Effects of nanoclay cloisite 20A and alkali treatments on structure-property relationships of bagasse/recycled polypropylene nanocomposites

Abstract

This study was aimed at evaluating the individual and combined effects of organically modified nanoclay cloisite 20A and alkali treatments on the valorization of totally waste-based bagasse fiber/recycled polypropylene nanocomposites. FTIR spectra indicated the reduction of bagasse fibers’ hydroxyl moieties due to hemicellulose removal resulting from alkali treatment. There were also trace diminutions in C-O peak intensities due to the degradation of little amounts of lignin. X-ray Scattering revealed the intercalation of nanoclay in the polymer matrix. The improved interaction of fiber-polymer interfaces brought about by alkali treatment was also confirmed by scanning electron microscopy. The physical properties of the nanocomposites were improved due to the barrier properties of nanoclay against water ingress. The flexural strength and modulus of elasticity increased by both individual and combined treatments; however, the impact strength decreased by the individual treatments. Thermogravimetric analysis demonstrated that the temperatures of first and second stages of composites’ thermal degradation increased due to the treatments via the formation of a carbonized char layer thermally insulating the deeper composite layers. Differential scanning calorimetry showed some slight increases in the melting temperature, melting enthalpy, crystallisation temperature, crystallisation enthalpy, and crystallinity index of the treated composite formulations due to the nucleating effect of nanofillers. The overall results confirmed that the combination of nanoclay cloisite 20A and NaOH treatments could significantly improve the overall properties of the studied composites. This was due to some interesting synergistic effects of the given treatments on the nanocomposites converting the bagasse/recycled PP composites to high performance materials of choice.

Keywords

Recycled polypropylene bagasse nanoclay cloisite 20A chemical treatment structure-property relationships

Introduction

Incorporation of natural fibers in to polymer composites has attracted much attention in recent years due to environmental restraints, and the fact that natural fibers are biodegradable, cost effective, and widely available.^1,2 Reprocessing plastic scraps and adding them in to wood plastic composites (WPC) as the polymer matrix reduce the adverse effects of dumping plastics in nature. These inexpensive wastes not only hold a large share of municipal wastes across the globe but also are suitable for WPC production due to their low melting and processing temperature being below the degradation temperature of natural fibers (<200°C).^3,4 Water absorption is a major drawback in wide application of WPCs which is affected by the kind and amount of WPC constituents. The hydrophilic nature of natural fibers introduces phenomenal problems in WPCs which are poor interfacial adhesion of the given fibers with the hydrophobic plastic matrix as well as providing a proper condition for fungal attack. Thus, the surface of natural fibers need to be treated in order to reduce their hydrophilicity leading to enhanced stress transfer from the polymer matrix to the fibers and the resulting improved physico-mechanical performance. Sodium hydroxide (NaOH) can remove the surface impurities of fibers and increase their unevenness, thus opening and making accessible more hydroxyl and other functional groups on cellulosic fiber surfaces.^5–7 Sodium hydroxide can also react with reachable –OH moieties, thus facilitating other chemical reactions⁸ as given:

Cellulose - OH + NaOH \to Cellulose - O^{-} {Na}^{+} + H^{2} O + impurities

(1)

Bagasse fiber (sugar cane residue) alkali treatment with 1% NaOH has been reported to improve tensile, flexural, and impact strength properties as well as the aspect ratio (fiber length to diameter) of the manufactured biocomposites. Scanning electron microscopy (SEM) micrographs showed improved interfacial adhesion among the alkali treated bagasse fiber and the polymer matrix.⁹ The existing archival publications demonstrated that NaOH treatment led to increased rates of fiber surface fibrillation, thus providing more accessible reaction sites on fiber surfaces for subsequent chemical reactions; this improved the tensile strength and elongation at break of the polypropylene bagasse fiber composites.^10,11

Nanotechnology is rapidly on the growth in different industries.^12–20 One of the cheapest and widely available naurally occurring mineral fillers, which can be used to improve the overall properties of WPC, is nanoclays (i.e., montmorillonite). They are one to two layer aluminosilicate groups, having exchangeable cations (e.g., Na+ and Ca2+) which can fill the spaces between the layers. The exchangeable cations can also be replaced by organic cations like quaternary ammonium salts and produce organophilic clay holding a much higher compatibility and chemical affinity to bond with polymers.²¹ Wang et al. (2001) stated that the impact of nanoclay on composites depended on its shape, size, aspect ratio, type, rate and quality of dispersion, and interfacial adhesion. They also reported that the addition of nanoclay could improve thermomechanical properties as well as dimensional stability of the studied composites.²² Wu et al. (2007) found out that the incorporation of only 2 wt% nanoclay into pine flour/high density polyethylene (HDPE) composites could increase flextural and tensile strengths by 4.4% and 1.2%, respectively; however, water absorption and thichness swelling decreased by 5%–7%. They also profferred that the temperature of glass transition, moduli of storage and loss, and cystallinity degree of the studied composites dramatically increased due to nanoclay. This brought about improved thermal stability and fire retardancy for the studied pine flour/HDPE composites.²³

To the best of the authors’ knowledge, the present study, for the first time, addresses the synergistic effects of alkali treatment and organically modified nanoclay addition on structure-property relationships of fully waste-based environmentally compatible bagasse fiber/recycled polypropylene composites through a wide range of analyses including water absorption, thickness swelling, flexural and impact strengths, modulus of elasticity, SEM, FTIR, XRD,TGA, and DSC tests.

Materials and methods

Materials

The bagasse fiber used in this study was purchased from Khuzesta Dash Laleh Nasr Co., Iran. Polypropylene (PP) grade V30 G with melt flow index (MFI): 16 g/10 min and density: 0.9 g.cm-3 was purchased from Maroon Petrochemical Co., Iran. Maleic anhydride grafted ploypropylene (MAPP) with MFI: 10 g/10 min and density: 0.95 g.cm⁻³ was purchased as the compatibiliser from Aria Polymer Pishgam, Iran. Organically modified montmorillonite grade cloisite 20A with bulk density: 0.118 g/cc and average particle size: <10 μm was provided from Southern Clay Co., USA. Sodium hydroxide (99%) was purchased from Merck Co., Germany.

Treatment of sugarcane bagasse fibers

The sugar cane bagasse fibers were first soaked in hot water for 1 hour. The fibers were then air dried for 72 h and placed in an oven (103°C for 24 h) to be well died. The fibers were then processed in a rotary chipper, and the 30–100 mesh size portion of the fibers were selected for further experiments. Alkali treatment of bagasse fibers was conducted by soaking the fibers in 5wt% NaOH at room temperature for 2 h. Thereafter, the fibers were washed and rinsed with distilled water, air dried for 3 days, and then placed in an oven at 103°C for 24 h. The so-treated bagasse fibers were sealed in plastic bags for further WPC manufacture.

Preparation of recycled PP

Vigin PP was extruded through a single screw extruder (barrel length: 1.5 m) equipped with five thermal zones manufactured by Pouya Polymer Amirkabir Iran Co., Iran and cut in to recycled PP granules coming out of the extruder. Extrusion was carried out at 190°C, and the produced granules were pulverized and sealed in plastic bags for further WPC manufacture.

Composite processing

All composite materials were placed in an oven at 103°C for 12 h prior to material mixing for composite manufacture and sealed in plastic bags to prevent from probable moisture content alterations. The materials were then weighed to the nearest 0.001 G and mixed based on their weight pecentages (Table 1) in a rotary mixer, Santam Co., Iran for 5 min. The obtained mixture was poured in a die of 28 × 28 × 1.3 cm and pressed under a hydraulic hot press, Santam Co., Iran at 185°C (35 bar, 18 min). The manufactured samples were then cold pressed at the same pressure for 10 min to release the incurred stresses. The schematic flow chart of the experimental process is given in Figure 1.

Table 1.

Composition and codes of composite formulations.

