Thermogravimetric analysis of flax, jute, and UHMWPE fibers and their composites with melamine and phenol formaldehyde resins

Abstract

This article presents the thermal stability of flax, jute and ultrahighmolecular weight polyethylene (UHMWPE) fibers and their composites. Composites are fabricated using a combination of hand layup and compression moulding. Two types of resins namely phenol formaldehyde (PF) and melamine formaldehyde (MF) are considered. Stacking of eight layers of individual fabric was utilised to produce composite panels of 4 mm thickness. Alkali treatment of the flax and jute showed an enhancement in thermal behaviour, whereas UHMWPE fibers sustained the thermal degradation till 474°C. Among the two resin systems, neat MF resin degraded at 278°C with 84% mass loss, whereas PF resin had a final mass loss of 52.54% at 600°C. The thermogravimetric analysis confirmed an increase in thermal stability of the composites considering both resin systems. Density functional theory (DFT) calculation on a small molecular unit of PF, MF, and their complexes with cellulose and molecular unit of UHMWPE indicated that the composites with natural fibers and MF matrix exhibited better thermal stability, while PF resin showed better binding characteristics for composites with UHMWPE fibers. The plausible intermolecular interactions between fibers and the matrix are discussed. The proposed application of the composites is to be used as intermediate wall linings of the furnaces.

Keywords:

• Flax
• jute
• ultra-high molecular weight polyethylene
• Compression moulding
• thermogravimetric analysis
• density functional theory

1. Introduction

A prominent feature of the present manufacturing industry is the extensive usage of polymer composites (Czigány, 2005). Polymer technology caters to several environmental concerns, but primarily its ease of processing and recyclability to an extent is the one that drives its growth (A. K. Mohanty et al., 2000). Polymer composites may not cater to high-temperature applications, but they have significant features like high strength-to-weight ratio, low cost, good corrosion resistance, good thermal stability and the ability to generate complex shapes. Polymers have been deemed suitable for reinforcement with both natural fibers and synthetic fibers (Pillin et al., 2011; Yan et al., 2014). However, it is well known that synthetic fiber-reinforced composites do not degrade as they have a high molecular mass and hydrophobic nature.

On the other hand, natural fibers have certain advantages over manufactured fibers like low density, biodegradability, and low ecological impact, all while providing composites with good mechanical properties. Specific issues associated with natural fibers include low strength and poor interfacial bonding between the fiber and the matrix compared to their synthetic counterparts (A. K. Mohanty et al., 2004; Le Duigou et al., 2013; Mathew et al., 2005; Saba et al., 2015). The weak bonding is because structural compositions of fibers (cellulose, hemicellulose, lignin, pectin, and waxy substances) allow moisture absorption from the environment (M. Haque et al., 2010; M. M. Haque et al., 2009; Rezaur Rahman et al., 2010; S. L. Gao et al., 2006). To overcome this drawback, it is necessary to modify the fiber surface. The ineffective stress transfer that stems from the shortfalls can be controlled by certain chemical treatments on the surface of the fibers (Albinante et al., 2013; John & Anandjiwala, 2008; Kalia et al., 2009; Xie et al., 2010). These treatments are typically based on reagent functional groups that react to fiber structures and change their composition. The tendency of moisture absorption of the fibers is reduced, facilitating compatibility with the polymer matrix (Abdelmouleh et al., 2007). Treated natural fibers, together with their characteristics of being lightweight and nontoxic by nature, natural fiber-reinforced composites have become a massive attraction over conventional composites (Wibowo et al., 2004).

