Improving hydrophobicity and compatibility between kenaf fiber and polymer composite by surface treatment with inorganic nanoparticles

2.1 Materials

The National Kenaf and Tobacco Board (NKTB), Kota Bahru, Kelantan, Malaysia, provided the KF in the form of a mat. The unsaturated polyester Reversol P-9565 (UP) resin was utilized as the polymer matrix for the composites during the entire study. Synthomer PLC manufactured the resin, and Dr. Rahmatullah Sdn. Bhd., Bukit Mertajam, Malaysia supplied it. Methyl ethyl ketone peroxide (MEKP) was marketed under the trade name Butanox M-50 by Kaumjung Akzo Nobel and distributed by Hasrat Sdn. Bhd. Butterworth, Malaysia. Sigma-Aldrich (M) Sdn. Bhd. (Kuala Lumpur, Malaysia) provided rod-shaped zinc oxide nanoparticles (ZnONPs).

2.2 Treatment with Nano-ZnO

In this study, the kenaf mat was cut into 20 × 20 × 0.3 cm to length and width, respectively, to be used for further treatment. Surface treatment of KF by using ZnONPs, different percentages of the ZnONPs suspension in distilled water were used, namely 0, 1, 2, 3, 4, and 5 wt%. A moderate mixing speed (up to 2,500 rpm) was employed at room temperature to avoid the destabilization of suspension. A satisfactory dispersion was accomplished using a Ragogna mixer specifically manufactured for FPInnovations by Custom Machinery Ltd. (Ontario, Canada) with speed up to 5,000 rpm. The prepared KF was then submerged into a water-ZnONPs emulsion that had been made beforehand. To prevent the nanoparticles from sedimenting, the KF was promptly immersed in the solution having ZnONPs for 60 min at 60 °C (moderate heat was applied to open up KF voids and allow ZnONPs to penetrate the KF cells). The treated KF layers were dried overnight at 80 °C in a conventional oven. ZnONPs powder was mixed with distilled water in serial percentages, which are tabulated in Table 2.

2.3 Composite preparation

The hand lay-up method was utilized to make a composite material formed into a stainless-steel mold with dimensions of 200 mm × 200 mm × 5 mm. Untreated and treated KF layers were compressed using Instron Universal Testing Machine (Model 5569) at 90 °C. Initially, the KF mat was positioned in the mold. Next, 2% of MEKP was added to UPE resin based on the UPE weight. This chemical item is utilizing in small quantities as a trigger for initiating the cross-linking of UP resin during the composites manufacturing process. Table 3 has been tabulated the percentages of KF, UPE, and MEKP. The UPE resin and MEKP mixture were then poured over the KF mat. The resin was distributed and impregnated using a hand roller. KF loadings were arranged in four different loadings (wight %) of 10, 20, 30, and 40 for both treated and untreated KF. The mold was covered on top and bottom of the mold with two stainless steel sheets. As indicated in Fig. 1, the molding was pressed using a Geotech Testing Machine and left at room temperature for 24 h to cure. Finally, the composites were cut into a 20 × 20 × 0.3 cm specimen using an electric saw.

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Fig. 1

Fabrication of composites.

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3.1 Contact angle measurements

The contact angle is a prevalent method for measuring the surface wettability of the material. The contact angle was recorded by a Zeiss microscope which was equipped with a Goniometer Ocular. The fluid used to measure the contact angle was deionized water. 4 μL was the volume of the water droplet, and the measurement of the contact angle was repeated three times, and the average value was taken.

3.2 Water absorption

The samples were subjected to a water absorption test in accordance with ASTM D570-95. The samples were cut in the shape of a rectangular bar with a size of 30 × 5 × 2 mm (length × width × thickness). The samples were placed in water inside a closed bottle, and the experiment was conducted at ambient temperature. This experiment was conducted for ten weeks period with an interval of 1 week. This was followed by removing the samples from water, wiping them with a dry cloth, and weighing them on a digital balance to the nearest 0.001 g. This experiment was also performed on the UPE sample for comparison purposes. The percentage of water absorption was calculated following Equation (3).