Samples code	Fiber content (wt%)	PP content (wt%)	Coupling agent content (wt%)	Treatment	Nano (wt%)
PMB	50	48	2	Untreated	0
PMBA	50	48	2	Treated	0
PMBNc	50	44	2	Untreated	4
PMBANc	50	44	2	Treated	4

Figure 1.

Schematic flow chart of the experimental process.

Composites characterization

Water absorption and thickness swelling tests were carried out according to ASTM D7031-11 standard specifications with five replications and calculated based on equations (2) and (3).

W A = (\frac{m_{w} - m_{o}}{m_{o}}) \times 100

(2)

Where: m_w and m_O are sample wet and oven-dried weights

T S = (\frac{t_{w} - t_{o}}{t_{o}}) \times 100

(3)

Where: t_w and t_O are sample wet and oven-dried thicknesses

Mechanical tests included unnotched impact and bending strengths which were conducted according to ASTM D256 and ASTM D790 specifications, respectively, using an Instron testing machine model 4489 with five sample replications.

The functional groups of the untreated and alkali treated fibers were determined and analysed by a Fourier Transform Infrared Spectroscopy (FT-IR) Tensor 27 IR spectrophotometer (Bruker Co., Germany). The transmittance range of the scan was 4000–370 cm⁻¹ for each composite sample.

SEM was conducted on the fractured surfaces of the composites to analyse the interfacial adhesion and mophology of the treated and untreated bagasse fiber/PP nanoclay composites by a MIRA III model SEM (Tescan Co., Czech Republic). The fractured surfaces were coated with a thin layer of gold before scanning.

Small-Angle X-ray Scattering (SAXS) analysis for WPC samples was conducted with a Phillips model PW17 C (Germany) operating with scattering angle, 2θ, scanned from 5° to 90° via Cu Kα (λ = 0.154 nm) radiation. XRD analysis was performed at 2 $θ$ = 0.8–12°. The d-spacing of WPCN samples was calculated based on the Bragg’s equation (Equation 4):

2 d \sin θ = n λ

(4)

Where: d is interplanar distance, $θ$ is half of diffraction angle, n is the order of diffraction (n = 1), and λ is X ray wavelength (nm).

Thermogravemetric analysis (TGA) can determine the percent by mass ratio of a solute using heat and stoichiometry ratios. TGA analyses of the WPCs were conducted using a thermal gravimetric analyzer (STA 504 thermal analysis, Co., Bahr, Germany). The WPC samples were heated up at a rate of 10 °C/min from 25 to 600°C.

Differential scanning calorimetry (DSC) can measure major thermoplastic properties, including melting temperature (Tm), melting enthalpy (ΔHm), crystallization temperature (Tc), crystallization enthalpy (ΔHc), and crystallinity rate (Xc). Thermal analysis of the WPC samples was carried out on a differential scanning Q600, Co.,TA, USA. All DSC measurements were performed with 15 ± 0.2 mg of each composite sample under a nitrogen atmosphere with a flow rate of 20 mL/min.

The statistical analyses were conducted in a completely randomized factorial design using SPSS software. One way analysis of variance (ANOVA) was employed, and the means were grouped by Duncan’s Multiple Range Test (DMRT) at 95% confidence level.

Results and discussion

Composites physical properties

Hydrophilicity and polarity of lignocellulosic fibers as well as the hydroxyl and carboxyl moieties existing on fiber surfaces lead to the formation of hydrogen bonding between water molecules and natural fibers.^24,25 Figure 2 shows the percentages of water absorption and thickness swelling in the studied nanocomposites after 3 weeks of immersion in water. There is the same water absorption and thickness swelling trends in the studied composte formulations.

Figure 2.

Physical properties of bio-nanocomposite formulations.

PMB showed the highest percentages of water absorption and thickness swelling, being about 8% and 4%, respectively. The Addition of 4wt% nanoclay cloisite 20A to composite formulation (PMBNc) could slightly decrease water absorption and thus thickness swelling by 6.22% and 8.39%, respectively, which were statistically non-significant. This is due to the barrier properties of nanoclay particles preventing from water ingress in to composites as well as the fact that they also fill the micro and nano voids present within the composites, thus reducing both of the mentioned physical properties.^26–28 Archival publications show that proper dispersion of nanoclay, hydrophobic nature of silicates present in the given nano, increased crystallinity rate due to the addition of nanoparticles can also lead to the immobilization of water and vapor molecules and thus reduced water absortion and thickness swelling rates in PMBNc composites.^29,30 However, Yeh and Gupta³¹ reported that the addition of >2% nanoparticles could increase composites’ water absorption rates due to the coagulation and agglomeration of nano fillers.

Alkali treatment reduced water absorption rate from 8% (PMB) to 7.21% (PMBA). Also, thickness swelling decreased by 10% in PMBA composite formulation, as compared to the reference one (PMB). This is due to the reduction of polar hydroxyl groups by alkali treatment which creates stronger bonds between the now less polar fibers and the apolar polymer matrix.^32–34 SEM micrographs (Figure 3) also confirm the better wettability and adhesion of NaOH treated bagasse fibers and the polymer (PMBA) vs. the reference composite formulations (PMB).

Figure 3.

SEM micrographs of fracture surfaces of composites, (a) PMB, (b) PMBNc, (c) PMBA, (d) PMBANc.

Alkali treatment also releases the fibers entangled in fiber bundles through dissolving the surface hemicellulose, pectin, and aliphatic compounds (e.g., waxes) on fiber bundles,^35–37 thus increasing not only the efficient fiber surface area to bond with the polymer but also the fiber distribution in the polymer matrix. Furthermore, the NaOH treated fiber surfaces grow rougher, thus enhancing fiber-matrix wettability by providing added fiber sites for mechanical entanglement.^35–38 Statistical analysis revealed that there were no significant differences between the water absorption and thickness swelling rates of PMBA and those of the PMBNc at 95% confidence level, whereas significant differences were observed when the given properties of PMBA composites were compared to those of the reference (PMB). PMBANc composite showed the highest reduction rates of water absorption and thickness swelling (∼20%). This can be attributed to the combined positive effects of nanoclay and alkali treatments on the mentioned physical properties. Except for thickness swelling rates of PMBA and PMBNc composites, there were significant differences between the water absorption and thickness swelling rates of PMBANc and all the other composite formulations at 95% confidence level.

Composites mechanical properties

Flexural strength

Flexural strength is defined as a material’s potential to resist deformation under static load. This represents the highest stress exerted on the material at the moment of rupture.³⁹ The flexural strength of composites depends on the properties of each composite material as well as the interfacial adhesion between the matrix and reinforcing and/or filler agents.

Figure 4 displays the flexural strengths of the studied composite formulations. As observed, the given strength slightly increased in PMBNc samples (29.67 MPa) as compared to that of the reference one; however, the given increase was not statistically significant at 95% confidence level. This increase can be attributed to miscellaneous reasons including the high aspect ratio of nanoclay, formation of intercalation and exfoliation nanoclay structures, filling the empty spaces and void in composite structure, thus introducing a denser and more rigid texture in nanocomposites,^40–42 strong interaction between the polymer and silicate layers due to formation of hydrogen bonds in the gallery spaces of nanoclay;^31,43–45 furthermore, heterogeneity and high ratio of surface to volume of the organically modified nanoclay cloisite 20A improve the reinforcing effect of nanoclay in the studied composites by increasing the interfacial area and the resulting interactions between the polymer matrix and bagasse fibers.²³ The existing literature demonstrates that the content of nanoclay addition in composite formulations plays a key role in the overall effects of the nanoparticles; it has been reported that agglomeration of nanoparticles leads to impaired interfacial adhesion between the phases and the resulting weak mechanical performance in nanocomposites.⁴⁰ When nanoclay concentration is increased, the solid nano particles may adversely affect the absorption of fracture energy rendering the nanocomposites more brittle; this is attributed to the harder and denser structure of nanos as well as their restrictiing effect on natural fiber and polymer entanglement and interlocking. Also, the nanofillers would attract much more compatibilizer and prevent from the proper bonding of the compatibilizer with wood fibers, undermining the mechanical strength of nanocomposites.^31,41

Figure 4.

