fields such as medicine, agriculture, food production, water treatment, and numerous other industries. These materials have the potential to promote environmentally sustainable innovation and support a range of initiatives aimed at improving human health and the environment.

Chitin is the second most common biopolymer in the world after cellulose, making it a captivating subject of research. Structurally, it is a linear polysaccharide composed of repeating units of β-(1 → 4)-2-acetamido-2-deoxy-β-D-glucose, characterized by an acetamide group at the C₂ position (Figure 1).¹ Unlike the acetamide group in chitin, its derivative, chitosan, has a distinct identity. Chitosan’s structure features repeating units of β-(1 → 4)-2-amino-2-deoxy-β-D-glucose, each containing a strong primary amine group (Figure 1).² The transition from chitin to chitosan involves the deacetylation of N-acetylglucosamine units, which alters solubility, crystallinity, and mechanical behavior. The literature demonstrates that both molecular weight (MW) and degree of deacetylation (DDA) are critical parameters that govern the properties of chitosan and its performance in composites. Higher DDA increases the number of free amino groups, improving solubility in acidic media and enabling stronger interfacial interactions with polymers or nanoparticles. Higher MW chitosan exhibits greater chain entanglement, leading to increased viscosity and stronger mechanical properties in films or composites. Very high MW can reduce processability, while very low MW may reduce mechanical reinforcement ability in composites. Optimal combinations of MW and DDA are needed for effective dispersion in polymer matrices, proper interfacial bonding, and enhanced toughness, strength, or barrier properties.³

The nano chitosan prepared in this study exhibited a degree of deacetylation (DDA) of ∼82%, which provides a high density of free amino groups to enhance solubility and chemical reactivity.⁴ Using viscometry and the Mark–Houwink equation, the viscosity-average molecular weight (Mv) was estimated to be approximately 100 kDa (∼100,000 g/mol.), corresponding to a degree of polymerization (DP) of ∼593 units (Supplementa l Material). These molecular characteristics indicate sufficient chain length and functional group density to ensure effective dispersion and strong interfacial bonding within the unsaturated polyester resin matrix. The combination of measured DDA and estimated MW provides a quantitative basis to correlate chitosan’s structure with its performance in polymer composites, supporting its role in enhancing mechanical properties.⁵ Unsaturated polyester resin provides excellent mechanical strength and easy processing, making it an ideal matrix for nano chitosan.

Unsaturated polyester resins (UPR) are a type of linear, short-chain condensation polymer that is cured by a free radical source. Maleic anhydride, o-phthalic anhydride, and 1, 2-propanediol are the most common components of UPR (Figure 2). Considered a very adaptable material, UP resins are still actively employed in a wide number of applications, spanning from household to the aircraft sector. Major use occurs in the marine, construction, and transportation industries. UPRs are appealing due to their low production costs, ease of handling, and rapid curing via a free-radical mechanism.⁶ Recent studies highlight efforts to enhance its performance and sustainability through nanofiller reinforcement and hybridization with bio-based materials, expanding its role in advanced functional composites.⁷ Rice-husk-derived biosilica nanoparticles incorporated into a bio-based UPR matrix improved tensile strength by nearly 88% at an optimal loading of 2.5 wt%, owing to good dispersion and strong interfacial bonding.⁸ Similarly, eggshell-derived CaCO₃ fillers enhanced impact resistance, demonstrating improved energy absorption even with slight reductions in tensile strength.⁹ Modified lignin and cellulose-based fillers have also been used to adjust flexibility and toughness, illustrating that bio-derived fillers can simultaneously reinforce UPR and increase its ductility.¹⁰ Generally, bio-based nanofillers, especially nanocellulose and nano chitin, have compatibility issues with hydrophobic unsaturated polyester resins. Olja J. Pantic et al. introduced a method to address this challenge in which dimethyl itaconate acts as an esterification agent and solvent. The subsequent reaction mixture was employed directly as a reactive diluent, eliminating the need for a harmful solvent and an additional purification step. The addition of such modified nanocellulose leads to outstanding performance enhancements.¹¹ Slavko Mijatov developed green composites using camphoric acid-based unsaturated polyester resin and reinforced with non-woven bamboo/flax mats, showing improved mechanical properties.¹² Ahmad H Ragab et al. developed a novel self-cleaning nanofiltration membrane for the efficient removal of the cationic dye methylene blue (MB) from industrial wastewater. The membrane is composed of vinyl resin (VR), cellulose nanofibrils (CNF), and titanium alpha aluminate (TAAL) nanoparticles.⁴ Mahmoud F Mubarak et al. demonstrate the exceptional ability of an integrated CDs@ MS-NF nanocomposite coating to mitigate mineral scale formation and reduce membrane fouling under environmentally relevant scaling conditions.⁷