3.3 Tensile test

The tensile characteristics were depicted by Instron Universal Testing Machine (Model 5569), using a crosshead speed of 2 mm/min, while the sample size was 20 × 20 × 0.3 cm with a gauge length of 14 cm. The tensile strength and tensile modulus of the UPE/KF composites before and after surface treatment with zinc oxide nanoparticles were determined according to the ASTM D3039 (ASTM, 2014). Each composite was tested five times, and its average value is reported. Tensile strength and tensile modulus were calculated using Equations (1) and (2).

3.4 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) was utilized to obtain the size, shape, and diameter of the nanoparticles (ZnONPs). TEM micrographs of the samples were obtained using Hitachi H7500 with a 70 kV accelerating voltage. The nanoparticles were dispersed initially in dry acetone solution, then placed the emulsion on carbon-coated 400 mesh copper grids. The sample then was left to dry at ambient temperature for around 5 min before the analysis. The obtained micrographs were then used to examine the structure of the nanoparticles.

3.5 Field Emission scanning electron microscopy (FESEM)

The size, shape, and diameter of the nanoparticles (ZnONPs) were determined using a Field Emission Scanning Electron Microscope (FESEM), a Carl Zeiss Supra 55VP, and a JEOL JSM-7600F. Platinum was used to cover the samples to prevent electron charging during the testing.

3.6 Scanning electron microscopy (SEM)

Using a JEOL JFC6460LA, the surface morphology of the tensile fractured surface of the prepared UPE/KF and UPE/KF-ZnONPs composites was examined. Before the test, the specimens were coated with a layer of platinum.

3.7 Atomic-Force microscopy (AFM)

Veeco di CP-II Atomic Force Microscope was used to examine the topography of the composites. The surface of the specimen was scanned in 6 points (two points at the center of the specimen, two points at perimeter of the specimen, and two points at half distance between the center of the specimen and perimeter in contact mode with CONT20A-CP tips. 1 Hz scan rate and 256 × 256 resolution was maintained to obtain topography on a 5 × 5 μm scanning area.

3.8 X-ray diffraction (XRD) analysis

The microstructure and crystallinity of specimens were determined using a high-resolution X-ray Diffractometer (XRD) device model Phaser-D2 manufactured by Bruker company (X’Pert PRO), equipped with a monochromatic Cu-Ka radiation filter with wavelength = 0.154 nm. The experiment was run at room temperature with an angle range (2θ) from 5 to 70° at a scanning rate of 2°/min.

3.9 Fourier transform infrared spectroscopy (FT-IR)

Infrared spectra of the samples were obtained by a Perkin- Elmer Spectrum 65 Infrared Spectrometer with a scan number of 10 and resolutions of 4 cm⁻¹. All the composites were analyzed through an ATR technique and the wavenumber was in the range between 4000 and 500 cm⁻¹. In addition, FT-IR analysis was conducted to study the impacts of surface treatment of KF with ZnONPs on the chemical composition of KF after treatment.

4.1 Contact angle (CA) measurement

The wettability properties of the treated KF-ZnONPs were characterized by measuring their contact angle. Fig. 2 shows the images of a water droplet on untreated KF and KF-ZnONPs with 1%, 2%, 3%, 4%, and 5% of ZnONPs. Understandably, KF is a hydrophilic material since it originated from natural fiber. It’s hydrophilic nature has been reported by several research communities (Farahani et al., 2012). However, after treatment with ZnONPs, this hydrophilicity diminished. Treatment of KF with ZnONPs exhibited a hydrophobicity trend with a contact angle of 74.40° for the untreated KF. A linear increment in the contact angle is noted as treatment with ZnONPs from 1% to 5%. The contact angles were 80° for 1% ZnONPs, 82° for 2% ZnONPs, 85° for 3% ZnONPs, 95.2° for 4% ZnONPs, and 104.4° for 5% ZnONPs as shown in Fig. 2. These values were much higher than that of the untreated KF. The hydrophilic characteristics of the KF were found to be reduced by the ZnONPs.