Natural fibers can be from various plant species and different parts of the plant body (leaf, stem, and seed). Some of the natural fiber resources that are well recognized as good potential reinforcements for engineering fiber composites are sisal, coconut coir, jute, hemp, ramie, palm, cotton, rice husk, bamboo, banana, wood, and flax. Among available natural fibers, flax is considered as one of the strongest and readily available fibers that can be replaced with conventional fibers such as glass fibers (John & Anandjiwala, 2009). Cultivated chiefly in South Asia, jute fibers produced from the bast or skin of plants are 100% biodegradable, environmentally friendly, and recyclable. They have relatively high tensile strength and tensile modulus making them a desirable candidate (Hu et al., 2010; Wis et al., 2020). Accordingly, jute fibers have been extensively explored as reinforcement in polymer composites, with both thermosetting and thermoplastic matrices (Das et al., 2018). It has been observed that untreated natural fibers generally begin degrading above 240°C. Structural constituents of the fiber are sensitive to a different range of temperatures. To enhance the thermal stability of the natural fiber, one can remove certain portions of hemicellulose and lignin present in natural fibers with the help of chemical treatments (Sgriccia et al., 2008).

As far as synthetic fibres are concerned, E-glass, carbon and kevlar have shown good thermal stability with different resin systems. A synthetic fiber, namely Ultra High Molecular Weight Polyethylene (UHMWPE), is a long-chained polyolefin with molar mass ranging from 3 to 5 million. UHMWPE has superior tensile strength and chemical resistance in comparison to other materials (Chin et al., 2009; Forster et al., 2016). The high-performance characteristics make UHMWPE suitable for manufacturing protective clothing, bullet armlets, waterproof cloth, canvas, and filter materials. Despite its favourable properties, polyethylene is generally flexible and has a low glass transition temperature, even for UHMWPE (Berger et al., 2003; S. Gao & Zeng, 1993). Understanding the thermal degradation behaviour of natural and synthetic fibers is of prime importance for developing composites to be used as intermediate wall linings of the furnaces. Parameters such as composite moulding technique, curing time and temperature, reinforcements and resins used are very significant for good service life conditions of the material (Lotfi et al., 2021). One of the methods to interpret the composites thermal stability is by conducting a thermogravimetric analysis. Thermogravimetric analysis (TGA) gives us an understanding of the thermal stability of the composites, wherein the weight loss of samples is measured as a function of rising temperature. The greater the decomposition temperature of a sample, the more suitable it is for high-temperature applications (Monteiro et al., 2012).

Zheng et al. (Zheng et al., 2015) reported the thermal properties of the flax fibers untreated and treated with six kinds of scouring methods and observed from TGA and DTG analysis that the thermal stability became better after scouring. Therefore, a thermal analysis study is necessary to determine the influence of different refined treatments on the stability of natural fibers. Mohanty et al. (S. Mohanty et al., 2006) investigated the TGA and DTG characteristics of Jute fibers and Jute/High-Density Polyethylene. The authors reported that cellulose degradation was observed for jute fibres at 380°C, whereas for HDPE resin, the decomposition was in the range of 430°−515°C. The untreated jute fiber/HDPF composites decomposition was at 515°C, and that of treated jute fiber/HDPF composites was 530°C. Treatment with maleic anhydride grafted polyethylene improved the thermal stability of the composites. El-Shekeil et al. (El-Shekeil et al., 2012) studied the thermal characteristic of kenaf and kenaf/thermoplastic polyurethane composites with fiber loadings of 20, 30, 40 and 50 weight percent. The researcher reported that the decomposition of lignin, hemicellulose and cellulose for the kenaf fiber was in the range 305°C−386°C, TPU resin degradation was around 363°C, for kenaf/TPU composites, with significant degradation in the range of 445°C–509°C. An increase in fiber loading decreased the thermal stability of the material.

Aji et al. (Aji et al., 2012) evaluated the thermal stability of the kenaf fiber, pineapple leaf fiber and kenaf/pineapple leaf hybrid composites with HDPE as the matrix. The authors concluded that the thermal stability of kenaf and pineapple leaf composites was in the range of 160°C to 384°C, while HDPE degraded above 384°C. Hybridization of the composites decreased the thermal resistance with an increase in fiber loading. Muralidhar (Muralidhar, 2013) investigated the flax woven and rib knitted preforms for TGA. Hybrid composites were fabricated with different combinations using epoxy as the resin. The authors revealed that the significant weight loss for neat epoxy was between 340°C and 480°C. The hybrid combination had a lower decomposition temperature of around 340°C.