The increasing demand for enhanced mechanical properties and environmental sustainability in sectors such as construction, automotive, electronics, and medical devices has led to the advancement of bio-based polymer nanocomposites, commonly referred to as biocomposites. In this context, Hyeri Kim and colleagues have demonstrated the effectiveness of chitosan nanowhiskers (CsWs) sourced from abundant chitin as a promising nanofiller to enhance the strength and durability of poly (butylene succinate) (PBS). The distinct polar surfaces of CsWs engage intensely with polymer chains, fostering a fibrillar and micro-void architecture that enhances the filler’s ability to reinforce the polymer matrix. This distinctive, all-organic nanocomposite exhibits exceptional resistance to external tensile and tear stresses, positioning it as a promising solution for the creation of environmentally friendly, biodegradable disposable products.^5,14 A novel flame retardant, CS-MA-SPEPA, derived from chitosan, was developed by Jiang et al. The incorporation of this material into unsaturated polyester resin significantly reduced flammability, inhibited the emission of harmful gases, and enhanced char formation, demonstrating the potential of chitosan-derived additives to improve fire resistance in unsaturated polyester matrices.¹⁵

The addition of nano-chitosan as a filler in polymer-resin systems offers great potential, but its effectiveness is strongly governed by how well the particles are dispersed and how compatible their surface chemistry is with the resin matrix. Poor dispersion or weak filler–matrix adhesion leads to agglomerates, voids, and stress-concentration sites, which undermine mechanical performance. Ensuring uniform nano-chitosan distribution and strong interfacial interaction (for example via amino–ester bonding or hydrogen-bonding with the unsaturated polyester resin matrix) is therefore critical for achieving reinforcement rather than deterioration.^16,17

Islam et al. (2024)¹⁸ developed polyester/chitosan/silver nanocomposites and observed significant improvements in tensile strength and antimicrobial activity with a 2.5 wt% chitosan loading. Other natural fillers, such as nanoclay and cellulose nanocrystals (CNCs), have been extensively studied within UPR systems. Nanoclay improves thermal and barrier properties but often requires surface modification to ensure proper dispersion and compatibility with hydrophobic matrices.^19,20 CNCs exhibit significant stiffness and reinforcement capabilities; however, they tend to encounter issues with moisture absorption and agglomeration. In contrast, nano chitosan combines mechanical strength, processability, and environmental friendliness, mainly when derived from marine biowaste. Nanoclay plays a crucial role in enhancing both thermal and barrier properties; however, dispersion challenges often necessitate the use of surface modification techniques. CNCs provide notable stiffness and reinforcement capabilities; however, their high hydrophilicity can lead to moisture sensitivity and processing challenges. The eco-friendly and functional benefits of chitosan have led to its application in various composite systems.²¹ Wolanski et al. (2023)²² created UV-cured acrylate/chitosan composites and observed improvements in thermal stability and surface characteristics. Sarmin et al. (2023)²³ demonstrated that chitosan significantly enhanced the storage modulus and viscoelastic properties in bio-epoxy/date-palm fibre composites.

Fragassa et al. (2025)²⁴ initiated their investigation by examining the effects of saltwater immersion on the mechanical properties and longevity of bio-composites strengthened with powdered seashell fillers, including those from mussel, oyster, and clam shells. Their findings indicated that although saltwater exposure causes matrix degradation, the addition of appropriate quantities of seashell powder can improve stiffness and toughness, thus partially alleviating deterioration effects. This research highlights the potential of utilizing seashell-derived waste in the development of sustainable composites, particularly for marine or coastal applications. Despite this advancement, there is a notable scarcity of studies that have employed nano chitosan specifically sourced from marine biowaste, especially from the shells of Crassostrea madrasensis oysters. Transforming shell waste into nano chitosan and incorporating it into UPR enhances mechanical properties and promotes the reuse of marine waste. This research addresses a notable gap by presenting a dual-benefit strategy that enhances polymer matrices and supports environmental sustainability. Converting oyster shell waste into nano chitosan for UPR is an eco-friendly method that aims to strengthen composites and reduce marine pollution.²⁵

The objectives of this paper are to prepare chitin and nano chitosan from discarded Crassostrea madrasensis oyster shell wastes, to characterise the prepared samples by using IR, XRD, SEM and TEM, to reinforce the unsaturated polyester matrix using the prepared nano chitosan filler, and to study the mechanical properties of unsaturated polyester-nano chitosan composites and are found to be enhanced. This dual strategy reflects our dedication to environmental responsibility while fostering material innovation.