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Fig. 2

Micrograph images of water dropping of KF with different percentage of ZnONPs. (a) untreated, (b) 1%, (c) 2%, (d) 3%, (e) 4%, (f) 5 wt% ZnONPs.

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The reason why surface treatment with ZnONPs did increase the hydrophobicity of KF is that the inorganic nanoparticles were integrated into the cell wall of KF and likely occupied the space (micropores) inside the cell wall that would have been accessible to water molecules, leading to a reduction in the hygroscopicity of the treated natural fibers (Donath et al., 2004; Hill, 2007). Furthermore, incorporation of the inorganic nanoparticles into cell walls reduces the capability of the cell wall to swell due to bulking, therefore improving the dimensional stability of the impregnated natural fibers. Therefore, only inorganic nanoparticles integrated into the cell wall are predicted to influence the hygroscopicity and dimensional stability of natural fibers significantly. In contrast, those deposited in the cell lumen are expected to have a negligible effect (Tshabalala et al., 2011).

On the other hand, the hydroxyl groups of cell wall components that are primarily responsible for moisture absorption may have been blocked during hydrolysis by creating hydrogen bonds with the inorganic nanoparticles, which may have contributed to the decreased hygroscopicity. In other words, the most notable functional group of natural material is the OH group which is well known to be taking part in extensive hydrogen bonding (Gao and McCarthy, 2008). This allows the ZnONPs to interact with the –OH–. Consequently, the –OH– group is fully occupied with no further available sites for interactions with moistures on the surface of the composite. Thus, the hydrophobicity of the composite is increased. Furthermore, the increase in contact angles may also be due to the presence of excess non-polar group ions on the surface of the composite brought by the treatment with ZnONPs.

4.2 Water absorption

We examined the effect of treating KF with ZnONPs on the water absorption of UPE/KF-ZnONPs composites. After measuring the contact angle of the UPE/KF composite after treatment with ZnONPs, the hydrophobicity has significantly increased by increasing ZnONPs concentration. Therefore, it is necessary to evaluate the water absorption of UPE/KF-ZnONPs composites before and after treatment, as shown in Fig. 3. In light of the overall outcome, it is clear that water absorption for the composites before treatment augmented as KF loadings increase. Immediate water absorption is observed in the specimens for up to 14 days before it begins to slow down. The hydrophilic property of natural fibers, activated by hydroxyl groups, is mainly responsible for this quick water absorption (Akil et al., 2011). This result corroborates the findings by Salem et al. that the augment in water absorption is attributable to the hydrophilic nature of the natural fibers and the high interface area between the fibers and matrix. Furthermore, the development of water transport routes inside the composites resulted in micro gaps between the fibers and the resin matrix (Salem et al., 2017).

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Fig. 3

water absorption test of (a) UPE/10 wt% KF- ZnONPs (b) UPE/20 wt% KF- ZnONPs, (c) UPE/30 wt% KF- ZnONPs and (d) UPE/40 wt% KF- ZnONPs after treatment with different concentrations of ZnONPs.

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The dry specimens absorb a greater quantity of water at the start of the immersion process, causing the fiber cell wall to inflate and rapidly acquire weight. After three weeks of testing, it was seen that all composites had attained equilibrium. Water absorption rose significantly more with increasing KF loadings. This increase was mainly due to the porous structure and hydrophilic characteristics of KF. However, water absorption decreased considerably as the ZnONPs treatment concentration was increased in all periods of immersion, indicating that the composite became hydrophobic and lacked affinity for high water absorption.

