Effect of montmorillonite nanoclay on temperature dependence mechanical properties of naturally woven coconut sheath/polyester composite

Abstract

The untreated, NaOH and silane-treated naturally woven coconut sheath/polyester composites have been developed using compression molding technique by varying the weight percentage (1, 2, 3, and 5 wt%) of organically modified montmorillonite (MMT) nanoclay to find the mechanical properties as a function of temperature. X-ray diffraction, scanning electron microscope and transmission electron microscope have been performed to understand the distribution of nanoclay mechanism and morphological structure of treated coconut sheath fiber. Dynamic mechanical analysis revealed a decrease in storage modulus (E′) with the increase in temperature by a considerable fall in the temperature range of 50℃ to 110℃. The loss modulus (E″) and damping peaks (tan δ) were found to be higher after the chemical treatment of coconut sheath. The maximum increase in E′ and E″ values were found in the case of silane-treated composites. In all types of composites, the dynamic factors were observed to increase with the incorporation of nanoclay but the anomalous trend was noticed according to the wt%. A slight increase in shift of glass transition temperature value was noticed in association with the E″ peaks for silane and NaOH-treated composites compared to untreated composites.

Keywords

Nanoclay coconut sheath dynamic mechanical analysis transmission electron microscope scanning electron microscope

Introduction

The improved strengthening mechanism by the incorporation of particle in nanometer scale to polymers has been used as an alternative matrix for making new class of composites. Due to their nanoclay sizes and uniform dispersion they can create extensively large surface area available for interactions with polymers and exhibit markedly enhanced performance when compared to the pristine polymers or their conventional composites.^1–3 These include increased modulus and mechanical strength, flame resistance, chemical resistance, thermal stability and gas barrier properties. Hence, these polymer nanocomposites (PNC) are aimed to have wide range of applications in the field of electronic sector, aerospace, packaging industries and automobile industries due to their superior performance.^4,5 In general, PNC can be classified into three types according to morphological state namely, unmixed, intercalated and exfoliated. Different kind of processing methods have been employed to prepare the exfoliated/intercalated type of nanocomposites, because, it can produce enhanced properties in various aspects compared to unmixed state of nanocomposites.^6–8 To achieve a well-dispersed clay nanocomposite in thermoset resin, the conventional mixing process has been identified as highly efficient and environmental friendly in achieving exfoliated/intercalated state of dispersion within a short period of time. Vaia et al.⁹ found that the degree of exfoliation could be improved through the aid of conventional shear devices such as extruders, mixers, ultra sonic agitators and shear mixture. Hackman et al.¹⁰ reported that the epoxy/clay nanocomposite can be widely used in civil engineering application by sieving in moisture barrier and life time of the material can be enhanced by slow absorption of moisture. They concluded that the reinforcement of the nano fillers into the polymer matrix reduces the permeability. Among the various nano-particles, the montmorillonite (MMT) nanoclay has been used by several researchers due to the dispersion capability, swelling nature, curing mechanism high-aspect ratio, plate morphology, natural availability and low cost.^11–13

Natural fibers offer an eco-friendly option for manufacturing composite components while providing performance comparable to traditional composites in some applications. In recent years, the natural fiber-reinforced composites have shown better properties not only on mechanical but also in transport and viscoelastic behavior in polymer matrix.^14–16 Natural fiber-reinforced polymer composites have been replacing suitable components in different industry sectors like automobile, construction, house-holding equipments and food packaging. Hence, it is essential to study the behavior of mechanical properties of nanoclay/natural fiber-reinforced polymer composites under varying temperature condition.