Flexural Strengths of bio-nanocomposite formulations.

Figure 4 shows that the flexural strength of NaOH treated bagasse fiber composites (PMBA) increased (30.21 MPa) as compared to that of the reference, being statistically significant at 95% level. This is probably due to the dissolution of fibers’ surface impurities such as pectin and waxes, and likewise the improved wettability (polymer encapsulation), interfacial adhesion, and distribution of alkali treated bagasse fibers in the polymer matrix;⁴⁶ this can also be observed in SEM microgrphs of the fractured surfaces of PMBA composites (Figure 3).

The combined effects of alkali treatment and nanoclay introduced a marked increase (11%) in the flexural strength of PMBANc composites versus PMB ones, which was statistically different with those of the other studied formulations. The given treatments improved the interfacial adhesion between the bagasse fibers and the polymer, thus increasing flexural strength in PMBANc formulations. This can be attributed to the formation of a more homogeneous composite structure due to the synergistic effects of the combined treatments; this leads to better stress transfer, thus fewer stress concentration points in the given nanocomposite materials when a static load (stress) is introduced on the given samples.

Modulus of elasticity

Modulus of elasticity (MOE) is a measure to describe stiffness in solid materials; in other words, it measures a material’s resistance against elastic deformation under stress. Figure 5 shows that alkali, nanoclay, and the combined treatments had positive effects on the MOE of composite formulations. As Figure 5 displays, there is an increase in the MOE of PMBNc nanocomposites (5248.34 MPa), as compared to that of the reference (5057.33 MPa). MOE of a composite is a function of the MOE values of the composite’s individual components.^47,48 The MOE of nanocomposites depends on a variety of factors such as nanoclay content, surface to volume ratio, aspect ratio (nano filler length to diameter), formation of intercalation and exfoliation structures, strong entanglement of nanoclay with the polymer, and the nucleating effect of well-dispersed orgnically modified nanoclay particles in the polymer matrix, which generally improve the MOE and strength performance of nanocomposites, owing to the stiffness of nanoclay silicate layers.^49,50 Studying the effects of nanoclay (Cloisite 20A) on wood fiber/HDPE composites, Lee et al. (2010) reported that the incorporation of nanoclay could increase MOE, such that the exfoliated composites had larger MOE values than the intercalated ones. Also, Compatibilizers hold the potential to reduce and near the surface energy of natural fibers to that of the polypropylene; this leads to better wettability and encapsulation of fibers by the polymer, thus increasing the entanglement and adhesion of nanoparticles with both composite phases.⁵¹ Similar results have also been reported by other researchers.^52,53

Figure 5.

Modulus Elasticity of bio-nanocomposite formulations.

As Figure 5 depicts, there is an increase in the MOE value of PMBA composites (5306.33 MPa), compared to that of the reference. This is due to the reaction of NaOH with the hydroxyl moieties of bagasse fibers converting them to opened-up microfibrils; this phenomenon (i.e., fibrillation) introduces increases in the efficient surface area of fibers to bond with the polymer matrix, fiber wetting and encapsulation by the polymer, bagasse fiber distribution in the polymer, interfacial adhesion between the two phases. These are the reasons behind the enhanced mechanical strengths of alkali treated composites.^51,54 Mercerization of bagasse fibers with sodium hydroxide also makes the fibers more rigid and fragile, engendering an additional increase in the MOE value of PMBA formulations. The combined treatment led to a statistically significant difference of the MOE value (5644.3 MPa) of PMBANc composites with those of the other composite formulations at 95% level. The combined treatment seems to have more efficiently increased the interfacial adhesion between the bagasse fibers and the recycled polypropylene. This probably led to facilitated stress transfer between the two mentioned phases; therefore, a larger stress load is needed should the composites be fractured.^55,56 The interactive effect of nanoclay and alkali treatment could possibly improve particle dispersion in the resulting composite materials, which can be observed in the corresponding SEM micrographs (Figure 3).

Impact strength

Impact strength of a composite is affected by an array of factors such as the hardness of the reinforcing agent, the nature of the interfacial surface, and the frictional energy engaged in pulling out the fibers from the polymer.^57,58 As shown in Figure 6, thers is a slight reductioin in the impact strength value of the nanoclay treated composites (7.24 kj/m²), as compared to that of the reference (7.41 kj/m²). This is in line with the results reported by Khanjanzadeh et al. (2011); they related the reduction of impact strength to the restriction of polymer chain movements due to the effect of nanoparticles, which undermines the ability of composites to absorb the applied impact energy.^6,59 Furthermore, the formation of non-dispersed fiber bundles and voids would impair impact strength.^45,60–62 The lowest impact strength was observed in alkali treated composites (PMBA), being 7.23 kj/m²; however, there was not a statistically significant difference between the mentioned value and those of the PMB and PMBNc composites. The sodium hydroxide treatment of bagasse fibers reinforces the interaction between wood-polymer phases, bringing about strong adhesion; thus, the composites would be fractured at relatively lower impact energy values. On the other hand, composites with weaker interfacial adhesion show higher impact strength values due to the formation of micro-cracks in the impact test. The mentioned micro-cracks propagate along the fiber/polymer interface and cause fiber/matrix debonding, imparting a higher energy-absorption capacity to the composites.^63,64 However, the reduction of impact strength in the alkali treated composites may be due to the immobilization of the macromolecular polymer chains introduced by bagasse fibers; this undermines the ability of composite materials to adapt to the deformation, thereby causing the composites to be more brittle.^65,66 As Figure 6 illustrates, there is an interesting improvement in the impact strength of PMBANc composites, which is statistically sifnificant at 95% confidence level, as compared to the impact strength values of the other composite formulations. As previously mentioned, there are contradictory results for the impact strength of WPC in the archival publications, which needs to be further studied. Also, the unnotched impact strength test is highly dependent on the surface properties of composite samples such that in case of the presence of a hard and rigid substance on the sample surface, the impact strength is expected to rise.⁶⁷

Figure 6.

Impact Strength of bio-nanocomposite formulations.

Bagasse fiber characterization

The effects of chemical treatment on bagasse fiber structure was analysed by Fourier transform infra-red spectroscopy (FTIR). Figure 7 displays the results of FTIR analysis of bagasse fibers prior to and after NaOH modification in the wavelength range of 500–4000 cm⁻¹ The band at 3440 cm⁻¹ shows the –OH groups stretching in bagasse fibers, which is observed at a lower intensity of infra-red absorbance in the treated fibers, as compared to that of the untreated fibers.^68,69 This is ascribed to the reduction of hydroxyl moieties due to alkali treatment, rendering the fibers less hydrophilic and thus more chemically reactive towards the hydrophobic polymer matrix. There are also strong peaks at 2923 cm⁻¹, which belong to the C-H stretching of aromatic methoxyl and methylene groups existing in lignin.^70,71 The given peaks did not have significant differences in the alkali treated and treated fibers, demonstrating that the treatment had almost no effect on the removal of lignin polymer components. The absorption bands at 1738 cm⁻¹ can be attributed to the vibrational stretching of unconjugated carbonyl functional groups (C = O), present in fiber hemicelluloses.^72,73 As observed in Figure 7, the given band has disappeared in the alkali treated fibers due to hemicellulose removal by sodium hydroxide treatment. Furthermore, the increase in absorption intensity at 1245 cm⁻¹ is relative to C-O stretching of acetyl groups in hemicelluloses,⁷⁴ which is eliminated in the alkali treated fibers. The peak at 1605 cm⁻¹ can be related to the bending mode of the absorbed water and also some contributions from carboxylate moieties present in hemicelluloses structure, having transferred to 1590 cm⁻¹; this can be referred to the removal of bound water from hemicellulose due to the given treatment.⁷⁵ The peak at 1432 cm⁻¹ is for the stretching C-H bonds in the amorphous and crystalline parts of cellulose chains, having both transferred to 1429 cm⁻¹ and increased in intensity; this can be due to the release of C-H groups from fiber surfaces after alkali treatment.⁷⁶ The peak at 1047 cm⁻¹ relates to the stretching C-C and C-O bonds in hemicellulose xylan, and C-O-C bond in the glycosidic linkages of hemicelluloses,⁷⁷ having remarkably increased in intensity after alkali treatment. This can be ascribed to fibrillation phenomenon caused by NaOH treatment, which opens up the once-interwoven cellulose and hemicelluloses fibers, thereby making the above-mentioned stretching bonds accessible to absorb the infra-red light, introduced on the test specimens, in FTIR analysis. The vibrating peaks at 898 cm⁻¹ are due to β-glycosidic bonds of glucose rings in cellulose, being symmetric in polysaccharides.^78–80

Figure 7.