By comparing untreated and treated KF, untreated UPE/KF composites have a higher capacity for water absorption than treated UPE/KF-ZnONPs composites. The surface treatment with nanoparticles was expected to have a significant impact on the water absorption of UPE/KF-ZnONPs because the incorporation of inorganic nanoparticles into the micropore structure of the fiber cell walls could reduce the volume of microvoids within the fiber as well as minimize the formation of air bubbles throughout the composite manufacturing process. Additionally, inorganic nanoparticles block the hydroxyl group of cell wall components and act as a shield for fiber cell wall components, obstructing the pathway for a water molecule to enter matrices. This claim was also made by Li et al. (2017), who said that these inorganic nanoparticles were used to block the way for water infiltration, resulting in decreased water absorption by natural fibres. Table 4 illustrates the standard deviation of the water absorption according to the treatment %.

4.3 Mechanical properties

Fig. 4 demonstrates the influence of varying the proportion of zinc oxide nanoparticles on the tensile strength of UPE/ KF composites. As shown, treated KF reinforced-composites consistently outperformed untreated KF reinforced-composites at all different KF loadings and treatment with 1% and 2% ZnONPs. This observation is explained by the fact that treated KF with UPE exhibit superior interfacial adhesion to untreated KF, as illustrated in Fig. 5. Interfacial bonding between the fiber and matrix improves stress transfer between the components, allowing composites to reach their maximum strength (Hadjadj et al., 2016). However, the tensile strength of the UPE/KF-ZnONPs composites decreases at treatment with 3%, 4%, and 5% ZnONPs concentration at all different KF loadings. This is due to nanocomposites with a more significant nanoparticle weight fraction having a lower ultimate tensile stress, which is attributed to nanoparticle agglomeration at a higher nanoparticle weight fraction (Shokrieh et al., 2012). When the nanoparticle content of the nanocomposite exceeds a certain amount, particle agglomeration occurs, which serves as a barrier to stress transmission from the polymer matrix to the fiber, resulting in a decrease in mechanical performance in composites having a more significant proportion than 2% of inorganic nanoparticles, the same effect observed by Wang et al. (2011). The findings were consistent with those of Cosnita et al. (2017) that tensile stress decreased as nanoparticles loading increases. However, the dilemma of insufficient interfacial bonding between polymer matrix and fiber resulted in the ineffective transmission of stress between the polymer matrix and fiber, leading to a decline in tensile strength value (JA et al., 2016).

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Fig. 4

Tensile strength of the fabricated treated and untreated UPE/KF composites.

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Fig. 5

SEM micrographs of UPE/KF-ZnONPs composite treated with 2% ZnONPs.

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Additionally, Fig. 4 reveals that the impact strength of untreated UPE/KF composites rises with increasing KF loading. This assertion is consistent with the broad observation that the mechanical strength of polymer composites increases with increasing KF loadings up to specific loading before its lowering (Mohammed et al., 2018). In this investigation, the tensile strength rose dramatically at 30% KF loading incorporation. The optimal loading of KF was 40 wt% KF loading inclusions with a slight increase in tensile strength. Additional loadings of KF at 45% loadings diminished the tensile strength of the UPE/KF composites. The explanation for this is that the resin matrix is insufficient to wet the reinforcement KF effectively. Insufficient fiber saturation can trigger inadequate interfacial bonding strength. It was consistent with the findings of Salman et al. (Salman et al., 2016), who identified that the composite comprising 40% KF loadings exhibited the largest tensile strength value before reducing with increased KF loadings. The matrix could not adequately wet the fiber at higher KF loadings (45 wt% and 50%). Such a condition impairs the capability of the matrix to convey the load to the reinforcement fiber. For comparison purposes, the percentage difference in the tensile strength of UPE/KF composites before and after treatment of KF with different percentages of ZnONPs is tabulated in Table 5. It was found that an increasing rate of ZnONPs has a tangible impact on the increments in the tensile strength up to treatment with 2% of ZnONPs. However, values greater than 2% caused a reduction in tensile strength.

