Enhancing mechanical, thermal, and flame-retardant properties of polyester/flax composites with magnesium and calcium hydroxide nanoparticles

ABSTRACT

This study investigates the synergistic effect of magnesium hydroxide (MH) and calcium hydroxide (CH) nanoparticles on the mechanical, thermal, and flame-retardant performance of polyester / flax composites. Composites with 45 wt.% flax fabric and varying nanoparticle loadings (1–3 wt.%) were fabricated using the hand lay-up method. FTIR (Fourier transform infrared) spectroscopy confirmed strong chemical interactions between the nanoparticles and the polyester matrix, while SEM (Scanning electron microscope) revealed uniform nanoparticle dispersion and improved interfacial bonding. The optimized 2 wt.% hybrid composite exhibited notable improvements, including 37.7% higher tensile strength, over 320 % increase in flexural strength, enhanced impact resistance, and superior thermal stability with a 4.3 % rise in onset degradation temperature. Flame-retardant tests demonstrated significant improvements, with composites achieving LOI (Limited Oxygen Index) values above 24.4%, UL-94 V-0 rating, and HB classification, indicating excellent self-extinguishing behavior and reduced burning rates (<45 mm/min). The combined action of MH and CH nanoparticles promoted char formation, reduced heat release, and improved ignition resistance, surpassing the performance of single-filler systems. These results highlight the potential of dual nanoparticle reinforcement for developing lightweight, sustainable composites with balanced mechanical, thermal, and fire - safe properties, suitable for safety-critical applications in automotive, aerospace, and construction sectors.

Keywords:
Flax fiber; Mechanical properties; Fire retardant; Thermal properties; Magnesium Hydroxide.

1.INTRODUCTION

Natural fibers, including flax, jute, and coir, are renewable and environmentally benign materials that are frequently used in composites for aerospace, construction, and automotive applications. Flax fiber is an excellent choice for lightweight applications, enduring composites due to its exceptional mechanical properties, like high tensile strength and stiffness. The 45wt.% flax matrix composites showed magnificent improvement in mechanical properties [¹]. With flame retardant treatments, flax fiber composites achieve enhanced fire resistance, making them suitable for safety-critical applications. Calcium (Ca) hydroxide and Magnesium (Mg) hydroxide (MH) are known for their flame-retardant characteristics, releasing water molecules when heated, which helps in fire suppression and also it improves thermal stability by forming a protective char layer. It enhances bonding with flax fiber, improving mechanical properties. Both hydroxides are non-toxic and environmentally friendly compared to synthetic flame retardants. The study focuses on enhancing flax fiber composites by adding Expandable Graphite to improve mechanical performance [²]. Plywood and honeycomb structures demonstrated superior fire resistance, highlighting their applicability in fire-sensitive environments [³]. This review explores the role of textile-based natural fibers in polymer composites for automotive lightweighting. It highlights their environmental benefits, mechanical suitability, recyclability, and applications in interior and structural vehicle components. Key focus areas include surface treatments, matrices, thermal behavior, and processing methods for improved composite performance [⁴]. Flax fiber composites treated with Dimethyl Methyl phosphonate (DMMP) and modified with nano-MMTs showed improved flame retardancy and reduced water absorption. [⁵]. The methodology involves a comprehensive review of surface treatments like plasma treatment, alkalization, polydopamine coating silane treatment, and acetylation [⁶]. Flax fiber composites underwent pre-treatment with diverse fire retardants (FRs), and composites were fabricated using polypropylene (PP) and polylactic acid (PLA) matrix [⁷]. This study reviews state-of-the-art repair methods for structural polymer composites used in automotive applications. It covers non-destructive testing, machining, surface preparation, and scarf patching to address damage and restore component performance. The paper emphasizes the need for innovation in Structural polymeric composite repair to enhance safety and durability in service [⁸].

Water jet machining was used to extract specimens for tensile testing from the composite boards, in accordance with ASTM guidelines [⁹]. The fabrication involved using marble dust as a filler and coconut fibers as reinforcement in different weight percentages (4, 8, 12, and 16 wt.%). The epoxy resin-based composites were prepared through the hand lay-up method [¹⁰]. The silicon nanoparticles were dispersed in the polymer matrix through mechanical mixing and sonication before being added into the composite structure. This ensured uniform distribution of nanoparticles and fibers in the matrix [¹¹]. Enset ventricosum fibers (EVF) and Limonia acidissima shell powder (LASP) were treated with 4% NaOH to enhance their compatibility with the epoxy matrix [¹²]. The research investigates the effects of graphene nanoplatelets (0–1 wt.%) on the mechanical, thermal, and viscoelastic properties of glass and basalt fiber textile composites. Optimal GNP loading enhanced tensile, flexural, and impact strength, with basalt-based composites showing superior performance. The results confirm these nanocomposites as potential alternatives to metal panels in automobiles [¹³]. Flax fibres were modified with silanes, polysiloxanes, and alkali treatments, then integrated into polylactic acid (PLA) composites with 20% fiber content evaluated using SEM, and tensile tests to assess fiber-polymer adhesion and property improvements [¹⁴]. This study examines resin type, alkali treatment, and textile structure affect the properties of sisal fiber-reinforced composites for automotive applications. Epoxy with 6 wt.% NaOH-treated unidirectional sisal fibers yielded the best mechanical and dynamic performance. Finite element modeling validated the experimental findings for structural design and optimization [¹⁵].