Dynamic mechanical analysis (DMA) has been accepted as a widely used method for the purpose of measuring the temperature dependence properties such as storage modulus (E′), loss modulus (E″) and damping factor (tan δ). In general, the incorporation of reinforcement in a polymeric resin provides the constraint to the mobility of molecular chain around the fiber surface leading to higher glass transition temperature. The parameters such as fibre content, addition of filler, chemical treatment of fiber, fibre orientation and the mode of testing affects the temperature dependence properties of fiber-reinforced composites.¹⁷ Several works^18–20 have been reported on the DMA behavior of synthetic fiber-reinforced polymer composites. They have reported the role of bonding mechanism between the matrix and fiber reinforcement using the output of DMA. Ghosh et al.²¹ have studied the DMA for the combination of synthetic and natural (glass and jute) fibres reinforced epoxy composite. Rana et al.²² carried out the dynamic analysis of jute/polypropylene composites and they observed the role of compatibiliser on the dynamic response of the composites. They concluded that the loss modulus increased and the tan δ peak shifted to a higher temperature with the increase of the fibre loading. The visco-elastic behavior of fiber reinforced polymer composites is significantly influenced by the chemical treatment of natural fibers due to the improvement of interfacial adhesion between the fiber and matrix. According to Ray et al.²³ the rate of fall of loss modulus depends upon the percentage of defect concentration during alkali treatment. In the case of alkali-treated jute fiber composites, glass transition temperature (T_g) value showed a decreasing trend. Gassan et al.²⁴ have developed the jute/epoxy composites for DMA and observed an enhancement in storage modulus by the inclusion of treated jute fiber in epoxy. Saha et al.²⁵ carried out comparative studies on the damping of as-received and chemically treated jute fiber reinforced polyester composite. Chowdhury et al.²⁶ have reported the effect of varying weight percentage nanoclay of carbon/epoxy composites on temperature dependence mechanical and thermal properties. They have reported that the addition of nanoclay upto 2 wt% shows the increased modulus compared to unfilled clay composites whereas no change in glass transition temperature was noticed for all 5 wt% of nanoclay addition. The enhanced storage modulus and the shift of peak towards right of T_g have been found in glass fiber-reinforced vinyl ester composites upto the addition of 3 wt% of nanoclay.²⁷ Biswal et al.²⁸ carried out the DMA analysis for banana fiber/polypropylene/nanoclay composites which showed enhanced thermal stability, storage modulus and damping factor by the strong interaction between nanoclay/banana and compatibilizer in the natural fiber-reinforced nanocomposite.

Extensive studies on DMA have been reported for synthetic and natural fiber-reinforced composites. Similar studies on MMT nanoclay as secondary reinforcement in natural-fibre-reinforced hybrid composites have been scanty. In this investigation, naturally woven coconut sheath (along with MMT nanoclay) has been used as reinforcing element, which is identified as huge waste from the coconut tree. The temperature dependence mechanical properties such as storage modulus, loss modulus and damping factor of naturally woven coconut sheath/polyester composites have been evaluated as a function of varying wt% of nanoclay and chemical treatment of fiber. The morphology of interfacial adhesion between the fiber and matrix were also observed from the tensile fractured surface of the composites.

Experimental details

Materials used

Unsaturated isophthalic polyester resin obtained from M/s Vasivibala resins (P) Ltd, Chennai, India, has been used as the polymer matrix. Nanomer 1.31 PS, the organically modified MMT with 15–35 wt% octadecylamine and 0.5–5 wt% aminopropytriethoxysilane procured from M/s Sigma Aldrich (P) Ltd, Bangalore, India, has been used as nanoclay. Naturally woven coconut sheath in the form of woven mat extracted from outer basks of the coconut tree was used as a reinforcing agent. The sheath consists of fine fiber mat which is sandwiched between two layers of coarse fibers. The woven architecture of thick and thin fibers in coconut sheath is shown in Figure 1.

Chemical treatment

Alkali treatment

The coconut sheath fiber mats were treated with 4 wt% NaOH solution²⁹ in a water bath at room temperature for 60 min. The treated mats were washed several times with fresh water until all the NaOH deposition over the fiber surface was removed. The mats were then left to dry at room temperature for 24 h followed by drying in an oven at 80℃ for next 1 h.

Silane treatment

Before the silane treatment, the coconut sheaths were pretreated with NaOH solution. The 0.5 wt%²⁹ of trichlorovinyl silane [C₂ H₃ Cl₃ Si] solution was made in aqueous ethanol liquid. Coconut sheath fibers were immersed in that solution for 1 h. Silane aqueous solution was acidified to pH 3.5 with acetic acid. The treated fibers were kept in room temperature condition prior to use for about 24 h to ensure complete dry condition.

