Introduction

Over the last decade, many industries, scientists and academicians are paying more attention in the development on new eco-friendly materials due to the environmental awareness [1]. The major concern is to reduce the pollution caused by use of man-made synthetic materials like aramid, nylon, etc. In this regard, recently, natural fibers had gained a huge interest due to its remarkable properties over synthetic materials concerning environment awareness [2, 3]. From various research, it is noticed that the natural fibers can easily replace the synthetic materials due to its potential properties like eco-friendly, biodegradability, availability, low density, low cost and considerably good mechanical and physical properties compared to synthetic fibers [4, 5]. Nowadays, industries are implementing the natural fibers and natural fiber-based composites in versatile applications, revolutionizing the use of biomaterials, reducing the environment hazards and improving the employment opportunities. Natural fibers has made its way towards many applications like automotive, electrical devices, sports products and many lightweight products [6, 7].

In general, natural cellulose fibers are extracted from various parts of cellulosic materials like stem, bark, leaf, roots, fruits, stalks of the plant or tree [8]. The properties and performance of the cellulose fibers are mainly governed by the properties like extraction mode, fiber structure and chemical composition [9, 10]. The main compositions of natural fiber are cellulose, hemicellulose, and lignin therefore, natural fiber has been known by common name as cellulosic fiber. Cellulose is a linear polysaccharide polymer comprising of glucose monosaccharide groups [11]. Generally, the higher weight cellulose ratio shows better mechanical and thermal behavior. There are various research works that have been undertaken on the extraction and characterization of a cellulosic fiber and used as a potential reinforcement in polymer matrices [12, 13].

The development of a good polymer composite with good properties needs a keen awareness about the properties and type of the natural cellulose fibers. Recently, there are various natural fibers like Grewia damine stem bark fiber, Luffa vine, Hierochloe Odarata, Acacia nilotica L., Muntingia Calabura, Kigelia Africana, etc., are identified as a potential alternative resource of reinforcement for polymer matrices [4, 9, 14,15,16,17]. However, the major drawback is that each plant fibers are limited to a particular geographic location. Hence it is very necessary to find potential new resource of raw material to satisfy the demand [11].

The utilization of natural fibers has some drawbacks such as biocompatibility, hydrophilic nature, moisture absorption, low wettability, and heterogenous structure [18]. These problems makes it difficult to use the natural fibers as reinforcement in polymer matrices because it hinders the performance of the composites. Various literatures reported that surface modification techniques help to overcome this limitation. There are various fiber treatment techniques like peroxide, acetylation, silane, alkali, corona, laser etc., Among them, alkali treatment is a common technique which is mostly used because of its low cost and efficiency to remove excess amount of amorphous constituents by breaking down hydroxyl groups, thus improving the interfacial bonding between the fiber and matrix. Hence, alkali treatment is chosen as a surface modification technique in this current research.

In a research, it was, reported that the 5% alkali treatment on Perotis indica fiber for 60 min increased the cellulose wt% increased by 5.78% and reduced the hemicellulose wt% by 27.96% [19]. When Urena lobata fibers were treated with 6% alkali solution, the results revealed that there was an 6.07% increase in cellulose wt%, and improved the tensile strength up to 238 ± 0.015 MPa [20]. In another research when Dichrostachys cinerea fibers were treated with 5% NaOH, the weight % of cellulose was increased up to 78.4% and increases the crystallinity index to 65.63% [21]. Similarly, when red banana peduncle fiber was treated with 5% alkali solution, it improved its thermal stability of the fiber up to 320 ℃ and increased the crystalline index up to 65.47% [22]. There are various reports stating that the alkali treatment is good and effective surface modification technique, which enhances the properties of the natural fibers. The treatment enhances the bond between fiber and matrix thus improving the performance of the composites.

To meet the demands of raw material supply and to protect the environment, Zmioculus Zamiifolia (ZZ) was identified as a potential resource of raw cellulose material. Research on ZZ fiber as a potential resource of reinforcement in polymer composites was not yet been explored till date. Therefore, in current work, Zmioculus Zamiifolia fiber was analyzed for its phyco-chemical and thermomechanical properties and its suitability for reinforcement in polymer matrices. The research focusses on investigating the ZZ fiber for its chemical, physical, thermal, mechanical and morphological properties by using X-Ray diffraction test, Fourier Transform Infrared Spectroscopy (FT-IR), chemical analysis, thermogravimetric analysis (TGA), differential Scanning Calorimetry (DSC), single fiber tensile test, contact angle and scanning electron microscope images (SEM).