Composites were fabricated using melt pressing of fire-retarded flax/PP and flax/PLA fibers that were treated with organo phosphonate flame retardants. This research showed that nanofiller concentrations under 1 wt.% failed to significantly alter UL-94 fire ratings or HRR, underlining the inefficacy of sub-1 wt.% additions in fire performance improvements [¹⁶]. A novel flame-retardant strategy inspired by volcanic lava was developed for PFRP composites, forming a dense ceramic protective layer upon combustion. Combustion behaviours were assessed through LOI, UL-94, and heat release tests. [¹⁷]. Liquid engines with convergent-divergent nozzles undergo long-duration hot tests to assess endurance. To prevent this, a spiral wound gasket is designed, analyzed, and evaluated for its impact on the system [¹⁸].

Hybrid flax-based PP composites were produced by vacuum bagging with aluminium trihydrate and alumina microparticles using ultrasonication at various filler ratios. This work reported that excessive filler content (above 3 wt.%) resulted in filler agglomeration and reduced interfacial adhesion, compromising thermal insulation and mechanical uniformity [¹⁹]. Authors analysed that expandable graphite (EG) significantly enhanced both thermal and dynamic properties compared to pure flax fiber composites [²⁰]. The mercerization process and modifications improved the thermal stability and flammability with visible purified and coated fibers in SEM images and stable silicon-flax fiber bonds confirmed by FTIR [²¹]. FE-SEM, EDAX, FTIR, and Raman spectroscopy were employed to analyse the layer-by-layer fire retardant coatings that were applied to wood ply and PP/flax composites [²²]. Regenerated cooling flax fibers with 60 wt.% Boron nitrides showed a tensile strength of 121.68 MPa, elongation of 13.37%, and thermal conductivity of 0.35751 W/mK, making them ideal for summer cooling textiles [²³]. Flax/epoxy composites with 4 wt.% graphene nanoparticles improved the thermal conductivity enhancements ranged from 0.22 W mK^–1 to 0.286 W mK^–1 with small rod-shaped fillers achieving a 59.6% improvement [²⁴]. The addition of magnesium carbonate hydroxide pentahydrate (MCHPH) reduced the hardness by 36% and improved thermal stability, while maintaining biodegradability with significant weight loss in soil over four weeks [²⁵]. Dynamic mechanical analysis results showed increased storage modulus and reduced damping factor with the 50% PALF composite achieving the highest flexural stress (78.2 MPa) and flexural modulus (6503 MPa) [²⁶]. Fiber pullout tests revealed increased maximum load due to enhanced chemical bonding with the cementitious matrix, confirmed the effectiveness of the carboxylate styrene-butadiene rubber (XSBR) treatment [²⁷]. The optimized silicone rubber composite demonstrated potential for applications in high-rise building fire protection, such as sealing fire zones, cable bridges, and pipeline wells [²⁸]. Coir fiber/polypropylene (CF/PP) composites with 30 wt.% CF achieved a V-0 rating in UL-94 horizontal burning tests [²⁹, ³⁰]. Flame-resistant cellulosic panels composed of discarded newspaper, borax, boric acid, and soy protein attained low density (120 to 170 kg/m³) and bending strength (0.06 to 0.64 MPa) [³¹]. Basalt fiber-reinforced polymer (BFRP) composites reduced burning rate by 8%, and jet fire tests showed an 80°C lower maximum surface temperature compared to woven structures [³²]. Jute fiber/polylactic acid (JF/PLA) composites showed higher frequencies reduced water absorbency, improved flame retardancy, and lowered biodegradability due to a denser network structure [³³]. Epoxy/glass/ramie (EGR) hybrid composites with ammonium polyphosphate (APP) achieved V-0 grades in UL-94 vertical burning tests at 15–20 wt.% APP [³⁴]. Furthermore, research on synergistic filler systems has demonstrated that combinations of organic and inorganic additives can lead to improved fire performance compared to individual fillers, as shown in vinyl ester/flax systems with blended flame retardants [³⁵]. Advances in tailored magnesium hydroxide fillers with improved dispersion have also been reported to significantly increase LOI and reduce heat release rates in polymer composites [³⁶], supporting the rationale for exploring hybrid MH–CH nanoparticle systems in this work.

Although flax fiber composites offer sustainability and competitive mechanical properties, their practical application in safety-critical sectors such as automotive, aerospace, and construction remains limited. The primary challenges are their low flame resistance, poor thermal stability, and variability in interfacial bonding, which compromise durability under fire and elevated temperatures. Conventional approaches, including surface treatments or incorporation of single fillers such as magnesium hydroxide, calcium hydroxide, expandable graphite, or nano-clays, have shown improvements in either fire retardancy or mechanical performance. However, these methods often lead to trade-offs: an increase in flame retardancy at the expense of mechanical strength, or vice versa. Moreover, most existing studies focus on single-nanoparticle systems, leaving the potential of dual or hybrid nanoparticles underexplored.

To bridge this gap, the present study proposes the simultaneous incorporation of magnesium hydroxide (MH) and calcium hydroxide (CH) nanoparticles into polyester/flax composites. The rationale is that MH provides effective flame suppression through endothermic decomposition, while CH contributes additional hydroxyl groups that enhance interfacial adhesion with the flax fibers. Their combined use is expected to deliver synergistic improvements in mechanical, thermal, and flame-retardant properties, overcoming the trade-offs seen in single-filler systems.

Despite extensive studies on single magnesium hydroxide or calcium hydroxide flame-retardant systems, effective fire resistance in natural fiber composites is typically achieved only at high filler loadings, often at the expense of mechanical performance. The novelty of the present work lies in demonstrating that a hybrid MH–CH nanoparticle system provides a synergistic flame-retardant mechanism rather than a simple additive effect, enabling simultaneous improvements in mechanical strength, thermal stability, and flame resistance at low filler contents (1–3 wt.%). The distinct decomposition temperatures of MH and CH facilitate sequential heat absorption and sustained water release, while the combined MgO–CaO residue forms a denser and more stable char barrier, effectively suppressing heat and volatile transfer. This synergistic behavior clearly differentiates the proposed hybrid approach from conventional single-filler systems and establishes its superiority beyond incremental improvement.