FTIR spectra of untreated and alkali treated bagasse fibers.

Morphological analysis

SEM is an analysis conducted on the fractured surfaces of composite materials to study the interfacial adhesion between the reinforcing agent and the polymer matrix; this is a qualitative measure showing the reasons behind the overall evolution of composite properties. As observed in Figure 3, there are some gaps and cavities in the fracture surface of PMB composites (Figure 3(a)) showing fiber pull-outs due to the wealk interfacial adhesion between bagasse fibers and the polymer. The incorporation of nanoclay in composite formulations incurred some alterations in the fracture surfaces of PMBNc composites (Figure 3(b)). Nanoclay particles can be observed in the given figure leading to better fiber-polymer interfacial adhesion, since the organically modified nano particle could make some linkages beween composite phases. Although few gaps are also seen, the fiber fractures observed in Figure 3(b) reveal the better fiber-polymer interlocking in the PMBNc composites than the PMB ones confirming the improved physical and mechanical (except for impact strength) performance of the nanoclay cloisite 20A treated composites versus the untreated counterparts. The effects of NaOH treatment on bagasse fibers are seen in Figure 3(c) (PMBA). The given composite shows a different fracture surface compared to that of the reference. There are fewer and smaller cavities but remarkably more fiber pull-outs in PMBA composites than the PMB ones demonstrating the better fiber-polymer bonding in the former composites than the latter ones. The dissolution of fiber surface impurities (e.g., pectin and waxes) could also improve the wettability and thus interfacial adhesion between fiber-polymer phases in the alkali treated composites versus the reference ones.⁸¹ Furthermore, the alkali-induced fibrillation in PMBA composites could increase fiber surface areas bonding with the other composite components (i.e., compatibilizer and polypropylene).^82–85 There is a marked difference in the fracture surfaces of PMBANc composites with those of the other composite formulations (Figure 3(d)). The whole surface is fairly uniform and smooth showing a good fiber-polymer wetting. There is almost no distinct cavities and gaps between the two phases. Also, The nanoclay particles are well dispersed through out the fractured surfaces. The fiber fracture observed in the given micrograph is another reason behind the significantly better overall properties of PMBANc composites than other formulations.

Small-Angle X-ray Scattering (SAXS) analysis

X-ray diffraction spectra, provide invaluable data on structure, type of materials, and likewise their dispersion quality. Table 2 shows the X-ray diffraction results of wheat/bagasse recycled PP composites containing Nano-clay particles at the angles of 0.8–12°. The SAXS analysis contributed to measure the intercalation and/or exfoliation rates through analysing the main diffraction peaks in the crystalline layered structure of organically modified nanoclay.

Table 2.

SAXS analysis of nanoclay and its composites.

Sample code	2θ°	d-spacing (A°)
Nc	4.8	18.33
PMBNc	4.30	20.53
PMBANc	4.20	21.95

As shown in Table 2, 2θ = 4,80° pertains to neat nanoclay with basal spacing of 18.33 A°. In PMWNc, PMWANc, PMBNc, and PMBANc formulations, the peaks were shifted to lower 2θ angles, and therefore higher interplanar d-spacing values than cloisite 20A demonstrating the formation of intercalation structures. The shifting of the peak to lower angle can be assigned to the increase in the interlayer spacing of silicate layers present in the structure of the incorporated nanoclay in the composite formulations. This reveals that the crystalline chains of polypropylene were intercalated into the silicate layers of the nanoclay. Diffraction peak shifting to a lower angle has also been reported for nano clay based HDPE biocomposite.⁴³

To note that the reported increases in the interplanar d-spacing values of the studied nanocomposites indicate the occurrence of only low intercalation (vs. exfoliation) structures in the given composites, since interplanar distances > 70 Å are attributed to total exfoliation phenomenon, which refer to 2θ values close to 0°.¹⁴

Composites thermal properties

TGA

Thermogravimetric analysis was conducted to study the thermal stability of composite materials. The results are tabulated in Table 3. As observed, the initial degradation temperature (Ti), the thermal degradation temperature at 10% and 50% weight losses (T10 and T50), and residual char yield were measured by TGA analysis. The results indicate that the thermal degradation of baggase (Ti) started at about 240°C,^86,87 relative to the low degradation stage of cellulose and hemicellulose, and lignin. The degradation temperatures of hemicelluloses, cellulose, and lignin are between 150–350°C, 275–350°C, and 250–500°C, respectively.^88–91 Thus, the main sources of the first stage of thermal degradation in the studied composites are hemicelluloses, cellulose, and lignin.^89,92 From the literature, as hemicelluloses’ thermal degradation start, CO, CO₂, condensable vapors, and organic acids also start to form.⁹³ The formation of the resulting organic acids accelerates the degradation of other polysaccharides, thereby intensifying wood decomposition in the inert atmosphere.^94,95

Table 3.

TGA analysis of bagasse fiber/polypropylene nanocomposites.

Sample code	T_i	T_d1	T_d2	T₁₀ (°C)	T₅₀(°C)	Char (%)
PMB	242.1	321.0	421.5	320.1	450.1	12.5
PMBA	242.0	321.0	421.4	319.7	450.0	11.8
PMBNc	242.8	324.6	431.5	325.1	451.2	13.6
PMBANc	244.1	326.1	445.7	325.5	451.4	13.5

The TGA results reveal that the alkali treatment (PMBA) slightly reduced the fibers thermal behavior, which can be attributed to composites’ lignin content. The FTIR spectra demonstrate that lignin was not efficiently degraded through alkaline treatment.^96,97 Alkali treatment is reported to impair thermal performance as a result of the interactive effects of transformation of cellulose, crystallinity, etc., which need be further studied.⁸⁶ However, the given treatment has also led to the improved themal stability of composites.^74,98 Their results showed that alkali treatments degraded portions of hemicellulose and lignin from the fiber. Thus, the decomposition process mainly occurred on the cellulose, thereby raising the overall degradation temperature. Also, the alkali treated composites release water at higher temperatures as a result of better encapsulation of the finely separated fibers by the polymer matrix. This led to stronger polymer-fiber interfacial adhesion, thus downsizing the accessible surface area for moisture release. This engenders delay in moisture evaporation bringing about elevated thermal decomposition temperatures.⁶⁴ TGA results also indicate that the incorporation of nanocly improved the thermal properties of PMWNc, as compared to those of the PMW and PMWA composites. Different mechanisms can play a role in this regard, briefed as follows: The silicate layers of the studied nanoparticle expand the tortuous path, thus delaying the diffusion of degraded volatile compounds throughout the composites due to their high aspect ratio as well as formation of intercalation structure)⁹⁹ A char layer is formed on the external surfaces of composite materials by nanoclay/silica, and this leads to the insulation of the sub layer, therby preventing the nanocomposite from burning and the resulting higher degradation temperatures and better thermal stability)¹⁰⁰ Strong interaction between the nanoclay silicate layers and polypropylene chains can limit chain mobility and thus delay conveying free radicals formed in the course of polymer fragmentation brought about by thermal degradation, thus enhancing the thermal stability of nanocomposites)^101,102 Rigid, impervious nanoclay layers may decrease heat conduction; thus, their presence in nanocomposite formulations not only limits the mobility of polymer chains but also inherently improves composites thermal performance)¹⁰³ The pores created by the crystalline structure of nanoclay cloisite 20A act as some thermal insulation sites leading to enhanced thermal performance.^104–106