Fabrication of coconut sheath/MMT nanoclay reinforced hybrid composites

The hybrid nanocomposites have been fabricated using compression molding technique by the following process. The polyester nanoclay mixer was prepared in the first step containing nanoclay in x wt% (x = 1, 2, 3 and 5 wt%) and then it was added to the polyester resin by hand stirring until all the clay was immersed. This clay-filled polyester resin was again stirred at 500 r/min for 2 h using a variable speed four blade radial rotor using mechanical shear mixing process. The mixing speed and time in shear mixing process were selected from the optimization process based on preliminary investigations.³⁰ Due to the rotation of rotor in clay mixture the possibility of entrapped air into the viscous mixture can occur. Thus, it can form voids in the stagnant position while the curing was taking place. Hence, the clay mixture was then degassed using vacuum desiccator for about 2 h. One wt% of methyl ethyl ketone peroxide (MEKP) and 1 wt% cobalt naphthenate were used as catalyst and accelerator respectively, for room temperature curing. The mats of coconut sheath fibers were sized and placed in a mold by keeping one by one successively upto six number of layers (48 ± 2 wt%) having a total dimension of 300 mm × 125 mm × 3 mm, and it was wetted by pouring the polyester/clay solution mixed with curing agents on to the mat and squeezed using a roller to remove the air bubbles. The mold was then closed and curing was done at room temperature for 24 h under a constant pressure of 150 kgf/cm².

Characterization of materials

The structure of the coconut sheath fibers and nanocomposites were studied using scanning electron microscope (SEM) and transmission electron microscope (TEM). TEM images were obtained using Technai Sprit, FEI (Netherlands) with a 20–120 kV operating voltage on a 50-nm thin section from samples. The chemically treated fiber surface of coconut sheaths is scanned by SEM (M/s FEI, The Netherlands and model XL30 ESEM). A Fourier transformer infra-red (FTIR) spectrum was taken to characterize the chemical modifications of fiber with an infra-red spectrophotometer (Shimadzu, Japan).

Dynamic mechanical analysis

Dynamic mechanical thermal analyzer (DMTA) measurements were carried out on a TA Instruments DMA 983. Rectangular specimen having size 50 mm × 10 mm × 3 mm was used for the dynamic mechanical experiments. The test specimen was clamped between the ends of two parallel arms, mounted on low force flexure pivots allowing motions only in the horizontal plane. The samples were tested in a nitrogen atmosphere in a fixed frequency mode, 1.0 Hz (oscillation amplitude 0.3 mm) and an increased heating rate of 5℃/min. The samples were evaluated in the range from 30℃ to 400℃.

Results and discussion

IR Spectrum

The IR spectra of the untreated and treated (NaOH, Silane) coconut sheath are given in Figure 2. The broad spectrum at 3316 cm⁻¹ is the characteristic band for O-H stretching and the peak at 2900 cm⁻¹ is the characteristic band for C-H stretching. The broadness of the peak at 3316 cm⁻¹ is related to OH stretching for hydroxyl group present in polysaccharide.³¹ Somewhat narrower peak corresponding to the hydroxyl group at 3316 cm⁻¹ was observed for the untreated coconut sheath (UTC). Hence, it could have had low hydrophilicity. The subsequent increase in broadening of the peak for the same wave number was noticed for silane and alkali-treated coconut sheath (ATC). The peak at 3316 cm⁻¹ owing to O-H stretching in the IR spectrum of the silane-treated coconut sheath (STC) is less broad than that for the NaOH-treated fiber, and broader than that of the UTC. As a result of silane treatment, the hydrophilic nature of the coconut sheath is partially reduced, and the water content therefore decreases. However, this peak gets broader and less intense in the case of alkali-treated fiber. This may be due to the formation of more OH groups as a result of the breaking of cross-links of hemicellulose with cellulose after alkali treatment.³² It could have more moisture content and ultimately high polarity on fiber surface.