Materials and methods

Materials

Crassostrea madrasensis oyster shells were collected from the Vembanad lake shore, Kochi, Kerala, India. UPR, methyl ethyl ketone peroxide, cobalt octoate were obtained from Riotech limited. Pala and other chemicals were used as received. Essentially UPR is a polystyrene cross-linked terephthalic and maleic ester of PPG with Mn-2600 g/mole [information from suppliers] and unsaturation eqv. Weight of 401 g/equiv. Styrene content: 35–37% by weight of the polymer. Reagents like hydrochloric acid, sodium hydroxide, and hydrogen peroxide were sourced from Merck India Pvt. Ltd.

Methods

Preparation of chitin

Preparation of Crassostrea madrasensis oyster shell powder involved thoroughly washing the shells multiple times with distilled water and oven-drying at 100°C for 24 h. Once dried, the shells were ground into a fine powder.

For alkaline treatment, 100 g of this oyster shell powder were autoclaved with a 1:10 mixture of 1M NaOH solution (10 ml per gram) at 10 bar (150 psi) for 60 min. Afterward, the mixture was cooled to a neutral pH of 7, then filtered and washed with distilled water. In the acid treatment step, the alkaline-treated powder was subjected to autoclaving again with 1M HCl (10 ml per gram) at 10 bar (150 psi) for 60 min. The final product was carefully filtered and washed to remove all acid residues, resulting a pH-neutral powder. Then the obtained chitin residue was bleached using acetone. A solid-to-liquid ratio of 1:1 w/v was used (100 g chitin in 100 ml acetone), and the mixture was stirred at room temperature (∼25°C) for 30 min. The bleaching step was repeated three times until the chitin achieved a bright white color. After each bleaching cycle, the chitin was filtered and washed with fresh acetone. Finally, the bleached chitin was oven-dried at 50–60°C until constant weight.

After the bleaching process, the powder that was left over was autoclaved with 3M HCl at a ratio of 1:10 g/ml for 60 min. After centrifuging the suspension (5000 rpm), it was dialyzed against running water until a pH of 7 was reached. The suspension was homogenized and lyophilized.

Preparation of nano chitosan

The obtained chitin was subjected to autoclave-assisted alkaline treatment for deacetylation to produce nano-chitosan. The reaction was carried out using 50% (w/v) NaOH solution under a pressure of ∼10 bar (150 psi) for 60 min. After that, the chitosan is cleaned by being washed and rinsed in hot distilled water and heated to 90°C to remove any remaining sodium hydroxide. After autoclave-assisted deacetylation, the resulting solution was centrifuged and dialyzed against distilled water to remove residual alkali and salts. Cellulose dialysis tubing with a molecular weight cutoff (MWCO) of 12–14 kDa was used. The solution was dialyzed at room temperature for 48 h, with the external water replaced every 6–8 h to ensure complete removal of impurities. This step ensures that the final product is free from residual NaOH and salts, improving purity and reproducibility, and then drying it for 16 h at 50°C.

Preparation of nano chitosan – UPR composite

The prepared nano chitosan was slowly added to the resin under continuous mechanical stirring at 500–800 rpm at room temperature (∼25°C) for 45 min. After homogenization and ultrasonication, the initiator ethyl methyl ketone peroxide (1.3% by weight) and the promoter cobalt octoate (0.5% by weight) were added. The final mixture was then carefully poured into a Teflon mould that had been treated with a releasing agent, and left to sit for 24 h at room temperature to begin the initial curing step. After that, post-curing was performed in an air oven at 80°C for 3 hours to facilitate complete crosslinking and consolidation. The composite was carefully cooled until it reached room temperature. Controlling stirring speed and temperature during mixing and curing ensures homogeneous dispersion of nano-chitosan, prevents aggregation, and leads to consistent mechanical properties.

Characterization

An FTIR spectrophotometer (IR Tracer-100, SHIMADZU) was used for the FTIR investigations.