However, Lei et al. (2007) attested that the presence of 2 wt% nanoclay slightly decreased the degradation temperature of wood flour/HDPE nanocomposites. They attributed this to the released low-molecular-weight compounds, by which the applied nanoclay particles were modified to become organophilic (i.e., nanoclay).¹⁰⁷

The combined treatment of PMW/BANc/Ns composites by NaOH and nanoclay/nanosilica reveals that the mentioned treatment engendered an interesting synergistic effect on increasing Ti, Td1, Td2, and char yield values of the studied nanocomposites, the individual alkali treatment had no distinct effect, though. This can be attributed to the degradation of certain amounts of wax, oil, lignin, and hemicelluloses compounds, the least thermally stable constituent of bagasse fibers ( by alkaline treatment) leading to the more and better accessibility of nanoclay/silica particles to the exposed cellulose fiber moieties, i.e., hydroxyl groups, and thus stronger interfacial adhesion and higher crystallinity values (confirmed by DSC results given in Table 4 due to the nucleating effect of nanoplatelets.⁹⁷ Moreover, the partial dissolution of cementing polymers resulting from the rupture of ester linkages between polyuronic acid and lignin can play a role in imparting a more crystalline structure to the alkali-treated wood fibers.¹⁰⁸ These two reasons are possibly behind the considerably improved thermal properties of the studied nanocomposites after the combined alkali-nano clay/silica treatment, compared to other composite formulations.

Table 4.

DSC analysis of bagasse fiber/polypropylene nanocomposites.

Sample code	T_m(°C)	T_C (°C)	X_C (%)	ΔH_C (j/g)	ΔH_m (j/g)
PMB	171.2	121.5	30.1	65.1	60.1
PMBA	170.0	121.2	30.1	64.9	59.5
PMBNc	172.6	122.4	30.2	67.2	60.5
PMBANc	174.5	123.4	30.8	68.8	62.5

DSC

The results of DSC analysis including melting temperature (Tm), melting enthalpy (ΔHm), crystallization temperature (Tc), crystallization enthalpy (ΔHc), and degree of crystallinity (Xc) of all the studied composite formulations are given in Table 4.

The DSC results attest that the alkali treatments very slightly impaired the thermal properties of PMBA composites. This is due to lignin, which serves as a cementing agent between the fibers, thus preventing from the penetration of sodium hydroxide into these areas.^96,97 The FTIR results (Figure 7) also confirmed that alkaline treatment could not efficiently remove lignin in the composites.

It has been reported that higher concentrations of alkali treatment would significantly weaken thermal stability due to the combined effects of transformation of cellulose and crystallinity, which need be further studied. Also, the decreased crystallinity of WPCs was attributed to the hindering effect of wood fibers on the formation of polymer crystals.⁹⁶ Kallakas et al. (2015) declared that Tc and Xc of PP/WF composites treated with 5wt% NaOH were higher than those of the untreated composites due to better nucleatin ability of the treated wood fibers,¹⁰⁹ which expedite polymer crystallisation, thereby increasing crystallisation temperature (Tc).¹¹⁰ Furthermore, Alkali treatment can increase ΔHc by increasing crystallisation in heterogeneous nucleation spots. This, therefore, enhances crystallization rate (Xc) in alkali treated composites, as compared to that of the untreated ones.¹¹⁰ The crystallinity index of coir fibers have been reported to improve by alkali treatment due to a partial degradation of cementing materials imparted by the rupture of ester linkages between polyuronic acid and lignin.^108,111 PMBNc composites rendered significantly better thermal performance than PMB and PMBA composites.

The melting and crystallization temperatures of nanocomposites have been reported to increase by nanoclay incorporation into the composites.¹¹² This was attributed to the strong intyerlocking between the polymer molecules and the nanosilicate layers present in, and therefore the immobilization of polymer chains. These immobilised molecules of polymer are responsible for the crystallization process of nanocomposite. Therefore, the crystallization and melting temperatures of the polymer chains in nanocomposites shifted to higher values.¹¹³ To note that polymer chains can crystallize either via a self-nucleation process (homogeneous nucleation) or by incorpotating a nucleating agent, namely nano reinforcing or filler particles (heterogeneous nucleation).¹¹⁴ The enhanced crystallinity can be ascribed to the intercalation of polypropylene chains between MMT silicate layers and the possible interactions between them, where MMT nanoplatelets would serve as nucleating agents in PP/rice husk nanocomposite.¹¹⁵ It has also been asserted that the impervious crystalline regions improve stress transfer and energy absorption, thus enhancing composites’ overall mechanical performance.¹¹⁶ There are as well some contadicting results affirming that organoclay incorporation in to the polymer brought about an 18% decrease in the crystallininty rate of composites due to the creation of some disturbances in the crystalline structure of polymer matrix by the given nano fillers.¹¹⁶ The results of DSC analysis conducted on PMBANc composites demonstrated that the combined nanoclay and alkali treatment treatments could markedly raise melting temperature, melting enthalpy, crystallisation temperature, crystallisation enthalpy, and crstallinity rate of the given composites (Table 4). This can be attributed to the degradation of certain amounts of wax, oil, lignin, and hemicellulose compounds, the least thermally stable portions of wood fibers by alkaline treatment. This leadsto the more and better accessibility of nanoclay/silica particles to the exposed cellulose fiber moieties, i.e., hydroxyl groups, and thus stronger interfacial adhesion and higher thermal performance due to the nucleating effect of nanoplatelets.

Conclusion

The individual and combined effects of alkali and nanoclay cloisite 20A treatments on structure-property relationships of totally waste-based recycled polypropylene/bagasse fiber composites were studied; the results are as follows:

1. Alkaline and nanoclay treatments had positive effects on the flexural strength and MOE of the studied composites, in particular when a combination of both treatments was applied; this was attributed on one hand to the modification of fiber surfaces by NaOH leading to improved polymer-fiber interfacial adhesion and on the other hand to the resulting better dispersion of organically modified nanoclay in composite structure. The individual and, more specifically, the combined treatments could improve the physical properties (i.e., water absorption and dimensional stability) of the given composites.

2. The impact strengths of PMBA and PMBNc composites were lower than the reference composites due to the impaired impact energy absorption of the strong fiber-polymer interfaces in the former composites and the restriction of polymer chain movements by nanoparticles in the latter composite. It was interestingly observed that the impact strength increased when the combined treatment was introduced; this was ascribed to the probable coagulation of nanoclays in the interface of NaOH-treated fibers-polymer, weakening fiber-polymer interfacial adhesion, and thus, the occurrence and propagation of micro-cracks in the impact test would possibly increase the energy-absorption capacity of PMBANc composites. The sodium hydroxide treatment of bagasse fibers reinforces the interaction between wood-polymer phases, bringing about strong adhesion.

3. TGA and DSC analyses revealed the improved thermal properties of nanoclay and combined treatments on composite materials. Nanoclay hindered the progress of heat towards the deeper layers of composites by forming a carbonized heat insulating char layer. Although alkali treatment had no distinct effect on PMBA composites, the combined treatment could significantly enhance thermal properties in PMBANc samples.