In contrast, the flax hybrid preforms showed better stability ranging from 345°C to 365°C. Panthapulakkal and Sain (Panthapulakkal & Sain, 2007) conducted thermal tests such as Thermogravimetric Analysis (TGA) on hybrid short hemp fiber/glass fiber reinforced polypropylene composites. The authors found that hemp/PP degradation started around 250°C, whereas hemp/glass/PP hybridization improved the thermal properties with degradation at around 280°C. Poddar et al. (Poddar et al., 2016) fabricated areca nut leaf sheath short fiber reinforced polypropylene composites and analysed their thermal properties using TGA. The authors reported a considerable change in mass at a temperature ranging from 400°C to 500°C.

Boopalan, Niranjanaa and Umapathy (Boopalan et al., 2013) fabricated raw jute and banana fiber epoxy composites with various fiber weight fractions. TGA results showed that composites with equal amounts of jute and banana fiber had better thermal stability and resistance. Biswas et al. (Biswas et al., 2015) compared the TGA analysis of jute and bamboo fiber reinforced epoxy composites. The researchers concluded that the thermal decomposition temperature for jute and bamboo fiber epoxy composites was 255°C and 246°C, respectively. Among the two fibers, jute fiber epoxy composite had higher thermal stability than bamboo fiber epoxy composite. Srivastava (Srivastava, 2017) fabricated Ultra High Molecular Weight Polyethylene (UHMWPE)/High-Density Polyethylene (HDPE) blended nanocomposites with the addition of functionalized nanoscale alumina under various fractions by weight. The author reported that composites with higher percentages of nano alumina had higher initial decomposition temperatures. Li et al. (S. Li et al., 2018) used bamboo charcoal (BC) as filler for UHMWPE composites. TGA of the BC and BC/UHMWPE was determined. The investigators reported that the pure UHMWPE degraded significantly at a temperature ranging from 480° to 530°C, whereas the BC was very thermally stable. Both the weight loss and degradation rate of BC/UHMWPE composites decreased with the addition of BC, indicating that the presence of BC particles in the polymer matrix delayed the thermal degradation of UHMWPE. Chinnasamy et al. (Chinnasamy et al., 2020) compared the TGA of the glass fiber and Kevlar fiber with modified epoxy hybrid composites. The researcher reported that the TGA curves for kevlar and glass fiber showed that weight loss starts at around 300°C. In addition, there is a significant weight loss between 550 and 650°C, which is related to the pyrolysis of the materials. Ebrahimnezhad-Khaljiri (Ebrahimnezhad Khaljiri et al., 2017) investigated TGA of the Aramid/Semi-Carbon Fibers hybrid epoxy composites. High mass loss is noted by the researchers in the temperature range of 500°-620°C. The authors concluded that the mass loss shifted to higher temperatures by increasing the SCFs to aramid fibers ratio.

Now coming to the resin, which is a polymeric material used to bond the fiber reinforcements in polymer composites. Resins such as epoxy, polyester and vinyl ester are extensively utilised with natural/synthetic fibers for manufacturing composites (Rajak et al., 2019). In addition to these resins, the use of formaldehyde resins such as phenol, melamine and urea-formaldehyde is gaining momentum for developing polymer composites. Phenolic resins produced due to the reaction between phenol and formaldehyde under alkaline conditions have excellent mechanical strength, water resistance and chemical and thermal stability. Phenolic resins are used to produce moulded products, including billiard balls, laboratory countertops, hot oil filter applications such as the lube oil filters of automobiles, coatings and adhesives (Kariuki et al., 2019). Melamine formaldehyde resin obtained by condensation of formaldehyde with melamine has improved heat, moisture, scratch, and chemical resistance (Merline et al., 2013). Melamine resins are used to manufacture many products, including kitchenware, laminate flooring, laminate countertops, overlay materials, particleboards, and floor tiles. Melamine and its salts are also used as fire-retardant additives in paints, plastics, and paper (Kumar & Katiyar, 1990; Park & Jeong, 2010; Raval et al., 2006).