2. MATERIALS AND METHODOLOGY

2.1. Materials

Nanoparticles measuring 30 nm of both MH has a density of 2.36 g/cm³ and CH has a density of 2.21 g/cm³, as well as bi-directional flax fibers (Density: 1.5 g/cm³), were acquired from Go green enterprises, Chennai, Tamilnadu, India. Kovai Cheenu Enterprises, Coimbatore, Tamil Nadu, India supplied the hardener and accelerator for the polyester resin, which has a density of 1.07 g/cm³. Cobalt accelerators were implemented to accelerate the curing process, while methyl ethyl ketone peroxide was implemented as the hardener in the composites.

2.2. Composite fabrication

The polyester/flax composites were prepared using a hand lay-up method followed by compression molding. Magnesium Hydroxide (MH) and Calcium Hydroxide (CH) nanoparticles (average size: 30 nm) were initially dispersed in styrene monomer through ultrasonication for 30 minutes and magnetic stirring for another 30 minutes to achieve uniform dispersion and prevent agglomeration. The dispersed nanoparticle-styrene mixture was then blended with polyester resin (density: 1.07 g/cm³), along with 2 wt.% cobalt accelerator and 2 wt.% methyl ethyl ketone peroxide (MEKP) as the curing system. Bi-directional woven flax fibers (density: 1.5 g/cm³), cut into 300 × 300 mm² sections, were used as reinforcement, constituting 45 wt.% of the composite. Four layers of flax fabric were manually impregnated with the modified resin and stacked in a mild steel mold pre-coated with release wax. The mold was subjected to 2 MPa pressure and kept at room temperature for 72 hours to allow complete curing.

Following curing, the composite panels were post-cured in a hot air oven at 50°C for 2 hours to remove residual volatiles and improve dimensional stability. The resulting composite sheets had a uniform thickness of approximately 3 mm. MH and CH nanoparticles were incorporated at 1 wt.%, 2 wt.%, and 3 wt.%, both individually and in hybrid combinations. A summary of the composite formulations is provided in Table 1, and the fabrication procedure is schematically illustrated in Figure 1.

Table 1
Combination of composites used in this study.

S. NO.	NOTATION	POLYESTER (wt.%)	FLAX (wt.%)	MH (wt.%)	CH (wt.%)
1	S1	55	45	0	0
2	S2	54	45	1	0
3	S3	53	45	2	0
4	S4	52	45	3	0
5	S5	54	45	0	1
6	S6	53	45	0	2
7	S7	52	45	0	3
8	S8	54	45	0.5	0.5
9	S9	53	45	1	1
10	S10	52	45	1.5	1.5

Figure 1
Experimental process of the composite fabrication.

2.3. FTIR

A potential interaction between flax fiber, MH, CH, and the polyester matrix was investigated using FTIR. The research used a 4500–450 cm^–1 spectral range JASCO FTIR spectrometer (FT-IR-6300, UK). The experiment was carried out using the KBr disk method at room temperature.

2.4. Mechanical properties

In accordance with ASTM D3039, Universal Testing Machine (WDW-100E) was employed to conduct a tensile test on all specimens. The machine was outfitted with a 200 mm gauge length, a 2 mm/min crosshead speed, and a 100KN load cell. The universal testing machine was employed to conduct the 3-point bending test in accordance with ASTM D790, with a crosshead speed of 2 mm/ min and a span of 100 mm. The load cell employed in the experiment was 100KN. Following the guidelines laid out by ASTM D256, every specimen underwent an Izod impact test using a JP-300B model, which has a load arm weighing 14 kg. For every specimen, the impact energy was calculated. On one side of the testing samples, a V-notch measuring 2 mm deep and 45⁰ was created using a shaper machine. For each composite formulation, five specimens were tested for tensile, flexural, and impact measurements, and the reported values represent the average of the tested samples.

2.5. SEM analysis

SEM was conducted at a voltage of 20 kV using the German manufacturer of optical systems and optoelectronics Carl Zeiss, at Bannari Amman Institute of Technology, Sathya Mangalam, Erode, Tamil Nadu, India. Using the broken surfaces as a framework, researchers investigated the fiber/matrix bonding and composite failure mechanisms after tensile analysis.

2.6. Thermogravimetric analysis

The thermal stability of each composite was evaluated using a Perkin Elmer STA 6000 thermal analyzer. The samples were heated at a rate of 20 °C/min and flowed at a rate of 20 mL/min in a nitrogen atmosphere. Samples weighing between 9 and 12 mg were employed for the analysis. This experiment measured the mass loss of composites containing and removing nanoparticles with respect to temperature. In order to make the measurements, the composites were heated to temperatures spanning from 30 to 700 °C.