Utilising a Hitachi SU6600 FESEM scanning electron microscope, the surface morphology was examined. Samples were dried in a hot-air oven at 50–60°C. A small portion was mounted on aluminum stubs using conductive carbon tape and sputter-coated with gold (∼5 nm) to prevent charging.

A JEOL JEM 2100 high-resolution transmission electron microscope was used for TEM imaging. A dilute suspension of nano-chitosan (0.1–0.5 mg/mL) in distilled water was prepared. A drop of this suspension was placed on a carbon-coated copper grid and air-dried.

A Bruker AXS D8 Advance X-ray powder diffractometer was used for the XRD analysis. Samples were finely ground into powder using a mortar and pestle. Powder was evenly spread on the sample holder to form a smooth surface. Diffraction patterns were recorded using Cu-Kα radiation (λ = 1.5406 Å) over a 2θ range of 5°–50° at a scanning rate of 2°/min.

Tensile properties were assessed using a dumbbell-shaped specimen, following ASTM D638 Type V, and a Shimadzu Universal Testing Machine (UTM) equipped with a 10 kN load cell, at a displacement rate of 5 mm/min, at room temperature.

According to ASTM D 790, the flexural characteristics were measured using rectangular bars with dimensions of 127 mm × 12.5 mm × 4 mm on the same machine, moving at a speed of 5 mm/min.

At least five coupons were evaluated for each datum.

Using unnotched samples, the impact strength was determined according to ASTM D 4812–99. A sample is fixed vertically within the machine’s base. They release the pendulum. The tester directly reads the impact readings to determine the impact resistance or strength.

Impact Strength (E × 1000)/(b × d) kJ/mm². E Impact value.

b width of the specimen, and d its thickness.

All mechanical measurements were conducted under controlled conditions (25 ± 2°C, 50 ± 5% RH) to minimize the influence of ambient humidity and temperature on polymer matrix performance.

Results and discussion

FTIR analysis

The IR characterization of the chitin was conducted using an FTIR spectrophotometer (IR Tracer-100, SHIMADZU) within a frequency range of 4000–400 cm⁻¹. Figure 3 demonstrates the spectrum absorption patterns, which align with those identified in earlier studies, suggesting the successful production of high-quality chitin biopolymers. An in-depth examination of the spectrum indicates the presence of bands typical of chitin. A band appears in the spectra at 3415 cm⁻¹, which is prominently observed in the chitin spectrum, indicating the stretching vibration of the aliphatic O–H bond. The absorption peak at 2926 cm⁻¹ is attributed to the C–H vibration of –CH₂ and –CH₃ groups. The absorption band at 1633 cm⁻¹ corresponds to the stretching vibration of the carbonyl group, C=O, in acetamide (−NHCOCH₃). Characteristic absorptions for chitin are observed at 1550 and 1315 cm⁻¹, indicating the bending vibration of –NH and the stretching vibration of –CN from the acetamide group, respectively. The absorption peak at 1040 cm⁻¹ corresponds to the stretching vibration of the C-O-C in the glucosamine ring, while the peak at 878 cm⁻¹ characterises the ring stretching associated with β-1,4 glycosidic bonds. Acosta et al. (1993),²⁶ Abdou et al. (2008),²⁷ Lima and Airoldi (2004),²⁸ Liu et al. (2012),²⁹ and Zaku et al. (2011)³⁰ have also contributed evidence supporting this finding. The subsequent details outline the primary spectrum characteristics of chitosan (Figure 3): 3447 cm⁻¹ (O–H stretch overlapped with N–H stretch and inter-hydrogen bonds of the polysaccharide), 2918 and 2878 cm⁻¹ (C–H stretch), 1654.53 cm⁻¹ (amide I band, C=O stretch), 1427 cm⁻¹ (NH₂ bending), 1383 cm⁻¹ (amide III band, C–N stretch), 1323 cm⁻¹ (C–H bending), 1259 cm⁻¹ (C–N stretch), 1155 cm⁻¹ (bridge C–O–C stretch), and 1082 cm⁻¹ (C–O stretch). The findings of Suneeta et al.³¹ also supported this.

In the chitin spectrum, the distinct amide I and amide II bands appear prominently, confirming the presence of the acetamido groups (–NHCOCH₃) in the polymer chain. The amide I band (1633 cm⁻¹) is primarily associated with the C=O stretching vibration of the amide group. The amide II band (1550 cm⁻¹) arises from the N–H bending vibration coupled with C–N stretching, which is typical of the acetamide group in chitin.