Structure and morphology

Figure 3 shows the X-ray diffractograms of virgin MMT nanoclay and clay-filled polyester nanocomposite, with varying weight percentages of nanoclay. From the XRD pattern for pure nanoclay, a definite sharp peak at 4.5° (2θ) has been observed and it is attributed to the (0 0 1) crystallographic planes. However, on adding different weight percentages of MMT nanoclay with the polyester nanocomposite, it shows a completely amorphous nature, due to the uniform distribution of nanoclay. This shows that the interlayer distance of organically modified clay layers are expanded and randomly dispersed in the matrix. Hence, it can forms intercalated/exfoliated structure of the nanocomposite.

To understand the distribution of nanoclay in polyester matrix, TEM images have been taken and shown in Figure 4 (a–d). In the TEM images, the white region is the matrix phase while the dark region is the clay distributed phase. Figure 4 (a–c) reveals that intercalated/exfoliated clay arrangements were noticed for 1, 2, 3 wt% of clay addition and it showed the well delaminated clay platelets peeled off from staked arrangement of clay tactoids. In Figure 4(d), the thick dark band zones were noticed and it represents the agglomeration and clustering of nanoclay in nanocomposite containing 5 wt% of clay. The viscosity of resin–clay mixture increases due to the higher clay addition in polyester matrix. This attributed to increase in shear force and leads to improper mixing which results in agglomeration of particles. TEM with 5 wt% (Figure 4(d)) of nanoclay does not show any layer dispersions, which is termed as immiscible PNC.

Figure 5 displays the XRD pattern of untreated and chemically treated coconut sheath. From Figure 5 it is observed that the two peaks situated at 2θ = 15.4° and 2θ = 22.5° can be attributed to cellulose I and IV, both of which exhibit a monoclinic structure, which was already reported by many authors.^33,34 The two peaks in all the spectra are attributed to the (2 0 0) and (1 1 0) crystallographic planes.³⁵ The crystallinity index (CI) was estimated by equation (1) using the following expression³⁶:

CI = (I_{22.5} - I_{18.5}) / (I_{22.5})

(1)

where I_22.5 is the height of the peak at 2θ = 22.5°. This is due to the contribution of both the amorphous and the crystalline fractions. I_18.5 is the diffracted intensity at 2θ = 18.5° and is attributed to the amorphous fraction. The CI of the ATC (42%) exhibits the highest percentage of CI compared to silane (38%) and UTC (34%). It is evident that NaOH treatment removes the amorphous constituents (hemicelluloses, wax and impurities) and leads to better packaging of cellulose chain and thus increases the degree of crystallanity in the ATC. This may be attributed to the increase in the absorption capacity of the alkali-treated fiber.

The morphology of the untreated, alkali- and STC fiber is depicted in Figure 6(a–c). Figure 6(a) reveals the appearance of waxy layer over the coconut sheath which is identified from the white color cup like structure and it leads to the poor interfacial bonding between fiber and matrix. The alkali-soluble compounds like inherently existing waxy layers, artificial impurities and globular protrusions have been removed on the fiber surface as shown in Figure 6(b). This rough surface can provide adequate wetting of the fibers within the polyester matrix which further increases the cohesive coupling between the fiber and the matrix by mechanical interlocking mechanism. A non-homogenous thin film deposition of coating over the surface of STC could be seen in Figure 6(c). The layer of trichlorovinyl silane forms the chemical bonding with matrix which promotes a good physical adhesion between fiber and the polyester matrix. This strong bonding contributes to an increase in modulus with fiber-reinforced composites.

Dynamic mechanical properties

Storage modulus (E′)