The effect of varying the percentage of ZnONPs on the tensile modulus of different KF loadings UPE/ KF composites is demonstrated in Fig. 6. It reveals that the treated KF composites have a higher tensile modulus at 2% ZnONPs than untreated KF composites. It was explained as treatment with ZnONPs entered into the configurations of KF and the kenaf polymer interface. This intrusion resulted in an improvement in mechanical properties. The topic of polymer adhesion to natural materials has long been a source of worry for researchers working in this field, as the compatibility of the components of these materials has caused severe issues. However, significant adhesion between KF and matrix was established, most likely owing to the presence of the ZnO nanoparticles, which enhances the molecular interaction between KF and polymer matrix (Mohammed et al., 2018; Mohammed et al., 2019).

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Fig. 6

The tensile modules of the treated and untreated UPE/KF composites.

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The tensile modulus result revealed that as the proportion of ZnONPs was raised above 2%, the tensile modulus started to decline. The reason behind this is that the higher percentage of nanoparticles tends to agglomerate. The agglomeration of ZnONPs will lead to more interactions between the nanoparticles instead of the nanoparticles with KF, as seen in Fig. 7. Thus, it can be observed that ZnONPs clusters have been loaded onto the surface of the UPE/KF-ZnONPs composite, forming a more obvious micro nanostructure. As a result, the reduction in modulus occurs. Fu et al. (Fu et al., 2008) investigated the impacts of particle size, particle/matrix adhesion, and particle loading of nanoparticles on rigidity, hardness, and strength of nanocomposites. According to the researchers, all the three variables, notably particle/matrix adhesion, have a substantial impact on composite strength and toughness.

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Fig. 7

SEM micrographs of UPE/KF-ZnONPs composite treated with 5% ZnONPs.

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As seen in Fig. 6, the tensile modulus of untreated and treated KF composites augmented as the loading of KF rose. The explanation for this is that when the KF loadings rise, the quantity of cellulosic material in the composites increases, increasing the stiffness of the UPE composites (Mohammed et al., 2018; Mohammed et al., 2019). For the sake of comparison, Table 6 shows a percentage difference in the UPE / KF composite tensile modulus before and after KF treatment with different ZnONPs percentages. The increasing percentage of ZnONPs was found to substantially impact increases in the tensile modulus up to 2% of ZnONPs treatment. However, higher than 2% led to a descend in the tensile modulus. Based on these findings, it can be inferred that the mechanical properties were improved at 40% loadings of KF, but unfortunately, it comes with higher water absorption expenses. As fiber volume rises, the rate of moisture absorption increases proportionately. Therefore, when a natural fiber composite is used outdoors, the fiber content should be kept under control to achieve the required strength. This is to reduce moisture absorption and enhance the durability of composites (Azwa et al., 2013). The results so far obtained allow concluding that inorganic nanoparticles ZnONPs and fiber loadings have a synergistic impact on the mechanical characteristics of these composites.

4.4 TEM and FESEM of zinc oxide nanoparticles

TEM and FESEM morphology were conducted to confirm the nature of the nanoparticles (ZnONPs). Typical TEM and FESEM micrographs of the nanoparticles ZnONPs are illustrated in Fig. 8 and Fig. 9, respectively. Generally, TEM images revealed that the average diameter and length of ZnONPs nanoparticles were around 25 nm to 50 nm and 100 nm to 200 nm, respectively. This result demonstrates that the morphological structure of the ZnONPs is a rod-like shape in hexagonal form. These results were found in agreement with those provided by the FESEM results. In addition, the average size of nanoparticles was found to be around 45 nm.

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Fig. 8

TEM micrographs of ZnONPs.

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Fig. 9

FESEM micrographs of ZnONPs.