In contrast, in the FTIR spectrum of chitosan, the amide II band (∼1550 cm⁻¹) becomes significantly weaker or may even disappear. This change is a key spectral indicator of deacetylation, where the acetamido groups (–NHCOCH₃) of chitin are converted into amino groups (–NH₂) during alkaline treatment. The removal of acetyl groups reduces the intensity of the amide II band, while new peaks related to N–H bending of primary amines appear near 1590–1650 cm⁻¹, overlapping or masking the amide region (Table 1).

Table 1.

Comparative FTIR absorption bands of chitin and chitosan with references.

Functional group/Assignment	Chitin (cm⁻¹)	Chitosan (cm⁻¹)	Interpretation/Change	References
O–H/N–H stretching	∼3415	∼3447	Broad peak due to hydrogen bonding; increased overlap in chitosan	^26,27,31
C–H stretching (–CH₂, –CH₃)	∼2926	∼2918, 2878	Similar peaks with slight shifts	^28,29
Amide I (C=O stretching)	∼1633	∼1654	Slight shift; partial retention of acetyl groups	^27,29
Amide II (N–H bending, C–N stretching)	∼1550	Reduced/absent	Decrease confirms deacetylation	^26,30,32
NH₂ bending (primary amine)	—	∼1427	Appears due to the formation of amino groups	^31,33
Amide III (C–N stretching)	∼1315	∼1383	Shift indicates structural change	^28,32
C–H bending	—	∼1323	Observed in chitosan	³¹
C–N stretching	∼1315	∼1259	Shift due to deacetylation	^29,34
C–O–C (glycosidic bridge)	∼1040	∼1155	Backbone structure retained	^27,28
C–O stretching	∼1040	∼1082	Slight variation due to structural rearrangement	²⁹
β-1, 4 glycosidic bond	∼878	—	Characteristic of the chitin backbone	³⁰

These spectral shifts collectively confirm the effective deacetylation of chitin to chitosan and justify the reduction or absence of the amide II band in chitosan samples (Figure 4).^4,32–34

XRD analysis

Two peaks at approximately 9°–10° and 19°–21° are observed in the 2θ XRD examination of chitosan, which shows semi-crystalline chitosan. Islam et al.³⁵ observed broad diffraction peaks at 2θ = 10° and 21°, indicating semi-crystalline chitosan. Two characteristic crystalline peaks of chitosan at 9– 10° and 19–20° were reported by Yen et al.³⁶ Trung et al. also confirmed the presence of two identical peaks approximately at 10° and 20° for chitosan samples.³⁷

The XRD pattern of the synthesized chitosan (Figure 5) displays a broad diffraction peak centered around 2θ ≈ 20°, with a weak shoulder near 9–10°, corresponding to the (110) plane of chitosan. The broadness and reduced peak intensity compared to native chitin indicate partial loss of crystalline order following deacetylation. Using the Segal method, the crystallinity index (CrI) was calculated as:

where Ic and Iam represent the maximum intensity of the crystalline peak (at 2θ ≈ 20°) and the minimum intensity at the amorphous region (at 2θ ≈ 12°), respectively. The estimated CrI was approximately 60%, confirming the semi-crystalline nature of the chitosan sample.

C r I (%) = \frac{I c - I a m}{I c} \times 100

This reduction in crystallinity compared to chitin (which typically exhibits CrI values around 70–80%) reflects the disruption of hydrogen bonding and ordered packing during alkaline deacetylation, converting acetamido groups to amino groups. These results are consistent with previous studies.^38–40

The apparent crystallite size was estimated using the Scherrer equation:

2 θ = 20 °

Full width at half maximum (FWHM) = 5.43°.

D = K λ / β \cos θ

where D is the apparent crystallite size, K is the shape factor (0.9), λ is the wavelength of Cu Kα radiation (0.15,406 nm), β is the full width at half maximum expressed in radians, and θ is the Bragg angle.

The calculated apparent crystallite size was approximately 1.49 nm. This value represents the size of coherent crystalline domains within the chitosan structure rather than the actual particle size of the material. The small crystallite size suggests the presence of highly dispersed ordered regions embedded within a predominantly amorphous polymer matrix. Similar observations have been reported for nano-structured chitosan materials, where extensive disruption of crystalline packing results in broad diffraction peaks and reduced crystallite dimensions.