DMA is a technique where a small deformation is applied to a sample in a cyclic manner. DMA is also an efficient method to assess the interfacial adhesion between the fiber and matrix in fiber-reinforced composites. Figure 7(a) shows the effect of temperature on the storage modulus (E′) of the neat polyester/coconut sheath composite with UTC, ATC and STC fiber reinforcement. The chemical treatment of fibre surface has great influence on the interfacial adhesion and hence on the dynamic mechanical properties of the composites also. At lower temperature, the maximum improvement in storage modulus was found in the case of silane-coated coconut sheath-reinforced composites. It could be due to the improved compatibility between the fiber/matrix interface and the increase in stiffness of fiber. As the temperature increases, the decrease in modulus value continues sharply till a temperature 110℃ is reached. For the alkali-treated composites, the modified surface topography leads to improved wetting, which produces a strong mechanical interlocking with the matrix giving rise to a stiffer composite with high modulus compared to untreated composite. Another reason for high storage modulus is the formation of rough surface due to the NaOH treatment. It makes the fiber more reactive and quite apparently increases the cross-link density within the matrix–fiber network leading to high storage modulus.³⁷ In all types of composites, the sharp drop in storage modulus was observed with the increase in temperature. It could happen due to the formation of voids at the interface and composites of lower stiffness are produced by the change in interaction of water molecules on fiber surface.³⁸ The percentage of moisture content for untreated, alkali- and STC fibers are 8.79 wt%, 12.66 wt% and 11.50 wt%, respectively.³⁹ This increase in water content after the treatment was further confirmed from the broadening of the OH groups in IR spectra. However, in the glassy region (<110℃) the higher modulus was found in all cases of composites. In such a state, there exists a close and tight packing resulting in high modulus. It could happen due to the immobilization of polymer chain and it holds the fibers in tight packing arrangement. But in the rubbery region (>110℃) all the storage modulus values are merged. As temperature increases the components become more movable and lose their tight packing arrangement. From Figure 7(a), it is evident that the dynamic storage modulus values increase with chemical modifications and maximum E′ of 68% and 34% are exhibited by the composite prepared with STC and ATC fiber-reinforced composite compared with UTC/polyester composite. It is seen that the maximum improvement in storage modulus is observed in the case of silane-treated composites. This could be due to the efficient chemical linkage of STC as reinforcement in polyester composites and it can be further improved by the enhanced interfacial adhesion between coconut sheath and polyester. For the NaOH-treated composites, the large exposure of surface area to contact provides improved wetting. It produces a mechanical interlocking with the polyester matrix to obtain much stiffer composite which leads to high storage modulus values.

Figure 7(b–d) shows the effect of temperature on the storage modulus (E′) of the addition of different weight percentages of nanoclay with untreated (UTC+ × wt%), NaOH-treated (ATC + × wt %) and silane-treated (STC+ × wt%) coconut sheaths. In all the cases, the addition of nanoclay increases the dynamic storage modulus when compared with that of virgin coconut sheath/polyester composites. The exfoliated/intercalated type of dispersed clay particles imparts its secondary reinforcement effect with coconut sheath/polyester composites by providing effective stress-transfer between the fibre and matrix at nanoscale level. The enhanced modulus was observed in all types of hybrid nanocomposites owing to the large surface-to-contact ratio of clay with matrix and the intercalation of clay layers inside the polyester chain. The incorporation of coconut sheath and randomly disordered nanoclay acts as constraint to segmental molecular motion which can possibly cause an increase in the storage modulus.

In all the nanoclay-filled specimens, the decrease of fall in storage modulus was observed with the increase in temperature due to the enhanced thermal stability of nanocomposites by the incorporation of secondary reinforcement. Further, the addition of 2 wt% of nanoclay shows the highest modulus in all types of composites and that was already found in optimized condition in the mechanical strength of polyester nanocomposites.³⁰ To understand the interfacial bonding in microscopic level the SEM was performed for tensile fractured surfaces of treated and untreated composites in the case of without and with the addition of 2 wt% of nanoclay.

Figure 8(a–c) shows the SEM micrograph of interfacial bonding of tensile fractured coconut sheath/polyester composite specimens. Figure 8(a) reveals the rigidly coupled clay/polyester mixture around the fibers in nanoclay (2 wt%) filled silane-treated fiber reinforced composites. No fiber pullout was noticed during failure in the tensile test, which indicates better cohesive coupling between the fiber and matrix. The debonding at the interface between the fiber and matrix was observed in the clay-filled NaOH-treated composites although the closely packed resins hold the fibers in ordered manner (Figure 8(b)). From Figure 8(c), the fiber breakage, fiber pullout, matrix cracking and crack advance along the interface were noticed and it can split the NaOH-treated coconut sheath fiber from neat polyester. This is probably due to the absence of reinforcement effect of clay distribution in polyester matrix which can decrease the strength of the composite owing to the improper stress transfer from the matrix to the fiber.