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4.5 Surface morphology

Fig. 10 (a) and (b) display the microscopic photographs of the treated UPE/KF- ZnONPs with 1% and 5% of ZnONPs, respectively, at 5000× magnifications. As demonstrated, the nanoparticles were homogeneous and scattered evenly at 1% ZnONPs. On the other hand, the pattern of dispersion was different for considerably higher percentages, as for 5% ZnONPs, and was less consistent than for lower percentages, such as 1% ZnONPs. As shown in Fig. 10 (b), higher contents of ZnONPs showed the presence of agglomeration as nanoparticles have a high surface area and high surface energy. As a result, dispersing them equally on the KF surface is difficult (Pukánszky and Fekete, 1999). Fig. 10 (a) shows that treated KF has a rough and unblemished fiber surface. Hence, it will contribute to the improvement in the mechanical characteristics of the treated UPE/KF- ZnONPs, as depicted in Fig. 4 (Mohammed et al., 2019).

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Fig. 10

SEM micrographs of (a) UPE/KF-1% ZnONPs, (b) UPE/KF-5% ZnONPs, (c) UPE/ 10 wt% KF, (d) UPE/ 30 wt% KF, (e) UPE/KF- 2% ZnONPs composite.

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In comparison to the treated KF, impurities of the nonfibrous matter remain on the surface of the untreated KF. For example, Fig. 10 (c) and (d) of UPE/ 10 wt% KF and UPE/ 30 wt% KF composites show a prominent gap between the fiber and the matrix in untreated KF composites. This indicates impaired interfacial bonding between untreated KF and UP matrix resin, resulting in inferior tensile characteristics. Lai et al. (2016) presented a similar observation with a similar morphology appearance.

The mechanical characteristics of polymer matrix composites are impacted by the nature and structure of the fiber–matrix interface (Hadjadj et al., 2016). Fig. 10 (e) and (d) depict the interfacial bonding between KF and polymer matrix via microscopic photographs of cracked surfaces of treated and untreated KF composites. There were several fiber breakages between the treated KF and the UPE matrix, no fibers were pulling out, and fewer holes were observed. This finding suggests that UPE clings to the KF, enhancing the capability of composites to carry the load when force was exerted. However, compared to the untreated KF composites in Fig. 10 (d), cavities between the fibers and matrix were observed, indicating an inadequate bonding between the fibers and polymer matrix that exacerbates the reduced capability of composites to bear the smashing once the exertion of force.

4.6 Atomic force microscopy (AFM) of the fibers

The topological properties of the untreated and treated KF were also investigated using the AFM under the ambient environment. The two-dimension AFM micrographs of the untreated KF and treated KF-ZnONPs with 1, 2, 3, 4, and 5% ZnONPs are shown in Fig. 11 (a), (b), (c), (d), (e) and (f) respectively. Micrographs allowed for a good grasp of the topographical changes seen in the KF. According to the findings, the untreated KF showed a smooth surface. However, after treatment with ZnONPs, the surface roughness gradually augmented as ZnONPs content increase, indicating that more ZnONPs resided at the surfaces of the KF as more ZnONPs nanoparticles were embedded (Xiong et al., 2004).

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Fig. 11

AFM images (a) untreated KF, (b) KF- 1% ZnONPs, (c) KF- 2% ZnONPs, (d) KF- 3% ZnONPs, (e) KF- 4% ZnONPs, (f) KF- 5% ZnONPs.

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From the Fig. 11, bright areas represent increased adhesion forces between the tip and fiber, while the dark areas represent decreased adhesion forces. There were numerous large brighter areas in the AFM adhesion image of the untreated KF. The adhesion forces were distributed more uniformly on the surface of KF-ZnONPs than on untreated KF. After the treatment with ZnONPs, the adhesion force between the fiber and AFM tip was reduced because the roughness of the surface increased. This force is a combination of van der Waals and capillary forces (Butt et al., 2005). This revealed that the presence of ZnO NPs inorganic nanoparticles at the surface of the fiber causes the surface roughness to grow progressively. The investigation by Liang et al. (2014) confirms this out.