Having reviewed different works in the literature, one can conclude that the thermal stability aspects of natural fibers like jute and flax are extensively studied with epoxy and polyester resins. As a result, there is a knowledge gap for using these fibers with other less explored resins such as phenol formaldehyde, melamine formaldehyde and urea-formaldehyde. Thus, to explore this gap, the present research work investigates the thermal decomposition behaviour of untreated and treated jute and flax fibers, along with a synthetic reinforcement (UHMWPE fibers) for comparison. These fiber reinforcements are then blended with two types of resin systems: phenol formaldehyde (PF) and melamine formaldehyde (PF) to obtain jute, flax and UHMWPE fibre-reinforced composites. TGA and DTG analyses of the composites are carried out. The density functional theory (DFT) calculation on a small molecular unit of PF, MF and their complexes with cellulose and molecular unit of UHMWPE were performed to comprehend the intermolecular interactions between the fibers and the resin, leading to their increased thermal stability.

2. Materials and methods

2.1. Materials

Bidirectional fiber mats of UHMWPE, Flax and Jute are used as reinforcements. Technical specifications of the fiber mats are presented in Table 1. All the fibers are procured from e-commerce vendors. Natural fibers used in this study were subjected to 10% NaOH treatment (Aydın et al., 2011; Tanaka Razera et al., 2014) so as to facilitate better fibre-matrix adhesion. Industrial grade Phenol formaldehyde (PF50) of 1.15 g/cc density and Melamine formaldehyde (MF50) of 0.93 g/cc density are procured from local vendors.

Table 1. Specifications of the Fiber mats

Material	Areal Density (gsm)	Thickness (mm)
UHMWPE Fiber Mat (P)	240	0.51 ± 0.02
Flax Fiber Mat (Q)	360	0.87 ± 0.03
Jute Fiber Mat (R)	220	0.49 ± 0.02

2.2. Fabrication

The composites were fabricated using a combination of hand layup and compression moulding techniques. Figure 1 presents the composite fabrication details. Desired quantities of resins are used to wet the fibers, the fibers are arranged in stacks, and eight layers of the individual fibers are considered. The prescribed method for curing the resin was by applying temperature and pressure as specified by the supplier. Phenol formaldehyde resin (PF) was cured at 145°C, and the melamine formaldehyde (MF) was cured at 130° C. The laminates were subjected to compression moulding so as to obtain composite panels of uniform thickness of 4 mm. Post curing for about 6–7 h under room temperature, the composites were demoulded, and the edges were trimmed to obtain a finished laminate. Table 2 provides the technical details of the composites.

Figure 1. Composites fabrication (a) Mould preparation (b) Flax fiber mat (c) UHMWPE fiber mat (d) Jute fiber mat (e) Hand-layup (f) Compression moulding (g) Composite laminates.

Table 2. Technical details of the PF and MF composites

Stack Name	Orientation	Fiber Weight Fraction (%)	Thickness (mm)
PF1 (UHMWPE)	PP-PP-PP-PP	78	4.05 ± 0.01
PF2 (Flax)	QQ-QQ-QQ-QQ	65	4.10 ± 0.05
PF3 (Jute)	RR-RR-RR-RR	71	4.09 ± 0.03
MF1 (UHMWPE)	PP-PP-PP-PP	73	4.03 ± 0.04
MF2 (Flax)	QQ-QQ-QQ-QQ	62	4.15 ± 0.05
MF3 (Jute)	RR-RR-RR-RR	70	4.09 ± 0.04

2.3. Thermogravimetric analysis

Thermogravimetric analysis (TGA) helps to analyse a series of parameters like moisture loss, loss of solvent, loss of plasticizer, decarboxylation, oxidation, and decomposition of the substances. The working principle is based on the change in mass of the sample with a change in temperature or time (Bottom, 2008). A typical TGA instrument (Make: PerkinElmer; Model: TGA 4000) used in this study consists of a pan that rests on a sensitive analytical balance. The test sample is placed on the pan and is heated externally. A purge gas that may be reactive or inert is passed over the sample and exits through the exhaust. The sample’s heating rate is controlled, and the mass change over the entire period is monitored continuously. In the present research work, the thermogravimetric analysis of the fiber and composite samples is carried out. Approximately 15–18 mg of the sample weight is considered. The samples were initially heated from 50° to 700° C at a ramping rate of 10° C/min, under a nitrogen atmosphere. The software program of the TGA setup plots the mass loss curve and the first derivative curve, which is an essential tool to determine the point of most significant change on the mass loss curve. Figure 2 shows the TGA test setup.