2.7. Analysis of flammability test

To evaluate the vertical flame-retardant performance of the polyester/flax composites, Limited Oxygen Index (LOI) and UL-94 vertical burning tests (VBT) were conducted in accordance with ASTM D2863 and UL-94 standards, respectively. LOI testing involved specimens of dimensions 100 mm × 10 mm × 3 mm, placed in a calibrated chamber where the oxygen concentration was gradually reduced until the flame could no longer sustain. The minimum oxygen level required for combustion was recorded as the LOI percentage, based on the average of three trials. For the UL-94 VBT, conditioned specimens were vertically clamped and exposed to a 10-second flame using a Bunsen burner. After-flame durations (t₁ and t₂) were recorded following two flame applications, and observations were made for dripping behavior and ignition of cotton placed beneath. The burning rate was calculated from the flame propagation time. Based on performance, samples were classified as follows: V-0 (each flame extinguished ≤10 s, no flaming drips, total flaming ≤50 s), V-1 (flame ≤30 s, no flaming drips), V-2 (flame ≤30 s, flaming drips allowed), and NR (not rated if criteria were not met). In addition, horizontal burning tests were performed as per UL-94 HB criteria, with composites graded as HB if their burning rate was ≤45 mm/min; otherwise, they were designated as Not Rated. To further assess flame behavior, cone calorimetry was conducted in accordance with ASTM E1354 under a heat flux of 50 kW/m². Key parameters measured included heat release rate (HRR), peak HRR (PHRR), carbon monoxide (CO), carbon dioxide (CO₂), time to ignition (TTI), and total smoke production (TSP), providing a comprehensive profile of each composite’s combustion characteristics. UL-94 vertical burning tests were conducted on five specimens per condition, in accordance with the standard. Cone calorimetry tests were performed on three specimens (n = 3) for each composition, and the reported parameters, including TTI, HRR, and Total heat release (THR), correspond to the average values obtained from repeated measurements.

3. RESULTS AND DISCUSSION

3.1. FTIR analysis

FTIR was used to obtain whether the composites contained any functional groups. The results of this investigation are depicted in Figure 2. The different functional groups in nanoparticles and composites and the wavelengths at which they operate. Comparing the FTIR spectra of S1 composites with those of S3, S6, and S9 was done. The polyester/flax composites that result from MH, CH, or a mix of the two are denoted as S3, S6, and S9. The improved mechanical properties of these composites after adding both individual and hybrid nanoparticles led to their selection for FTIR research.

Figure 2
The FTIR spectrum of the composites.

The composite material consists of polyester and flax fiber shows unique absorption bands linked to different functional groups. At a wavelength of 2926.41 cm⁻¹, the hydroxyl group (O–H) in cellulose is detected, but the aromatic hydrocarbons in styrene produce an absorption band at 2848.47 cm⁻¹ (C–H). The carbonyl aldehyde group in lignin has a distinct peak at 2331.98 cm⁻¹ (C–H), while the acetyl ester group in hemicellulose reveals a signal at 1732.74 cm⁻¹ (C=O). Furthermore, the ester group associated with the carbon–carbon link (C=C) is detected at 1277.69 cm⁻¹, whereas the ester group involving carbon-oxygen bonds is noted at 996.52 cm⁻¹.

FTIR analysis confirmed the presence of key functional groups associated with MH and CH nanoparticles and their interaction with flax fibers. Characteristic O–H stretching peaks for MH and CH were observed at 3461.04 cm⁻¹ and 3464.71 cm⁻¹, respectively, while Mg=O and Ca=O stretching vibrations appeared at 1674.81 cm⁻¹ and 1647.24 cm⁻¹. In nanoparticle-reinforced composites (S3, S6, S9), a broadened O–H peak at 3318.14 cm⁻¹ indicated enhanced hydrogen bonding and physical interactions between nanoparticle hydroxyls and cellulose. The slight shift and overlap of C=O stretching bands near 1732.74 cm⁻¹ indicate enhanced interfacial compatibility and physical interactions between MH/CH nanoparticles and the polyester matrix, rather than the formation of new covalent bonds. The presence of oxygen-containing functional groups and metal–oxygen bonds suggest good interfacial adhesion and dispersion, supporting the improved mechanical and flame-retardant performance. FTIR analysis indicates the presence of oxygen-containing functional groups that may contribute to char formation and enhanced flame-retardant behavior, consistent with reported mechanisms [³⁵].

3.2. Mechanical properties

3.2.1. Flexural properties

Figure 3 indicates the flexural modulus and flexural strength of all the composites. Modulus and flexural strength were improved in the polyester/flax composite when nano MH and CH were added. Flexural characteristics of the S1 composites were enhanced by MH and CH interacting with flax fiber and by carefully dispersing nanoparticles throughout the polyester matrix. The interlocking of polymer molecules is made possible by nanoparticles, which impede their movement and consequently enhance their flexural properties. The S1 composite had a flexural strength of 4.26 MPa, while the S3, S4 andS9 composites have flexural strengths of 14.74 MPa, 18.04 MPa, and 16.52 MPa, respectively, that are 246.01%, 323.47%, and 287.79% higher. Improving the flexural modulus of the S1 composite was achieved by adding nanoparticles. After extensive testing, it was found that the maximum tensile modulus for S3, S7, and S9 composites was 2.49 GPa, 2.55 GPa, and 2.53 GPa, respectively. These enhancements are primarily attributed to the improved fiber-matrix interfacial adhesion facilitated by the uniform dispersion of nanoparticles. The nanoparticles act as physical crosslinks within the matrix, hindering polymer chain mobility and promoting better stress transfer between matrix and fiber, thereby increasing flexural resistance. Similarly, by adding 0.5% graphene improved the flexural properties of flax fiber composites. Increasing graphene content to 1.0% and 1.5% caused void formation and reduced mechanical performance. The composites retained structural integrity under elevated temperatures showing high suitability for industrial use [³⁶].

Figure 3
Evaluation of flexural properties of the hybrid composites.