Atomic force microscope images (AFM) revealed that the ZnONPs nanoparticles could successfully produce nano-level roughness, which was advantageous to the hydrophobic characteristics. A rough surface of the KF can increase UPE-KF adhesion by creating good interlocking with the surface of the UPE. This is demonstrated by the tensile modulus composite, which exhibits increased strength as the interfacial bonding between KF and UPE matrix increased (Liang et al., 2014). However, as the content of ZnO NPs nanoparticle increases, poor interfacial adhesion can be observed as a result of the encouragement of a wide debonding gap between fiber and polyester, as well as influence on the sample surface physical and topology that we were preferred to stop at 5% concentration of ZnONPs nanoparticles. It is critical to note that an increase in the mechanical characteristics of polymeric composites can be attributed to the compatibility of the fiber and matrix, as discussed in the section on mechanical properties above. The high surface area interaction from the ZnONPs, UPE, and KF causes an increase in interfacial adhesions and, therefore, a more homogenous surface of the composite. These findings are consistent with the outcomes of the mechanical characteristics, which revealed an augment in compression resistance due to the formation of a more compact interface and a high rough surface.

4.7 (X-RD) crystallinity of the UPE/KF-ZnONPs

XRD analysis was utilized to analyse the crystalline structure of the composites and the formation of a new crystalline compound due to the addition of ZnONPs nanoparticles. The comparison of the composite diffractograms of ZnONPs, untreated UPE/KF, and treated UPE/KF-ZnONPs composites is presented in Fig. 12. According to the XRD results, ZnONPs exhibited diffraction peaks at 2θ in the planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (1 1 2), respectively. These planes typically represent the hexagonal Wurtzite structure (Kulyk et al., 2010).

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Fig. 12

XRD pattern of ZnO nanoparticles, and (a) untreated UPE/KF, (b) UPE/KF- 1% ZnONPs, (c) UPE/KF- 2% ZnONPs, (d) UPE/KF- 3% ZnONPs, (e) UPE/KF- 4% ZnONPs, (f) UPE/KF- 5% ZnONPs.

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However, new peaks emerged in the composite architectures evidencing the presence of ZnONPs, particularly in the UPE/KF-ZnONPs composites with 5% ZnONPs concentrations which revealed the formation of new peaks at 2θ = 31.85°, 34.55°, and 36.35°. These peaks can be ascribed to (1 0 0), (0 0 2), and (1 0 1) ZnO crystal planes, respectively (Sui et al., 2006). This demonstrates the presence of ZnONPs along its c-axis on the KF surface. Peaks located at 46.98°, 56.33°, and 62.19° correspond to crystal planes (1 0 2), (1 1 0), and (1 0 3), respectively, and indicate the formation of hexagonal wurtzite ZnO structure (d’Água et al., 2018). Similar research performed by Arfaoui et al. (2017) found that the new peaks equally showed a new functional group as a marker of chemical interaction by adding the ZnONPs nanoparticles. This new chemistry indicated that specific interactions between nanoparticles and the composite were formed. The amorphous UPE/KF composite peak is present in all six diffractograms at 2θ ∼ 22.23°. However, a slight reduction in the intensity of the amorphous peak simultaneously with a new crystalline peak has developed at 2θ = 15.54°, 31.85°, 34.55°, and 36.35°. These findings indicate the emergence of a new diffraction peak in the UPE/KF composite, which is most likely the consequence of nanoparticles acting as nucleation centers in the KF. Therefore, it can be inferred that composites having ZnONPs nanoparticles may produce novel crystalline, water-repellent compounds that improve the ordering at the interface area. This aspect is essential for the dimensional stability of a composite that contains KF as a hydrophilic component.