4. It can be safely concluded that totally waste-based bagasse fiber/recycled polypropylene composites could be significantly valorized for different applications by the synergistic effects of nanoclay/alkali treatments.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Reza Naghdi

References

Nadali

Karimi

Tajvidi

, et al. Natural durability of a bagasse fiber/polypropylene composite exposed to rainbow fungus (Coriolus versicolor). Journal of Reinforced Plastics and Composites 2010; 29: 1028–1037.

Naghdi

Karimi

Latibari

, et al. Biological removal of chloro-organic compounds from bagasse soda pulp bleaching effluent by Coriolus versicolor. Global NEST Journal 2013; 15: 29–36.

Nadali

Layeghi

Ebrahimi

, et al. Effects of multiple extrusions on structure-property performance of natural fiber high-density polyethylene biocomposites. Mat Res 2018; 21: e20170301.

Nadali

Naghdi

. Effects of multiple extrusions on structure–property relationships of hybrid wood flour/poly (vinyl chloride) composites. Journal of Thermoplastic Composite Materials 2020; 35: 1076–1093.

Motaung

Mngomezulu

Hato

. Effects of alkali treatment on the poly(furfuryl) alcohol–flax fibre composites. Journal of Thermoplastic Composite Materials 2016; 31: 48–60.

Xiong

. Bagasse composites: A review of material preparation, attributes, and affecting factors. Journal of Thermoplastic Composite Materials 2018; 31: 1112–1146.

Zainal

Santiagoo

Ayob

, et al. Thermal and mechanical properties of chemical modification on sugarcane bagasse mixed with polypropylene and recycle acrylonitrile butadiene rubber composite. Journal of Thermoplastic Composite Materials 2020; 33(11): 1533–1554.

Yiga

Pagel

Lubwama

, et al. Development of fiber-reinforced polypropylene with NaOH pretreated rice and coffee husks as fillers: mechanical and thermal properties. Journal of Thermoplastic Composite Materials 2020; 33: 1269–1291.

Cao

Shibata

Fukumoto

. Mechanical properties of biodegradable composites reinforced with bagasse fibre before and after alkali treatments. Composites Part A: Applied Science and Manufacturing 2006; 37: 423–429.

10.

. Impact behavior of sawdust/recycled–PP composites. J Appl Polym Sci 2001; 81: 1420–1428.

11.

Vazquez

Dominguez

Kenny

. Bagasse fiber-polypropylene based composites. Journal of Thermoplastic Composite Materials 1999; 12: 477–497.

12.

Advani

. Processing and properties of nanocomposites 2006, World Scientifics.

13.

Nejat

Jalalinezhad

Hormozi

, et al. Hydrogen production from steam reforming of ethanol over Ni-Co bimetallic catalysts and MCM-41 as support. Journal of the Taiwan Institute of Chemical Engineers 2019; 97: 216–226.

14.

Pascual

Fages

Fenollar

, et al. Influence of the compatibilizer/nanoclay ratio on final properties of polypropylene matrix modified with montmorillonite-based organoclay. Polymer Bulletin 2009; 62: 367–380.

15.

Pattanakul

Selke

Lai

, et al. Properties of recycled high density polyethylene from milk bottles. J Appl Polym Sci 1991; 43: 2147–2150.

16.

Ratnayake

Haworth

. Polypropylene–clay nanocomposites: Influence of low molecular weight polar additives on intercalation and exfoliation behavior. Polym Eng Sci 2006; 46: 1008–1015.

17.

Smith

Wolcott

. Opportunities for wood/natural fiber-plastic composites in residential and industrial applications. Forest Products Journal 2006; 56: 4.

18.

Younesi Korekhili

Naghdi

Amiri

. The effect of nanoclay on physicochemical, mechanical and thermal properties of new urea- glyoxal resin. Iranian Journal of Wood and Paper Industries 2015; 6: 133–143.

19.

Youngquist

Myers

Muehl

, et al. Composites from recycled wood and plastics. Cincinnati, OH: US Environmental Protection Agency, 1995.

20.

Yousefi

Kartoolinejad

Naghdi

. Effects of priming with multi-walled carbon nanotubes on seed physiological characteristics of Hopbush (Dodonaeaviscosa L.) under drought stress. International Journal of Environmental Studies 2017; 74: 528–539.

21.

Ramos Filho

Mélo

TJA

Rabello

, et al. Thermal stability of nanocomposites based on polypropylene and bentonite. Polymer Degradation and Stability 2005; 89: 383–392.

22.

Wang

Zeng

Elkovitch

, et al. Processing and properties of polymeric nano‐composites. Polym Eng Sci 2001; 41: 2036–2046.

23.

Lei

Yao

, et al. Properties of HDPE/clay/wood nanocomposites. In: 2007 First international conference on integration and commercialization of micro and nanosystems, Sanya, China, 10–13 January 2007.

24.

Naghdi

Hosseini

Resalati

. Evaluation of paper properties obtained from maple juvenile wood through Kraft pulping process. Iranian Journal of Natural Resources 2008; 60: 1465–1472.

25.

Naghdi

Karimi

Latibari

, et al. Evaluation of the effect of rainbow fungus (Coriolus versicolor) bleach plant effluent color reduction. Iranian Journal of Natural Resources 2009; 61: 989–995.

26.

Deka

Maji

. Effect of TiO2 and nanoclay on the properties of wood polymer nanocomposite. Composites Part A: Applied Science and Manufacturing 2011; 42: 2117–2125.

27.

Kokta

Michalkova

, et al. Characteristics of wood-plastic composites reinforced with organo-nanoclays. Journal of Reinforced Plastics and Composites 2010; 29: 3566–3586.

28.

Turku

Kärki

. Research progress in wood-plastic nanocomposites: a review. Journal of Thermoplastic Composite Materials 2014; 27: 180–204.

29.

Ashori

Nourbakhsh

. Preparation and characterization of polypropylene/wood flour/nanoclay composites. Eur J Wood Prod 2011; 69: 663–666.

30.

Najafi

Kord

Abdi

, et al. The impact of the nature of nanoclay on physical and mechanical properties of polypropylene/reed flour nanocomposites. Journal of Thermoplastic Composite Materials 2012; 25: 717–727.

31.

Yeh

Gupta

. Nanoclay‐reinforced, polypropylene‐based wood–plastic composites. Polym Eng Sci 2010; 50: 2013–2020.

32.

Arrakhiz

Elachaby

Bouhfid

, et al. Mechanical and thermal properties of polypropylene reinforced with Alfa fiber under different chemical treatment. Materials & Design 2012; 35: 318–322.

33.

Bessadok

Marais

Gouanvé

, et al. Effect of chemical treatments of Alfa (Stipa tenacissima) fibres on water-sorption properties. Composites Science and Technology 2007; 67: 685–697.

34.

El-Abbassi

Assarar

Ayad

, et al. Effect of alkali treatment on Alfa fibre as reinforcement for polypropylene based eco-composites: mechanical behaviour and water ageing. Composite Structures 2015; 133: 451–457.

35.

Hosseini

Naghdi

. Evaluation on juvenile period and fiber length variation of maple wood. Acer Velutinum Boiss) 2004; 11: 7–16.

36.

Naghdi

Nadali

Younesi Kordkheili

. Evaluation of pulp and paper properties obtained from maple juvenile wood through organosolv alcohol method catalyzed by calcium and magnesium salts. Iranian Journal of Wood and Paper Industries 2015; 6: 31–40.

37.

Rahmati

Nourmohammadi

Naghdi

, et al. Effect of fungal degradation on physicochemical properties of exploited stumps of oriental beech over a 25-year felling period and the obtained Kraft pulp properties. J For Sci (Prague) 2019; 65: 96–105.

38.

Joseph

Mathew

Joseph

, et al. Dynamic mechanical properties of short sisal fibre reinforced polypropylene composites. Composites Part A: Applied Science and Manufacturing 2003; 34: 275–290.