In the case of the UTC/polyester composites, the dynamic modulus curves of the nanoclay incorporated composites show higher E′ value than unfilled composites above the T_g region in the rubbery plateau. At about 100℃, the E′ of neat coconut sheath/polyester without the addition of nanoclay is 812 MPa. By the incorporation of nanoclay as an additional reinforcement with the coconut sheath to the polyester the E′ value goes on increasing above 1580 MPa for 2 wt% and 3 wt% of nanoclay content. But no appreciable improvement in E′ was identified on further addition of nanoclay (5 wt%). However, it decreases the E′ value as shown in Figure 7(b). This could be probably due to the formation of agglomeration and clustering of nanoclay and it diminishes the effective stress-transfer between the fiber and matrix. The treatment of coconut sheath with NaOH removes the waxy layers and impurities on the fiber surface create mechanical interlocking with the resin and also impart the strength to the composites together with the addition of nanoclay. The maximum increase of 81% of raise in E′ value was noticed for 2 wt% of clay-filled silane-treated coconut sheath/polyester composites.

Loss modulus (E″)

The dynamic loss modulus is a measure of viscous behavior of the composite, which also relates to the energy dissipation of the material system. Figure 9(a–d) indicates the variation of dynamic loss modulus (E″) curve of the unfilled clay reinforced coconut sheath/ polyester composite in all treated conditions and different weight percentages of nanoclay with untreated (UTC+ × wt%), NaOH-treated (ATC+ × wt%) and silane-treated (STC+ × wt%) coconut sheaths composites. The effect of chemical modification on loss modulus is shown in Figure 9(a). From Figure 9(a), the sharp drop was also observed in loss modulus characteristic peaks between the temperature ranges of 30℃ and 100℃. This was mainly because of the weakening effect of fiber/matrix interface when the temperature increases. It occurred due to matrix molecular oscillation at interface by the movement of dissolved water molecules.³⁸ Consequently, it could reduce the tight packaging of fiber/matrix interface as well as frictional resistance at interface. Hence, the decrease in frictional resistance between the fiber and matrix was attributed to the reduction in heat loss and in turn the loss modulus also. The increased loss modulus was found in the order of silane-, NaOH- and UTC-reinforced composites similar to the case of storage modulus. From Figure 9(a), two well-defined peaks were observed, one around at 85℃ and the other at around 210° in all types of composites. For the increasing temperature, the cellulose network in the coconut sheath fibers moved towards their T_g and gave peak at 210℃.²² The T_g is usually interpreted as the peak of the loss modulus curves obtained during DMA.^18,40 In clay unfilled condition, the silane-treated coconut sheath reinforced polyester composite has shown higher loss modulus (E″) as well and the slight shift in peak height towards right was also identified from Figure 9(a). The decreased mobility of the matrix chains, which indicates the enhanced interfacial adhesion between the fibers achieved by the coupling effect of the silane, could be the reason for shifting of T_g to higher temperatures.

It can be observed from the Figure 9(b–d) that the loss modulus peak height increases with the incorporation of nanoclay content for entire test range of the temperature. In addition to the peak height, the broadening of loss modulus curve was also noticed when the clay content increases. This could be probably due to the change in physical state of the matrix by the addition of clay surrounding the fibers. Another reason for this slight broadening of the loss modulus peaks is due to the increase in energy absorption caused by the addition of fillers.

In general, when a composite is subjected to external load, the energy may be dissipated by friction at the interface in the form of heat.⁴¹ The randomly disordered nanoclay (crystalline phase) creates the constraint to the mobilization of the polymer (amorphous phase) molecules at the interfaces. Consequently, the various forms of physical coupling by the clay mixture can cause the restraint to the molecular motion near the surface of the fiber. It creates the high internal friction at the fiber/matrix interface that enhances the dissipation of energy. This could give rise to a higher or broader loss modulus curves as reported by other authors.^42,43 However, no linear increasing trend was observed by the increasing wt% of nanoclay. The maximum peak height and peak width were found for the STC/polyester composites with the 2 wt% of clay content. A slight decrease of E″ value for the nanoclay (3 wt%, 5 wt%) loaded coconut sheath reinforced composite was noticed in all the cases. This is due to the poor interfacial bonding between clay/polyester mixture and coconut sheath fiber by the presence of voids and aggregation of clay platelets.