SEM and TEM analysis

The SEM micrographs of chitin and chitosan reveal distinct morphological differences resulting from deacetylation. The SEM images of chitin (Figure 6) show irregularly shaped particles with compact, dense structures. In contrast, the SEM images of chitosan (Figure 7) exhibit a noticeable change in surface morphology. The chitosan sample displays a fibrillar, sheet-like, and porous structure with visible surface irregularities. The rough, fibrillated surface observed in chitosan suggests increased surface area and enhanced accessibility of amino groups generated during deacetylation.^36,41–43 TEM investigation revealed a layered, flaky structure in nano-chitosan.^44,45

HR-TEM images revealed that the CH-NPs primarily exhibit a nano-layered flaky structure, with sizes varying from 10 to 100 nm (Figure 8), offering significant insights into the internal structure of chitosan. The TEM micrographs reveal a layered structure within the chitosan particles. The layers manifest as separate lamellae arranged in a stacked formation, resulting in a flaky morphology. This layered structure indicates a meticulously organised and hierarchical arrangement at the nanoscale. The distinct layers exhibit a flaky structure, resembling thin, flat sheets. The boundaries of these flakes are distinctly outlined, enhancing the overall flat characteristics of the chitosan structure. This notable flakiness suggests the material’s capacity to create organised structures at the nanoscale. The identified layered flaky structure of chitosan presents noteworthy implications for its functional properties. This nanostructure has the potential to affect the mechanical strength, permeability, and interactions of the material with other substances. The layered arrangement may influence chitosan’s potential applications in fields such as drug delivery, nanocomposites, or as a framework for the synthesis of additional nanomaterials.

Mechanical properties

In terms of tensile, flexural, and unnotched impact capabilities, the effect of nano chitosan flakes on the UPR matrix’s mechanical performance was assessed. Every test involved the analysis of at least six distinct specimens. Figures 9 and 10 illustrates the variation in tensile and flexural strength of the produced nanocomposites. The figures demonstrated the toughening impact of the nano chitosan flakes. The following conclusions can be drawn from an analysis of these figures:

• The cured neat UPR showed tensile and flexural strength values of around 50 and 90 MPa, respectively. It was discovered that the cured pure UPER’s values were similar to those reported in the literature.

• The percentage increase in tensile strength, flexural strength, and impact strength of nano chitosan- UPR composites in comparison to the neat UPR is shown in Figures 9, 10 and 11.

The composite system exhibits a notable enhancement in tensile, impact, and flexural properties when nano-chitosan flakes are incorporated into the unsaturated polyester resin. The incorporation of 3% nano chitosan exhibited an increasing trend. The composites demonstrated enhancements in tensile, flexural, and impact strength of 70%, 89%, and 81%, respectively, when compared to pure UPR at this weight (Table 2).⁴⁶

Table 2.

Mechanical Properties of UPR-Nano chitosan composites.

Composite (UPR–Nano chitosan)	Tensile strength (MPa)	Flexural strength (MPa)
0% (UPR)	50 ± 2	90 ± 3
1%	58 ± 2	120 ± 3
2%	68 ± 1	150 ± 2
3%	85 ± 2	140 ± 3
4%	67 ± 1	123 ± 4
5%	53 ± 2	110 ± 3

The enhancement in tensile and flexural properties observed in the present study agrees with previous investigations on chitosan-polymer composites. Kumar et al. reported that the incorporation of nano-chitosan into polymer matrices improved mechanical strength due to better dispersion and increased polymer-filler interactions. Similarly, Wan Ngah et al. observed that chitosan-based nanofillers act as effective reinforcing agents by restricting polymer chain mobility and improving load transfer efficiency.^47,48

The addition of nano-chitosan flakes to unsaturated polyester resin (UPR) improved its mechanical properties. This can be attributed to several factors, chiefly the distinctive characteristics of nano-chitosan and the interactions between chitosan and the UPR matrix. In the composite system, nano-chitosan functions as a reinforcing agent. The uniform dispersion of nano chitosan within the UPR matrix is facilitated by their small size and considerable aspect ratio. The dispersion markedly enhanced the tensile, flexural, and impact characteristics of the composite, promoting more efficient stress distribution throughout the material. Due to its nanoscale dimensions, nano chitosan typically exhibits a significantly large surface area. The increased surface area facilitates the formation of additional sites for interaction with the UPR matrix, thereby enhancing the adhesion of chitosan and polymers. The enhancement of mechanical properties can be ascribed to an increase in adhesion levels. The improvement in mechanical properties at lower chitosan loadings is attributed to better dispersion and effective stress transfer between the matrix and filler, whereas higher loadings may lead to agglomeration, resulting in reduced performance.