4.8 FTIR spectra analysis

The FTIR spectra of untreated and treated KF with 1%, 2%, 3%, 4%, and 5% ZnONPs at various fiber loadings are shown in Fig. 13 (a), (b), (c), (d), (e), and (f), respectively. It has been found that the FTIR spectra for the untreated samples exhibit intensity bands in the regions 3340 cm⁻¹, 1732 cm⁻¹, 2800–3000 cm⁻¹, 1640, and 1000 cm⁻¹–1245 cm⁻¹ due to O—H stretching vibration, C⚌O stretching vibration, C—H stretching vibration, C⚌C stretching vibration, and C—O—C stretching vibration, correspondingly (Li et al., 2011). These absorbance bands are caused by hydroxyl groups in cellulose, acetyl ester carbonyl groups in hemicellulose, and carbonyl aldehydes and aromatic compounds in lignin.

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Fig. 13

FTIR spectra of (a) untreated UPE/KF, (b) UPE/KF- 1% ZnONPs, (c) UPE/KF- 2% ZnONPs, (d) UPE/KF-3% ZnONPs, (e) UPE/KF- 4% ZnONPs, (f) UPE/KF- 5% ZnONPs.

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The FTIR spectra of the treated KF samples have almost identical functional peaks to that of untreated KF. However, peak No.1 at wave number 3340 cm⁻¹ in Fig. 13 (a), (b), (c), (d), (e) and (f), the intensities decreased. This is most likely due to the use of ZnONPs nanoparticles in the treatment, which blocks hydroxyl (OH) functional groups responsible for moisture absorption. This OH stretching is related to hydroxyl (OH) functional groups in lignocellulose natural fibers made up of cellulose, hemicellulose, lignin, wax, and pectin. The peak for untreated KF has a broader intensity, indicating greater hydroxyl OH functional group content (Nosbi et al., 2011). The intensities decreased at wave numbers between 2800 and 3000 cm⁻¹ (peak No. 2) attributable to the nano treatment with ZnONPs nanoparticles. The intensities of peak No. 3 decreased because of the treatment (Ding et al., 2012; Pandey and Pitman, 2003).

As a result of the treatment, the intensities of the peak at about 1640 cm⁻¹ (peak No. 4) were reduced. This peak has been attributed to an aromatic skeletal vibration in lignin (C⚌C stretch) (Pandey and Pitman, 2003; Bak et al., 2012; Papadopoulos and Mantanis, 2011). Additionally, as ZnONPs concentration rose, there was a reduction in the peak at 1432 cm⁻¹ (peak No. 5), which appeared to be associated with C—H deformation in lignin (Pandey et al., 2012). The bands of peaks 4 and 5 are transformed into an association of three bands at about 1590, 1495, and 1451 cm⁻¹ with 5% (wt%) ZnONPs nanoparticles, demonstrating a chemical interaction between KF, UPE, and probably the inorganic nanoparticles. Furthermore, the band at 1030 cm⁻¹ was changed into an association of two bands at about 1124 and 1069 cm⁻¹, indicating a potential interaction between the components of the composite.

The intensity of the peak at wave number 1328 cm⁻¹ (peak No. 6), which has been related to C—H (CH3 bonding), and also, the intensity of the prominent peak (No. 7) between 1000 and 1245 cm⁻¹, which has been related to C—O stretching and C⚌O deformation in lignin and xylan for the syringyl ring decreased in the treated samples compared to that of the untreated KF (Ding et al., 2012; Pandey and Pitman, 2003). This peak (No.7) elucidates that hemicellulose and lignin are present in the untreated and treated KF (Sun et al., 2004).

FTIR analysis disclosed an increase in the number of the band after surface treatment with ZnONPs nanoparticles, suggesting new chemical interaction between the polymeric composite components UPE, KF, and ZnONPs nanoparticles. These findings corroborate the mechanical test results. Nevertheless, other authors have attributed ZnO to additional peaks, including 2865–2971 cm⁻¹ (Traistaru et al., 2012), 3419 cm⁻¹ (AbdElhady, 2012), and 3409 cm⁻¹ (Salehi et al., 2010).