Figure 2. TGA Test Setup (a) Top lid opening (b) Pan with composite sample (c) Placing pan and the sample in the ceramic furnace (d) Setting the analysis parameters (e) TGA analysis (f) Sample residue.

2.4. Density functional theory

The density functional theory (DFT) calculation on a small molecular unit of PF, MF and their complexes with cellulose and molecular units of UHMWPE were performed. Although these studies in gas-phase conditions may not corroborate exactly with material bulk properties, they are extremely useful to understand the fundamental nature of the involved inter- and intramolecular interactions responsible for bulk properties. The minimum-energy structure calculation followed by harmonic vibrational frequency calculation of the PF, MF and their complexes is performed using dispersion-corrected density functional methods. The B97-D3 (Grimme, 2004) and with the def2-TZVPP basis set, using Gaussian 16 (Frisch et al., 2016). The choice of DFT theory is based on its success for small-molecule 1-naphthol and its various complexes (Knochenmuss et al., 2019; Knochenmuss, Sinha, & Leutwyler, 2018, 2020; Knochenmuss, Sinha, Balmer, et al., 2020; Knochenmuss, Sinha, Poblotzki, et al., 2018). The structure optimizations were unconstrained and the threshold for SCF convergence was set to 10⁹ a.u., the convergence threshold for RMS force was set to 10⁶ a.u., the maximum force was set to 2 × 10⁶ a.u., the RMS displacement was set to 4 × 10⁶ a.u., and the maximum displacement was set to 6 × 10⁶ a.u. These parameters correspond to the VERY TIGHT option in Gaussian16 (Frisch et al., 2016). The harmonic vibrational frequency calculation on the ground state optimized structure of molecular units and complexes was checked to ensure the absence of any imaginary vibrational frequencies. The ground state binding energies De(S0) were obtained by subtracting the total energies of PF/MF and cellulose/unit of UHMWPE from the total energy of the complexes at their optimized minimum-energy geometry. The basis set superposition error is not calculated as, with the def2-TZVPP basis set, the BSSE effects start to be negligible (Grimme, 2004).

3. Results and discussion

3.1. Thermogravimetric analysis

The analysis of TGA thermograms of untreated flax fibers shows three distinct regions of weight loss, as evident from the first derivative plot in Figure 3a-c. The first weight loss of 13.95% is observed at a temperature of 276°C, while the second weight loss of 48.99% is observed at 360°C, and the third weight loss of 15.31% is obtained at 600°C. As evident from the literature, the weight loss at 276°C is attributed to hemicellulose degradation, while cellulose degradation may be observed at 360°C (Yang et al., 2004, 2007).

Figure 3. TGA thermograms of untreated and alkali treated flax fibers.

It may be observed that the TGA thermograms of alkali treated flax fibers, given as (Figure 3d-f), show increased thermal stability of the fibers, with the first degradation starting at 360°C with a mass loss of 54.43%, second mass loss of 16.97% at 419°C, while the final weight loss of 18.84% is observed at 605°C (Kabir et al., 2012). This observation is concurrent with the alkali treatment causing hydrolysis in the previously hydrogen-bonded molecules of hemicellulose, lignin and cellulose; however, it is to be noticed that the extent of hydrolysis is proportional to the strength of the alkali and the duration of the treatment (Ali et al., 2015; X. Li et al., 2007).

A similar observation is made in the untreated jute fibers whose TGA thermograms are given as (Figure 4a-c) showing the degradation of jute fibers starting at 288°C with a mass loss of 11.61%, which is pertaining to the breaking of hemicellulose molecules and the degradation of cellulose at 360°C with a mass loss of 56.09%. The fibers maintain a constant weight until a third mass loss of 17.92% is observed at 600°C.