39.

den van Oever

MJA

Bos

Van Kemenade

. Influence of the physical structure of flax fibres on the mechanical properties of flax fibre reinforced polypropylene composites. Applied Composite Materials 2000; 7: 387–402.

40.

Ashori

Nourbakhsh

. Effects of nanoclay as a reinforcement filler on the physical and mechanical properties of wood-based composite. Journal of Composite Materials 2009; 43: 1869–1875.

41.

Kokta

. Maximization of the mechanical properties of birch-polypropylene composites with additives by statistical experimental design. Journal of Thermoplastic Composite Materials 2010; 23: 239–263.

42.

Han

Lei

, et al. Bamboo–fiber filled high density polyethylene composites: effect of coupling treatment and nanoclay. J Polym Environ 2008; 16: 123–130.

43.

Faruk

Matuana

. Nanoclay reinforced HDPE as a matrix for wood-plastic composites. Composites Science and Technology 2008; 68: 2073–2077.

44.

Sun

Schork

Deng

. Water-based polymer/clay nanocomposite suspension for improving water and moisture barrier in coating. Composites Science and Technology 2007; 67: 1823–1829.

45.

Zhao

Wang

Zhu

, et al. Properties of poly (vinyl chloride)/wood flour/montmorillonite composites: effects of coupling agents and layered silicate. Polymer Degradation and Stability 2006; 91: 2874–2883.

46.

Kord

Danesh

Veysi

, et al. Effect of virgin and recycled plastics on moisture sorption of nanocomposites from newsprint fiber and organoclay. BioResources 2011; 6: 4190–4199.

47.

Girones

Pardini

Vilaseca

, et al. Recycling of paper mill sludge as filler/reinforcement in polypropylene composites. J Polym Environ 2010; 18: 407–412.

48.

Yildiz

Gümüşkaya

. The effects of thermal modification on crystalline structure of cellulose in soft and hardwood. Building and Environment 2007; 42: 62–67.

49.

Park

. Functional fibers, composites and textiles utilizing photothermal and joule heating. Polymers 2020; 12: 189–198.

50.

Mammeri

Bourhis

Rozes

, et al. Mechanical properties of hybrid organic–inorganic materials. J Mater Chem 2005; 15: 3787–3811.

51.

Lee

Kuboki

Park

, et al. The effects of clay dispersion on the mechanical, physical, and flame‐retarding properties of wood fiber/polyethylene/clay nanocomposites. J Appl Polym Sci 2010; 118: 452–461.

52.

Chowdhury

MJA

Wolcott

. Compatibilizer selection to improve mechanical and moisture properties of extruded wood-HDPE composites. Forest Products Journal 2007; 57: 46–54.

53.

Younesi

Kazemi

. A Comparison between steam and water absorption behavior in polypropylene-wood fiber composite. Journal of Wood and Forest Science and Technology 2011; 18: 129–134.

54.

Agunsoye

Aigbodion

. Bagasse filled recycled polyethylene bio-composites: morphological and mechanical properties study. Results in Physics 2013; 3: 187–194.

55.

Son

Kim

Lee

. Role of paper sludge particle size and extrusion temperature on performance of paper sludge–thermoplastic polymer composites. J Appl Polym Sci 2001; 82: 2709–2718.

56.

Väntsi

Kärki

. Different coupling agents in wood-polypropylene composites containing recycled mineral wool: a comparison of the effects. Journal of Reinforced Plastics and Composites 2015; 34: 879–895.

57.

Bledzki

Gassan

. Composites reinforced with cellulose based fibres. Progress in Polymer Science 1999; 24: 221–274.

58.

Zhou

Jiang

, et al. The reinforcement efficacy of nano-and microscale silica for extruded wood flour/HDPE composites: the effects of dispersion patterns and interfacial modification. Journal of Materials Science 2018; 2018: 1899–1902.

59.

Khanjanzadeh

Tabarsa

Shakeri

, et al. Effect of organoclay platelets on the mechanical properties of wood–plastic composites formulated with virgin and recycled polypropylene. Wood Material Science & Engineering 2011; 6: 207–212.

60.

Kord

Hemmasi

Ghasemi

. Properties of PP/wood flour/organomodified montmorillonite nanocomposites. Wood Sci Technol 2011; 45: 111–119.

61.

Hanumantha Rao

Forssberg

Forsling

. Interfacial interactions and mechanical properties of mineral filled polymer composites: wollastonite in PMMA polymer matrix. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1998; 133: 107–117.

62.

Yuan

Misra

. Impact fracture behavior of clay–reinforced polypropylene nanocomposites. Polymer 2006; 47: 4421–4433.

63.

Nourbakhsh

Ashori

. Wood plastic composites from agro-waste materials: Analysis of mechanical properties. Bioresour Technol 2010; 101: 2525–2528.

64.

Ray

Sarkar

Basak

, et al. Thermal behavior of vinyl ester resin matrix composites reinforced with alkali‐treated jute fibers. J Appl Polym Sci 2004; 94: 123–129.

65.

Unal

. Morphology and mechanical properties of composites based on polyamide 6 and mineral additives. Mater Des 2004; 25: 483–487.

66.

Valadez-Gonzalez

Cervantes-Uc

Olayo

, et al. Effect of fiber surface treatment on the fiber–matrix bond strength of natural fiber reinforced composites. Composites Part B: Engineering 1999; 30: 309–320.

67.

Othman

Ismail

Mariatti

. Effect of compatibilisers on mechanical and thermal properties of bentonite filled polypropylene composites. Polymer Degradation and Stability 2006; 91: 1761–1774.

68.

Cerqueira

Baptista

Mulinari

. Mechanical behaviour of polypropylene reinforced sugarcane bagasse fibers composites. Procedia Engineering 2011; 10: 2046–2051.

69.

Cui

Liu

, et al. Influence of steam explosion pretreatment on the composition and structure of wheat straw. BioResources 2012; 7: 4202–4213.

70.

Kalia

Kaith

Kaur

. Pretreatments of natural fibers and their application as reinforcing material in polymer composites—a review. Polym Eng Sci 2009; 49: 1253–1272.

71.

Xiao

Sun

. Chemical, structural, and thermal characterizations of alkali-soluble lignins and hemicelluloses, and cellulose from maize stems, rye straw, and rice straw. Polymer Degradation and Stability 2001; 74: 307–319.

72.

Liu

Zhong

Wang

, et al. Interactions of biomass components during pyrolysis: A TG-FTIR study. Journal of Analytical and Applied Pyrolysis 2011; 90: 213–218.

73.

Sgriccia

Hawley

Misra

. Characterization of natural fiber surfaces and natural fiber composites. Composites Part A: Applied Science and Manufacturing 2008; 39: 1632–1637.

74.

Kabir

. Effects of chemical treatments on hemp fibre reinforced polyester composites. Darling Heights: University of Southern Queensland, 2012.

75.

Peng

Ren

J-L

, et al. Fractional study of alkali-soluble hemicelluloses obtained by graded ethanol precipitation from sugar cane bagasse. J Agric Food Chem 2010; 58: 1768–1776.

76.

Simão

Carmona

Marconcini

, et al. Effect of fiber treatment condition and coupling agent on the mechanical and thermal properties in highly filled composites of sugarcane bagasse fiber/PP. Mat Res 2016; 19: 746–751.

77.

Wang

Jiang

, et al. Influence of steam pressure on the physico-chemical properties of degraded hemicelluloses obtained from steam-exploded lespedeza stalks. BioResources 2010; 5: 1717–1732.

78.

Jonoobi

Harun

Mishra

, et al. Chemical composition, crystallinity and thermal degradation of bleached and unbleached kenaf bast (Hibiscus cannabinus) pulp and nanofiber. BioResources 2009; 4: 626–639.

79.

Le Troedec

Sedan

Peyratout

, et al. Influence of various chemical treatments on the composition and structure of hemp fibres. Composites Part A: Applied Science and Manufacturing 2008; 39: 514–522.