3.2.2. Impact properties

Figure 4 displays the impact strength of the composites. The impact, tensile and flexural strengths of the materials with nanoparticles followed a pattern that was quite comparable. Impact energy absorption was best shown by S3 and S6, two of the particle-reinforced composites. When compared to other composites, the impact properties of the multi-particle filled S9 are clearly the best. The findings show that impact properties of the polyester/flax composites are improved by the combined effects of MH and CH nanoparticles. The impact energy absorption capacities of nanocomposites S3, S6, and S9 are 150.44% (16.98 kJ m^–2), 192.63% (19.84 kJ m^–2), and 222.57% (21.87 kJ m^–2) of that of S1 (6.78 kJ m^–2), correspondingly. These improvements are primarily attributed to the energy-dissipating role of well-dispersed nanoparticles, which promote better crack deflection and fiber-matrix interaction. The presence of nano-sized MH and CH particles at the interphase likely increased the toughness by impeding crack propagation, distributing stress more uniformly, and enhancing fiber pull-out resistance. This synergistic toughening effect is especially prominent in S9, where the balanced hybrid composition avoid filler agglomeration and provided optimum energy absorption. Alkali-treated flax fiber composites with 5% epoxidized palm oil and 0.5% graphene nanoparticles demonstrated enhanced mechanical properties. The composites achieved superior mechanical and thermal stability [³⁷].

Figure 4
Evaluation of impact strength of the hybrid composites.

3.2.3. Tensile properties

Figure 5 shows that adding MH, CH, or both to the polyester/flax composites significantly increased their tensile strength and tensile modulus. Results found that the optimum strength and elongation were attained when 2 wt % of MH and CH nanoparticles were added to polyester/flax composites. Figure 5 denotes an outcome of the evaluation of composites that contained 2 wt % of a mixture of MH and CH particles.

Figure 5
Evaluation of tensile strength and modulus of the hybrid composites.

The tensile strength (TS) (50.32 MPa) of S1 composites has been increased by 26.95% (63.88 MPa), 37.74% (69.31 MPa), and 16.51% (58.632 MPa) when compared to the S3, S6, and S9 nanocomposites, respectively. The hybrid composites that contained 30 wt.% hemp and flax fibers, 2–4 wt.% basalt material, and SiO₂ nanoparticles (NPs) demonstrated substantial mechanical enhancements. The TS increased from 25.36 MPa to 134.21 MPa, and the tensile modulus increased to 15.64 GPa [³⁸]. Compared to polyester/flax composites based on MH, those based on CH have higher tensile capabilities because CH includes more OH groups. The improved dispersion of nanoparticles in the polyester resin and their robust interfacial bonding with the flax fibers are the reasons for the improved tensile properties of the composites, as demonstrated by FTIR results. The tensile strength of the composites was reduced when the nanoparticle content exceeded 2 wt %. As nanoparticle concentration rises, increasing particle interactions generated by van der Waals forces cause aggregation and insufficient dispersion of nanoparticles inside the matrix. The composite of polyester and flax had an increased tensile modulus after nanoparticles were added. For each type of composite, the maximum tensile modulus values are as follows: S7 (2.66 GPa), S8 (2.62 GPa), S4 (2.47 GPa), and finally, S1 (1.56 GPa).

3.3. Scanning electron microscopy (SEM) analysis

Figure 6 denotes a tensile fractured surfaces results of the SEM analysis of polyester/flax composites, which aimed to determine that the presence of MH and CH nanoparticles affected the failure behavior of the materials. Figure 6a (Specimen S1) shows clear evidence of fiber pull-outs and substantial debonding at the fiber-matrix interfaces, indicative of weak interfacial adhesion in pure flax-polyester composites without nanoparticle reinforcement. Figures 6b, 6d, and 6f (Specimens S3, S6, and S8 respectively) reveal significant improvement in interfacial bonding due to the incorporation of 2 wt.% MH and CH nanoparticles. These composites exhibit better fiber-matrix adhesion, reduced fiber pull-outs, and more uniform dispersion of nanoparticles. Enhanced interfacial bonding facilitated effective stress transfer from the polyester matrix to the flax fibers, thereby improving mechanical properties.

Figure 6
SEM image of composite surfaces with tensile fractures a) S1, b) S3, c) S4, d) S6, e) S7, f) S8.

However, Figures 6c and 6e (Specimens S4 and S7) depict pronounced nanoparticle agglomeration due to an increased concentration of nanoparticles. Such agglomerations act as stress concentrators, potentially initiating microcracks and propagating fractures, which negatively impact mechanical performance and elongation.

Likewise, the hybrid material of ramie and flax fibers with SiC nanoparticles exhibited enhanced thermal stability, mechanical strength, and stiffness. Characterization through mechanical testing and SEM confirmed improved microstructure performance. The study validated the potential for sustainable and high-performance applications of the hybrid materials [³⁹].

3.4. Thermogravimetric analysis (TGA)

The thermal stability of the composites was examined by thermal gravimetric analysis (TGA). Figure 7 shows the TGA and DTG curves, while Table 2 has other pertinent information. The addition of MH and CH nanoparticles increased the onset degradation temperature (T_o) of the composites. The most significant rise in onset temperature was observed in the S4, S7, and S10 composites, which demonstrated increases of 4.34% (353.57 °C), 2.63% (347.76 °C), and 1.20% (342.92 °C) relative to the S1 composite (338.85 °C). Previous study shown that, the green bio-composites (GBCs) coated with ammonium polyphosphate and boric acid were analyzed for thermal stability using TGA at 350–900 °C. Intumescent coatings improved fire retardancy, raising degradation temperature to ~800 °C [⁴⁰].

Figure 7
TGA and DTG curves of the hybrid composites.

Table 2
The results of UL 94 vertical tests.