Figure 4. TGA thermograms of untreated and alkali treated jute fibers.

The alkali-treated jute fibers’ TGA thermograms (Figure 4d-f) show the first mass loss of 62.5% at 354.47°C and final mass loss of 23.12% at 604.47°C, indicating the increased thermal stability.

The TGA thermograms of UHMWPE given as (Figure 5a-c) shows a peak at 474°C pertaining to its complete degradation with a mass loss of 95.07%. It is worthwhile mentioning the thermal strength of UHMWPE, which displays its thermal stability up to 138°C, after which the thermal degradation of the polymer begins. The weak van der Waals forces primarily present between the hydrocarbons of the polymeric chains of these fibers get disrupted easily with heat treatment (McKeen, 2014; Tam & Bhatnagar, 2016).

Figure 5. TGA thermograms of UHMWPE.

The MF resin employed to prepare the composite shows a degradation temperature of 278°C, with a mass loss of 84%, which may be attributed to the breakdown of methylene bridges (Merline et al., 2013; Ullah et al., 2014). The TGA plot of MF resin is given as Figure 6a-c.

Figure 6. TGA thermograms of MF resin.

The thermal behaviour of the treated flax-MF composite indicates the weight loss of 5.46% pertaining to the removal of water at 106°C, produced during the crosslinking between the MF polymer chains on heat treatment with flax. Along with the crosslinking, intermolecular interactions occur between the —OH groups of the hemicellulose and cellulose components of flax and the MF resin. This is indicated by the second degradation peak obtained at 365°C, with a weight loss of 39.79%, which is much beyond the degradation temperatures of hemicellulose and the MF resin. The shift in the degradation peak for cellulose at 360°C in treated flax to 365°C in the composite and also the shift in the temperature from 600°C to 616°C (weight loss 30.66%) for the complete degradation of treated flax and the composite, respectively, suggests that composites of flax—MF resin possess higher thermal stability over the precursors. The TGA thermograms of the treated flax-MF composite are given as Figure 7a-c.

Figure 7. TGA thermograms of composites of treated flax and jute with MF resin.

A similar observation is made in the treated jute-MF composite, whose TGA thermograms given as (Figure 7d-f), shows a peak at 100.7°C with a weight loss of 6.62%, indicating the elimination of moisture, while the second weight loss of 35.07% is observed at 364.79°C pertaining to the degradation of the cellulosic components of the jute fibers. The next peak at 414.8°C with a weight loss of 11.47% is pertaining to the degradation of lignin of the jute. A peak at 616°C indicates the complete degradation of the composite with a weight loss of 32.05%.

The composite of UHMWPE with MF resin shows increased thermal stability from 474°C for pure UHMWPE to 504.66°C for the composite and the complete degradation occurring around 611.66°C. The corresponding weight loss is 70.30% and 16.46%, respectively. This indicates that there are possibilities of enhanced intermolecular interactions between the polar chains of MF resin and non-polar chains of UHMWPE compared to purely van der Waals forces present in UHMWPE molecular chains. The TGA thermograms of UHMWPE-MF resin composite are given as Figure 8a-c.

Figure 8. TGA thermograms of composites of UHMWPE with MF resin.

The TGA thermograms of PF resin indicates the first mass loss of 11.89% at 227°C, which is attributed to the removal of the terminal groups of the resin, second mass loss of 12.76% at 401°C may be due to the breaking of methylene bridges, third mass loss of 11.42% at 526°C is attributed to the oxidation of exposed aldehydes or carboxylic acids and final mass loss of 52.54% at 600°C may be due to the breaking of crosslinking bonds leading to the degradation of the polymer (Yang et al., 2007). The TGA thermograms of PF resin are given as Figure 9a-c.

Figure 9. TGA thermograms of PF resin.