The structure of chitosan features amino groups (-NH₂) that are capable of forming hydrogen bonds. Hydroxyl groups (-OH) and carbonyl groups (C=O) are commonly found in unsaturated polyester resins. The amino groups of chitosan and the functional groups of UPR can form hydrogen bonds, enhancing intermolecular interactions and thereby increasing the overall strength of the composite. Covalent bonds may form between reactive groups in chitosan and UPR under specific circumstances. For example, the amino groups of chitosan and the double bonds of unsaturated polyester resin may engage in reactions that lead to the formation of covalent bonds. The chemical bonding can enhance the mechanical properties of the composite, creating a strong connection between chitosan and the UPR matrix. Due to the presence of abundant hydroxyl and amine groups, hydrogen bonding in chitosan- and natural fiber-based composites is closely associated with increased moisture absorption, which can significantly influence interfacial adhesion and mechanical performance.⁴⁹

In this work, the interfacial cross-linking is understood to proceed via the primary amine (–NH₂) and hydroxyl (–OH) groups of the Chitosan backbone, reacting with the unsaturated and ester-functional segments of the Unsaturated Polyester Resin (UPR).⁵⁰ Specifically: The chitosan –NH₂ groups may form covalent amide or imine-type linkages with carboxylic anhydride/acid (–COOH/–COO–) groups or reactive maleate/fumarate double bonds in the UPR pre-polymer chain. The chitosan –OH groups (at the C3 or C6 positions) may engage in esterification or transesterification with ester moieties (–COO–) of the UPR, or participate in hydrogen-bond networks with carbonyl (C=O) or unsaturated (C=C) segments of the UPR matrix. On the UPR side, the unsaturated diacid or anhydride units (e.g., maleic/fumaric) provide vinyl (C=C) sites for radical‐crosslinking with styrene or other diluents, and the ester (–COO–) linkages in the polyester backbone provide polar carbonyl (C=O) sites capable of hydrogen-bonding or reacting with nucleophilic –NH₂/–OH from chitosan.

Conclusion

In the present study, nano chitosan was successfully synthesized from waste oyster shells and utilized as a reinforcing filler in unsaturated polyester composites. FTIR studies confirmed the deacetylation of chitin to chitosan, while XRD analysis revealed the semi-crystalline nature of the synthesized nano chitosan with highly dispersed crystalline domains embedded within an amorphous matrix. SEM and TEM analyses demonstrated significant morphological transformation and the formation of aggregated nano-flaky structures. The incorporation of nano chitosan improved the mechanical performance of the unsaturated polyester composites, indicating effective interaction between the nano filler and the polymer matrix. The inclusion of nano chitosan significantly improved the mechanical strength of UPR. This makes it particularly suitable for applications where toughness and resilience are crucial, especially in the transportation sector. Furthermore, the use of seafood shell waste as the precursor material provides an environmentally friendly and economically attractive approach for producing value-added biomaterials.

Despite these promising results, several challenges remain for the widespread application of nano chitosan-reinforced polyester composites. Realizing uniform dispersion of nano chitosan within the polymer matrix and minimizing particle agglomeration are important factors that require further optimization. In addition, large-scale production methods, detailed techno-economic analysis, long-term durability studies, and evaluation of thermal and environmental stability are necessary before industrial commercialization. Future research should also focus on surface modification of nano chitosan to improve interfacial bonding, optimization of filler loading, and the development of sustainable composite materials for engineering, biomedical, and environmental applications.

Supplemental material

Supplemental Material - Reforming composite materials: Nano chitosan from Crassostrea madrasensis oyster shell waste as a green toughener for unsaturated polyester resin

Supplemental Material for Reforming composite materials: Nano chitosan from Crassostrea madrasensis oyster shell waste as a green toughener for unsaturated polyester resin by Jeemol PA, Shibinimol PA, Fida Jabir in Polymers from Renewable Resources

Footnotes

Acknowledgements

We gratefully acknowledge the assistance of AI-powered tools in improving the language, grammar, and clarity of the manuscript

ORCID iD

Jeemol Poovathungal Abdulrahman

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Financially supported by the Muslim Educational Society, Kerala.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Supplemental material

Supplemental material for this article is available online.

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