SAMPLE	t1 (s)	t2 (s)	TOTAL FLAMING TIME (s)	DRIPPING	COTTON IGNITION	UL-94 RATING
S1	>30	>30	>60	Yes	Yes	NR
S2	14	12	26	No	No	V-1
S3	11	9	20	No	No	V-0
S4	6	5	11	No	No	V-0
S5	>30	>30	>60	Yes	Yes	NR
S6	7	4	11	No	No	V-0
S7	9	7	16	No	No	V-0
S8	12	10	22	No	No	V-1
S9	10	8	18	No	No	V-0
S10	8	6	14	No	No	V-0

Table 3 illustrates the significant thermal parameter, the highest temperature (T_max), at which weight loss becomes more pronounced. T_max1 and T_max2 are two peaks in the DTG plot that indicate the highest breakdown temperatures of the composite. The greatest weight loss occurred at T_max2 in comparison to T_max1. The addition of nanoparticles has considerably enhanced the T_max1 and T_max2 of the S1 composite. This is clearly attributable to the endothermic decomposition of nano MH and CH particles. The temperatures at which MH and CH decompose are 310°C and 345°C, respectively). At temperatures between 300 and 450 °C, an oxide layer forms on the surface of the composite material, which serves as insulation, as a result of the breakdown of MH and CH into water vapor. Because it reduces heat and mass transfer, this insulation makes buildings more thermally stable. The second thermal degradation peak temperatures (T_max2) of S4, S7, and S10 are 3.31%, 3.77%, and 4.73% higher than those of the S1 composite, respectively. The thermal properties of polyester/flax composites can be enhanced through the integration of nano MH and CH, as well as their synergistic effects.

Table 3
Properties of thermal degradation.

SPECIMEN	To (°C)	T maximum (°C)		Char residual (%) at 600°C
		Tmax1	Tmax2
S1	338.85	387.12	429.87	8.37
S2	342.38	399.57	441.58	9.82
S3	344.72	400.85	444.52	10.34
S4	353.57	397.87	444.11	10.96
S5	348.24	398.77	439.89	7.58
S6	346.02	394.74	441.21	9.23
S7	347.76	397.58	446.08	9.21
S8	349.96	400.12	443.84	8.18
S9	348.42	397.85	440.28	7.82
S10	342.92	399.87	450.21	9.86

The addition of nano MH and CH particles resulted in an increase in the quantity of char residue in S1 composites, as illustrated in Table 3 and Fig. 7. The S4, S7, and S10 composites exhibited the highest char residues at 10.96%, 9.21%, and 9.86%, respectively, while the S1 composite exhibited at 8.37%. This finding demonstrates that the addition of MH and CH nanoparticles enhances the thermal stability of polyester/flax composites compared to the neat S1 composite

3.5. Results on flammability test

The Limited Oxygen Index (LOI) test, a key indicator of a material’s resistance to ignition, revealed significant improvements upon the addition of flame-retardant fillers. The sample S1 exhibited a low LOI of 20.4%, indicating poor fire resistance and easy ignitability in atmospheric air. In contrast, samples reinforced with Magnesium Hydroxide (MH), Calcium Hydroxide (CH), or their hybrid combinations demonstrated substantial enhancement in oxygen index. Figure 8 shows, S4 (3 wt.% MH), S6 (2 wt.% CH), and S10 (1.5 wt.% MH + 1.5 wt.% CH) achieved LOI values of 25.3%, 25.1%, and 24.4%, respectively. These values exceed the 23% self-extinguishing threshold, indicating that the materials are capable of resisting sustained combustion under normal atmospheric conditions. This improvement is attributed to the release of water vapor from MH and CH decomposition, which dilutes combustible gases and promotes char formation. The LOI trend confirms the effectiveness of nanoparticle synergy in enhancing the ignition resistance of the composites.

Figure 8
Evaluation of Limited oxygen index results of the composite.

The UL-94 vertical flammability performance of the composites was evaluated by analyzing both burning rate and burn time, which are critical indicators for material classification under fire exposure. The sample S1 exhibited the poorest fire resistance, with a burning rate of 70.17 mm/min and an extended burn time of 0.58 minutes, rendering it ineligible for any UL-94 rating (Table 2). In contrast, samples reinforced with flame-retardant fillers demonstrated significantly reduced flammability. Figure 9 shows that, S4 (3 wt.% MH) achieved the lowest burning rate of 30.68 mm/min and a burn time of 0.11 minutes, satisfying the stringent UL-94 V-0 criteria.

Figure 9
Evaluation of vertical burning time and burning rate of the composites.

Similarly, S6 (2 wt.% CH) and S10 (1.5 wt.% MH + 1.5 wt.% CH) showed burn rates of 42.74 mm/min and 38.41 mm/min, with corresponding burn times of 0.10 and 0.12 minutes, respectively—both within the V-0 range due to extinguishing in under 10 seconds and absence of dripping. The improved flame resistance at low MH and CH loadings (1–3 wt.%) is attributed to nanoscale dispersion and synergistic flame-retardant action. MH and CH decompose endothermically at different temperatures, absorbing heat and releasing water vapor that dilutes flammable gases and delays ignition. The resulting MgO and CaO form a compact protective barrier that limits heat transfer and volatile release. Enhanced fiber–matrix bonding promotes stable char formation, reducing burning rate and heat release while enabling UL-94 V-0 performance at low filler content. Overall, the combined reduction in burning rate and time confirms the synergistic flame-retardant action of MH and CH nanoparticles and their effectiveness in achieving superior UL-94 vertical flammability classifications.

The composites were subjected to a horizontal burning test, with and without nanoparticle addition. Table 2 and Figure 10 display the rating, combustion time, and combustion rate. The combination of keratin fibers (KF) and chitosan (Ch) with PP-grafted ammonium polyphosphate (PP-gAPP) resulted in similar fire-retardant performance to improve magnesium hydroxide composites, with noticeable reductions in horizontal burning rates [⁴¹]. The presence of MH, CH, and their synergy enhanced the burning period of the composites; however, it reduced the burning rate. The gaseous phase is diluted because the composite surfaces are enveloped by water vapors, magnesium oxide and calcium oxide layers that have dissolved, and a barrier that prevents heat and mass from moving through the material (as shown in Eqs. 1 and 2). Compared to the S1 composite, the combustion time of nanocomposites S4, S7, and S10 have been increased by 140.94% (3.59 s), 80.54% (2.69 s), and 79.87% (2.68 s), respectively.