The TGA thermograms of the composite of alkali treated flax with PF resin show a mass loss of 4.85% at 90°C, indicating the removal of water produced during the initial stage of crosslinking in the PF resin on heat treatment for the formation of the composite with flax. The second mass loss of 41.20% is observed at 354°C may be due to the breaking of methylene bridges, although a shift in the temperature from 227°C in pure PF resin to 354°C is observed indicating an increase in the thermal stability, although a significant mass loss of 41.20% is observed. The third mass loss of 12.56% is observed at 461°C, and a final mass loss of 31.68% is observed at 610°C. The TGA thermograms of the alkali treated flax composite with PF resin are given as Figure 10a-c.

Figure 10. TGA thermograms of composites of treated flax and jute with PF resin.

Similar is the observation for the composite of alkali treated jute fiber with PF resin. It shows the first mass loss of 2.95% at 108°C, second mass loss of 35.21% at 337°C, third mass loss of 10.39% at 429°C and the final mass loss of 40.84% at 613°C. The TGA thermograms of the alkali treated jute composite with PF resin are given as Figure 10d-f.

The TGA thermogram of the composite of UHMWPE with PF resin shows the first mass loss of 71.20% at 514°C, while the final mass loss of 15.5% is observed at 609°C. The TGA thermogram of this composite is given as Figure 11a-c.

Figure 11. TGA thermograms of composites of UHMWPE with PF resin.

3.2. Interpretations of density functional theory

The plausible intermolecular interactions between the treated flax, jute and UHMWPE fibers with MF and PF resins are interpreted through DFT calculations. The DFT calculated structures of molecular units and complexes are shown in Figure 12. DFT calculations are carried out by considering the cellulose component of the natural fibers and a portion of the UHMWPE fibers to study the composite structure’s orientation, intermolecular interactions, and stability.

Figure 12. The DFT calculated structures of molecular units and complexes.

The DFT calculations show the intermolecular interactions between the cellulose component of the natural fibers with the MF and PF resin, wherein intermolecular interactions are more pronounced in the MF resin with the natural fibres than the interactions of the PF resin with the same component of the natural fibers. It is observed that MF is making a better interaction with natural fibers. The binding energy of the MF-cellulose complex is 95.31 kJ/mol and the dispersion energy (DE) id −0.14874 H. The higher binding energy indicates a strong interaction between MF and cellulose, whereas a higher negative value of DE confirms that apart from the H-bonding interaction, other weak non-covalent interactions also play a very important role in the stabilization of the complex structure. Compared to the MF-cellulose complex, the binding energy of the PF resin with natural fiber is 37.24 kJ/mol, and DE is−0.106072 H. The lower binding energy, in this case, could be because the ring system in the unit of PF is chemically more inert for H-bonding than MF. However, the DE still plays a vital role in the stabilization of the complex structure.

It is interesting to note that the composite of PF with UHMWPE is showing a relatively highest thermal stability up to 514°C as compared to the composite of MF with UHMWPE, which is stable up to 504.66°C although the BE for PF-UHMWPE composite is slightly lower than that for MF-UHMWPE composite as it is evident from the DFT calculations, which records the BE of 32.67 kJ/mol and DE of−0.104454 H for the composite of UHMWPE with MF resin, while the BE of 30.17 kJ/mol and DE of−0.075984 H is recorded for the composite of UHMWPE with PF resin. It may be argued that in the present study MF is a better resin of choice to bind the natural fibers to obtain a composite as it demonstrates better thermal stability, while PF resin demonstrates better thermal stability with the synthetic fiber, UHMWPE.

4. Conclusion

• • TGA has proven to be a versatile tool to study the thermal stability of composite materials.

• • Higher thermal stability is observed for the composites of natural fibers with MF and PF resins as compared to UHMWPE with MF and PF resins.

• • MF resin acts as a better choice of resin to bind natural fibers, while PF resin showed a better binding property to UHMWPE.

• • The development of new intermolecular interactions between the fibers and the resins enhances the thermal stability of the composites.

• • The composites of MF resin with natural fibers are found to possess the highest binding and dispersion energy, indicating the highest stability amongst the other composites of the present study.

Disclosure statement

No potential conflict of interest was reported by the authors.