80.

Łojewska

Miśkowiec

Łojewski

, et al. Cellulose oxidative and hydrolytic degradation: in situ FTIR approach. Polymer Degradation and Stability 2005; 88: 512–520.

81.

, et al. Interfacial studies of sisal fiber reinforced high density polyethylene (HDPE) composites. Composites Part A: Applied Science and Manufacturing 2008; 39: 570–578.

82.

Joseph

Thomas

Pavithran

. Effect of chemical treatment on the tensile properties of short sisal fibre-reinforced polyethylene composites. Polymer 1996; 37: 5139–5149.

83.

Nadali

Tajvidi

Naghdi

. Effects of fungal biodegradation on structure–property relationships of medium density fibre board and hybrid polypropylene composite made from sugar-cane residue. International Wood Products Journal 2021; 12: 152–163.

84.

Naghdi

. Advanced natural fibre-based fully biodegradable and renewable composites and nanocomposites: a comprehensive review. International Wood Products Journal 2021; 12: 178–193.

85.

Younesi

Naghdi

Honarbakhsh

. Investigation on some of physical and mechanical properties of polypropylene fiber/wood/cement composites. Iranian Journal of Wood and Paper Industries 2016; 7: 207–217.

86.

El-Shekeil

Sapuan

Khalina

, et al. Effect of alkali treatment on mechanical and thermal properties of Kenaf fiber-reinforced thermoplastic polyurethane composite. J Therm Anal Calorim 2012; 109: 1435–1443.

87.

Zhang

Jiang

Wilkie

. Thermal and flame properties of polyethylene and polypropylene nanocomposites based on an oligomerically-modified clay. Polymer Degradation and Stability 2006; 91: 298–304.

88.

Kim

H-S

Kim

H-J

, et al. Thermal properties of bio-flour-filled polyolefin composites with different compatibilizing agent type and content. Thermochimica Acta 2006; 451: 181–188.

89.

Ndiaye

Tidjani

. Effects of coupling agents on thermal behavior and mechanical properties of wood flour/polypropylene composites. Journal of Composite Materials 2012; 46: 3067–3075.

90.

Tajvidi

Takemura

. Effect of fiber content and type, compatibilizer, and heating rate on thermogravimetric properties of natural fiber high density polyethylene composites. Polym Compos 2009; 30: 1226–1233.

91.

Yang

H-S

Wolcott

Kim

H-S

, et al. Thermal properties of lignocellulosic filler-thermoplastic polymer bio-composites. J Therm Anal Calorim 2005; 82: 157–160.

92.

Zabihzadeh

Ebrahimi

Enayati

. Effect of compatibilizer on mechanical, morphological, and thermal properties of chemimechanical pulp-reinforced PP composites. Journal of Thermoplastic Composite Materials 2011; 24: 221–231.

93.

Sinha

Jhalani

Ravi

, et al. Modelling of pyrolysis in wood: a review. SESI Journal 2000; 10: 41–62.

94.

Brosse

El Hage

Chaouch

, et al. Investigation of the chemical modifications of beech wood lignin during heat treatment. Polymer Degradation and Stability 2010; 95: 1721–1726.

95.

Esteves

Pereira

. Wood modification by heat treatment: a review. BioResources 2008; 4: 370–404.

96.

Chen

Yuan

Song

, et al. Genetic analysis of LRRK1 and LRRK2 variants in essential tremor patients. Journal of Wood Science 2018; 22: 398–402.

97.

Zierdt

Theumer

Kulkarni

, et al. Sustainable wood-plastic composites from bio-based polyamide 11 and chemically modified beech fibers. Sustainable Materials and Technologies 2015; 6: 6–14.

98.

Rosa

Chiou

B-s

Medeiros

, et al. Effect of fiber treatments on tensile and thermal properties of starch/ethylene vinyl alcohol copolymers/coir biocomposites. Bioresour Technol 2009; 100: 5196–5202.

99.

Qin

Zhang

Zhao

, et al. Thermal stability and flammability of polypropylene/montmorillonite composites. Polymer Degradation and Stability 2004; 85: 807–813.

100.

Nafchi

Abdouss

Najafi

, et al. Effects of nano-clay particles and oxidized polypropylene polymers on improvement of the thermal properties of wood plastic composite. Maderas Cienc Tecnol 2015; 17: 0–54.

101.

Devi

Gogoi

Konwar

, et al. Synergistic effect of nanoTiO 2 and nanoclay on mechanical, flame retardancy, UV stability, and antibacterial properties of wood polymer composites. Polymer Bulletin 2013; 70: 1397–1413.

102.

Rajan

Upadhyaya

Chand

, et al. Effect of nanoclay on the thermal properties of compatibilized ethylene vinyl acetate copolymer/high-density polyethylene blends. Journal of Thermoplastic Composite Materials 2014; 27: 650–662.

103.

Leszczyńska

Njuguna

Pielichowski

, et al. Polymer/montmorillonite nanocomposites with improved thermal properties: Part II. Thermal stability of montmorillonite nanocomposites based on different polymeric matrixes. Thermochimica Acta 2007; 454: 1–22.

104.

Deveci

Sacli

Turkoglu

, et al. Effect of SiO2 and AL2O3 nanoparticles treatment on thermal behavior of oriental beech wood. Wood Research 2018; 63: 573–582.

105.

Kiaei

Amiri

Samariha

, et al. Effect of nanosilica on thermal, flammability, and morphological properties of WF/RPS-based nanocomposites. Cerne 2018; 24: 59–66.

106.

Mohaiyiddin

Ong

Akil

. Preparation and characterization of palm kernel shell/polypropylene biocomposites and their hybrid composites with nanosilica. BioResources 2013; 8: 1539–1550.

107.

Lei

Clemons

, et al. Influence of nanoclay on properties of HDPE/wood composites. J Appl Polym Sci 2007; 106: 3958–3966.

108.

Varma

. Thermal behaviour of coir fibres. Thermochimica Acta 1986; 108: 199–210.

109.

Kallakas

Shamim

Olutubo

, et al. Effect of chemical modification of wood flour on the mechanical properties of wood-plastic composites. Agronomy Research 2015; 13: 639–653.

110.

Ayrilmis

Dundar

Kaymakci

, et al. Mechanical and thermal properties of wood‐plastic composites reinforced with hexagonal boron nitride. Polym Compos 2014; 35: 194–200.

111.

Sreenivasan

Iyer

KRK

. Influence of delignification and alkali treatment on the fine structure of coir fibres (Cocos nucifera). Journal of Materials Science 1996; 31: 721–726.

112.

Kord

Ravanfar

Ayrilmis

. Influence of organically modified nanoclay on thermal and combustion properties of bagasse reinforced HDPE nanocomposites. J Polym Environ 2017; 25: 1198–1207.

113.

Ibrahim

Jamiru

Sadiku

, et al. Dependency of the mechanical properties of sisal fiber reinforced recycled polypropylene composites on fiber surface treatment, fiber content and nanoclay. J Polym Environ 2017; 25: 427–434.

114.

Essabir

Boujmal

Bensalah

, et al. Mechanical and thermal properties of hybrid composites: oil-palm fiber/clay reinforced high density polyethylene. Mechanics of Materials 2016; 98: 36–43.

115.

Majeed

Ahmed

Abu Bakar

, et al. Mechanical and thermal properties of montmorillonite-reinforced polypropylene/rice husk hybrid manocomposites. Polymers 2019; 11: 1557.

116.

Sardashti

. Wheat straw-clay-polypropylene hybrid composites. Waterloo: University of Waterloo, 2009.

117.

Williams

Hosur

Theodore

Netravali

Rangari

Jeelani

. Time Effects on Morphology and Bonding Ability in Mercerized Natural Fibers for Composite Reinforcement. Hindawi Publishing Corporation International Journal of Polymer Science. 2011; 9.