Figure 10
Evaluation of horizontal burning time and burning rate of the composites.

As illustrated in Table 4, the combustion rates of S1 and S5 composites were 70.17 and 58.34 mm/min, respectively, which exceed 45 mm/min. Therefore, no HB rating was obtained. Similarly, MH enhances flame retardancy, decreasing the horizontal burning rate to 11.6 mm/min. The limited oxygen index (LOI) is elevated to 25.3%, hence augmenting the composite’s ignition resistance. Enhancing fire resistance marginally diminishes mechanical qualities due to the incorporation of flame-retardant materials [⁴²]. The burning rate of all other composites was less than 45 mm/min, and they were all given an HB rating.

Table 4
Results for combustion of horizontal burning tests and cone calorimetry for all composite samples.

SAMPLES	EHC(MJ/Kg)	PHRR(kW/m2)	TTI(s)	CO2 g/s	THR(MJ/m2)	SPR(m2/S)	CO g/s	COMBUSTION DURATION (min)	COMBUSTION RATE (mm/min)	RATING
S1	20.65	469.04	16	0.338	100.37	0.191	0.0096	1.49	70.17	NR
S2	19.98	408.82	20	0.300	92.05	0.156	0.0083	2.39	43.69	HB
S3	18.32	402.73	28	0.296	75.66	0.153	0.0081	2.78	38.74	HB
S4	18.06	392.28	31	0.298	74.22	0.152	0.0077	3.59	30.68	HB
S5	20.09	423.54	18	0.296	90.87	0.161	0.0085	2.07	58.34	NR
S6	19.63	409.74	20	0.297	82.01	0.152	0.0082	2.89	42.74	HB
S7	18.21	394.21	31	0.291	77.58	0.152	0.0083	2.69	35.64	HB
S8	20.74	424.12	27	0.291	94.33	0.157	0.0084	2.44	43.69	HB
S9	20.69	413.14	22	0.295	90.11	0.155	0.0084	2.31	45.74	HB
S10	18.77	409.54	38	0.291	88.25	0.157	0.0078	2.68	38.41	HB

(1) $${\text{Mg (OH)}}_{\text{2}}\to {\text{MgO + H}}_{\text{2}}\text{O}$$

(2) $${\text{Ca (OH)}}_{\text{2}}\to {\text{CaO + H}}_{\text{2}}\text{O}$$

Cone calorimetry showed significant fire resistance improvements, with HRR reduced from 255 kW/m² to 161 kW/m². The addition of nano-SiO₂ reduced water absorption by up to 23.63%, making the composites suitable for maritime, outdoor, and packaging applications [⁴³]. TTI is a key phenomenon that signifies the rate of combustion in the specified samples. Because MH and CH decompose into water molecules, which helps cool and dilute flammable gases, the TTI for all other composites increased; however, the S1 composite had a TTI of 16 seconds. The most crucial attribute to consider while trying to understand composite fire behaviour is HRR. Figure 11 displays the HRR curves for all of the composites.

Figure 11
During combustion, HRR curves were plotted for each composite.

Rapid ignition of the S1 composite was achieved with a high of 469.04 kW/m². Adding nanoparticles significantly lowered the composites’ peak HRR. In related to the PHRR of polyester/flax (469.04 kW/m²), the PHRR of S4, S7, and S10 were 15.49% (392.17 kW/m²), 16.07% (389.45 kW/m² MPa), and 11.96% (408.54 kW/m2), respectively. Nano MH and CH particles’ char-forming capabilities rendered the polyester/flax composite ineffective in reducing PHRR. The formation of char layers significantly reduces the PHRR, hence enhancing fire resistant qualities. The combination of MH and CH, together with their additive effects, increases the density of the char layer, which in turn reduces heat transmission and makes the polyester/flax composite more fire retardant. Similarly, the MH reduces the combustion rate to 13.25 mm/min, substantially hindering flame propagation. The composite containing 35% sugar palm fiber (SPF) exhibits the maximum tensile strength of 9.69 MPa. [⁴⁴].

THR is a crucial measure for examining fire retardant characteristics. Figure 12 displays the THR for the entire composite. The thermal conduction of the composites was markedly reduced following the integration of nanoparticles. This reduction is attributed to the endothermic decomposition of MH and CH, which facilitates char formation and releases water vapor, thereby diluting flammable gases and slowing down heat and mass transfer.

Figure 12
THR of all composites during combustion.

A drop of 26.05% (74.15 MJ/m²), 22.71% (78.41 MJ/m²), and 12.08% (91.07 MJ/m²) has been seen in the THR of S4, S7, and S10, respectively, when compared to the S1 composite (100.37 MJ/m²). Combustion reduced THR levels because an oxide layer generated by dissolved MH and CH particles served as a char barrier. One of the most important factors for determining fire resistance is the smoke production rate (SPR), which is closely related to the CO2 and CO generation rates. After MH, CH, and their combination were included, these parameters followed a pattern comparable to those of PHRR and THR. At the optimal 2 wt.% MH and CH, nanoparticles are well-dispersed, enhancing interfacial bonding with flax fibers and the polyester matrix, while higher concentrations cause agglomeration, reducing effectiveness. The hydroxyl (OH) groups in MH and CH improve fiber-matrix adhesion, enabling better stress transfer, reducing fiber pull-out, and enhancing tensile and flexural properties. At high temperatures, MH and CH decompose into MgO and CaO protective layers, insulating the material, delaying degradation, and improving fire resistance. Their combined effect dilutes flammable gases, reduces heat release, and promotes char formation, leading to a lower HRR and improved burning test performance. Similarly, authors [²⁵] reported a 36% reduction in HRR using 9 wt.% magnesium carbonate hydroxide in jute/starch composites, while our dual system achieved a 15.49% HRR reduction at just 3 wt.% loading, offering a much higher efficiency per unit filler content.