Damping factor (tan δ)

The ratio of the loss modulus (E″) to the storage modulus (E′) is tan δ and is often called damping. Damping is a dimensionless property and is a measure of how well the material can dissipate energy. The variation of Tan δ values measured over a range of temperatures for the UTC, ATC and STC fiber reinforced composites without and with varying wt% of clay content is shown in Figure 10(a–d). The energy dissipation of complex material depends upon the various factors such as structure of matrix and fiber, state of interface, damping due to frictional resistance at contact surface between fiber and matrix, fiber breakage and matrix cracking.⁴⁴ The peak height of the T_g (tan δ) indicates the damping nature of the specimens. According to refs [45,46], the damping peak height is related to the internal molecular motion and energy dissipation of the fibre/matrix interface. From Figure 9(a), it is apparent that silane and NaOH-treated composites have the highest tan δ value indicating large amount of energy dissipation and hence better damping characteristics. This increase in damping character of NaOH-treated composites could be due to the structural changes at the fiber surface by the removal of artificially covered inorganic dusts and waxes and thus increases the frictional damping at interfaces. The formation of vacuum sites between the fiber networks due to the removal of waxes after the treatment can accommodate the resin packets leading to the increase in relative thickness at bonded interface. Moreover, the removal of the lignin in UTC fibers-reinforced composites leads to change in the extent of hydrogen-network, affecting the tan δ of the composites compared to NaOH and silane-treated conditions.

Besides, the presence of unreactive organic groups covered on the coconut sheath fiber due to the treatment by the silane coupling agent can reduce the cross-linking density of the interfacial network, so that it can allow the movement of small groups in molecular network at specified locations leading to high damping characteristics. Similar results are reported by Huda et al.⁴⁵ As shown in Figure 10(b–d), the incorporation of nanoclay reduces the peak height of tan δ by restricting the molecular movement at the interfaces, which results in decreasing the viscoelasticity of matrix. In UTC/polyester composite, all the wt% of clay addition shows lesser damping value than unfilled composites, whereas, in the case of NaOH and silane-treated condition a slight increase in tan δ values were noticed with the addition 3 wt% of clay. It may be due to the non-uniform dispersion of clay and shell of residual coupling agent over the fiber surface. The same trend was also followed for second transition peak, which is attributed to the cellulose content in coconut sheath. A similar second transition peak at a temperature higher than the T_g was also reported by the author.²³

Conclusions

Dynamic mechanical properties of coconut sheath fiber-reinforced polyester composites are greatly dependent on the addition of nanoclay and chemical treatment of the fiber.

In the case of the treated composites, the silane-treated fibre composites exhibits a higher storage modulus, loss modulus indicating superior interfacial bonding strength and improved physical contact between the matrix and the fibre compared with the untreated composites. However, they further increased with the incorporation of nanoclay up to 3 wt%.

In connection with loss modulus (E″), the T_g of the composite has been shifted to high-temperature regions with the incorporation of clays and chemical treatment of coconut sheath.

Moreover, a highest peak occurs at STC in the tan δ curves, due to the specified molecular mobility at the interlayer surface. Addition of nanoclay lowers the tan δ peak height, which again points to the improved fiber/matrix adhesion attributed to the crack arresting mechanism by randomly disordered secondary reinforcement.

Finally, it can be concluded that by hybridizing MMT nanoclay and silane coconut sheath, we can prepare eco-friendly and cost-effective composite materials possessing significant modulus and damping behavior. Maximum increase in dynamic properties is observed at 2 wt.% addition of nanoclay.

Footnotes

Acknowledgements

The authors thank the Department of Science and Technology, India, for the funding through SR/FTP/ETA-92/2009 project. The constant encouragement throughout the work given by the Center for Composite Materials, Department of Mechanical Engineering, Kalasalingam University, for their kind permission to carry out the preparation and testing of the composites.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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