Table 5 summarizes the fire performance enhancement achieved through the reinforcement, clearly demonstrating the transition from poor flame resistance in the neat composite (NR) to UL-94 V-0 classification at low nanoparticle loadings, accompanied by increased LOI, reduced burning rate, shorter burn time, and lower heat release rate.

Table 5
Comparative study of fire performance improvements.

SAMPLE	LOI (%)	BURNING RATE (mm/min)	BURN TIME (s)	UL-94 HB	UL-94 VERTICAL	HRR REDUCTION (%)
S1	20.4	70.17	35.0	NR	NR	—
S4	>24.4	<40	<7.5	HB	V-0	15.49
S6	>24.4	<40	<7.5	HB	V-0	—
S10	>24.4	<40	<7.5	HB	V-0	—

Flax-based flame-retardant composites reported in the literature typically achieve LOI values of 22–25% and UL-94 HB–V-1 ratings at filler loadings ≥5 wt.% using expandable graphite or phosphorous-based systems [¹⁶, ¹⁹], whereas the present MH–CH hybrid composites attain LOI values of 24.4–25.3% and UL-94 V-0 classification at ≤3 wt.% total nanoparticle loading, indicating superior flame-retardant efficiency at lower filler content. The PHRR reductions of 10–25% and moderate THR suppression at higher filler contents (≈5–9 wt.%) [²⁵, ⁴³], the present MH–CH hybrid system achieves a PHRR reduction of up to 15.49% and a THR reduction of 26.05% at ≤3 wt.% total nanoparticle loading, demonstrating improved fire-retardant efficiency per unit filler content.

The conducted mechanical (tensile, flexural, impact), thermal (TGA), and flammability (HBT, cone calorimetry) tests were strategically chosen to evaluate the composites for structural and flame-sensitive applications such as automotive interiors, building panels, and electrical enclosures. Mechanical tests confirm the material’s strength, stiffness, and impact resistance for load-bearing and shock-absorbing components. Thermal and flammability tests assess fire safety compliance, essential for enclosed environments. FTIR and SEM validate chemical bonding and uniform dispersion, ensuring durability and interfacial stability. Together, these tests demonstrate the composite’s potential for use in lightweight, fire-retardant, and sustainable applications.

4. CONCLUSIONS

This study aimed to evaluate the effectiveness of magnesium hydroxide (MH), calcium hydroxide (CH), and their hybrid combinations (1–3 wt.%) in enhancing the mechanical, thermal, and flame-retardant performance of polyester/flax fiber composites. The results clearly demonstrate that the incorporation of MH–CH nanoparticles significantly improves composite performance, with optimal enhancements observed at 2 wt.% nanoparticle loading. Compared to the neat composite (S1), tensile strength increased by up to 37.74% (S6), flexural strength by 323.47% (S6), and impact strength by 222.57% (S9), confirming that low nanoparticle additions can substantially improve mechanical integrity. FTIR and SEM analyses verified enhanced fiber–matrix interfacial bonding due to hydroxyl group interactions and uniform nanoparticle dispersion, which directly contributed to these improvements. Significant gains were also observed in thermal and flame-retardant performance. The neat composite (S1) exhibited a low LOI of 20.4%, a high burning rate of 70.17 mm/min, and a burn time of 35 s, resulting in a Not Rated (NR) classification in UL-94 tests. In contrast, MH–CH hybrid composites (S4, S6, and S10) achieved LOI values exceeding 24.4%, burning rates below 40 mm/min, and burn times under 7.5 s, satisfying UL-94 HB requirements and attaining a V-0 rating in vertical burning tests. Furthermore, the onset degradation temperature increased by 4.34%, while the heat release rate decreased by 15.49% (S4), indicating improved thermal stability and reduced flammability due to effective char formation. Overall, these quantitative results confirm that the hybrid MH–CH nanoparticle system provides a synergistic improvement in mechanical strength, thermal stability, and flame retardancy at low filler contents, outperforming many single-hydroxide systems reported in the literature. The use of non-toxic flame-retardant fillers and renewable flax fibers further supports the development of sustainable, lightweight, and cost-effective composites for structural applications. Nevertheless, the study is limited by potential nanoparticle dispersion variability and the absence of long-term durability and high-temperature mechanical evaluations. Future work should focus on advanced dispersion strategies, aging behavior, and application-specific performance validation.

5. ETHICAL CONSIDERATIONS

Hereby, I S.Mayakannan consciously assure that for the manuscript “Enhancing Mechanical, Thermal, and Flame-Retardant Properties of Polyester/Flax Composites with Magnesium and Calcium Hydroxide Nanoparticles” the following is fulfilled:

• ¹⁾

  This material is the authors’ own original work, which has not been previously published elsewhere.

• ²⁾

  The paper is not currently being considered for publication elsewhere.

• ³⁾

  The paper reflects the authors’ own research and analysis in a truthful and complete manner.

• ⁴⁾

  The paper properly credits the meaningful contributions of co-authors and co-researchers.

• ⁵⁾

  The results are appropriately placed in the context of prior and existing research.

• ⁶⁾

  All sources used are properly disclosed (correct citation). Literally copying of text must be indicated as such by using quotation marks and giving proper reference.

• ⁷⁾

  All authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.

DATA AVAILABILITY

All data that support the findings of this study are included within the article.

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Publication Dates

• Publication in this collection
  29 May 2026
• Date of issue
  2026

History

• Received
  25 Oct 2025
• Accepted
  16 Mar 2026