Materials and Methods

Materials and Fiber Extraction

Zmioculus Zamiifolia (ZZ) is a tropical perennial plant that is native to eastern Africa. It is an herbaceous and ornamental plant growing up to 45–60 cms tall with fibrous stem. The plant can survive in drought conditions by storing water in its leaf and roots. For fiber extraction, the plant stems were collected locally from an ornamental plant vendor. The ZZ stems were cleaned, chopped to 10 cm length, and drenched in water for 3 days to facilitate easy fiber retting using a wire brush. Later, the fibers were cleaned using distilled water and dried in sunlight for 48 h to eliminate the moisture content. Finally, the fibers were segregated and subjected to chemical treatment.

Chemical Treatment

For alkali treatment on ZZ fibers, initially the fibers are thoroughly rinsed using deionized water to remove the impurities. Then, 2.5 molar concentrated alkali solution was prepared by mixing 101.01 g of NaOH pellets (99% Pure) in 1-L deionized water. The NaOH pellets have a molar mass of 40 g/mol. Now, 125 g of ZZ fibers were soaked in one liter 2.5 molar alkali solution for 4 h. Finally, the fibers were taken out, washed completely with deionized water till the pH was neutralized, and left for drying in a hot air oven at 60 °C for 24 h. The schematic representation of the chemical reaction between the fiber and the alkali solution is presented in Fig. 1.

Fig. 1
figure 1

Mechanism of alkali treatment on ZZ fiber

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From Fig. 1, it was noticed that the alkali solution breaks down the hydroxyl groups present on the fiber surface (OH groups in fiber components) releasing water with ONa+. The alkali treated fibers were finally neutralized by washing through deionized water. Due to the breaking of OH bonds the impurities and considerable quantity of amorphous constituents were removed, resulting in the formation of rough fiber surface and reduced fiber diameter.

Physical and Chemical Analysis

The fiber diameter was measured using an optical microscope (Model: Olympus/BX43, Company: Olympus, Country: Japan). The diameter reading was taken at 25 random places for each 25 random fibers and the average was reported. The fiber density was determined through pycnometer experimentation (toluene) according to the equation \({\rho }_{zz}=\left[\left({m}_{2}-{m}_{1}\right)/\left(\left({m}_{3}-{m}_{1}\right)-\left({m}_{4}-{m}_{2}\right)\right)\right]{\rho }_{t}\). Where m1 represents the mass of the pycnometer, m2 represents the mass of the pycnometer filled with ZZ fiber, m3 is the mass of the pycnometer with toluene, and m4 is the mass of pycnometer with toluene and ZZ fiber. All units are in Kg. ρt is the density of toluene 0.87 g/mL.

The cellulose, hemicellulose, wax, lignin, and moisture content before and after the chemical treatment was analyzed through chemical analysis. Kushner and Hoffer’s method was employed to determine the cellulose content. Initially, 150 g of crushed fibers are soaked in a mixture of 95% nitric acid and ethanol. Later the fibers were taken out and dried using a hot air oven at 60 °C (Model: redline RE115, Company: Binder, Make: Germany). The weight fraction of cellulose was estimated by calculating the weight of the insoluble weight fraction. The weight fraction of hemicellulose was calculated according to NFT 120-008. Klason method was used to calculate the weight percentage of lignin according to APPITA P11s-78 standard. Sartorius, model MA45 was employed to find out the moisture content present in the fiber.

X-Ray Diffraction Analysis

The (XRD) X-ray diffraction test was carried out to measure the (CI) crystallinity index and (CS) crystal size of the untreated and alkali-treated ZZ fiber. The test was carried out using an X-ray diffractometer (Model: SmartLab X-Ray diffractometer, Company: Rigaku, Country: USA). The monochromatic radiation from CuKα has a wavelength λ = 0.154 nm, and operates at 30 kV and 30 mA. The monochromatic radiation was recorded from 2θ = 10° to 80° at a rate of 5º/minute. The crystallinity index was calculated using Segal’s equation: \(CI=\left({H}_{200}-{H}_{AM}\right)/{H}_{200}\) and the crystal size was estimated according to the equation: \(CS=K\lambda /\beta cos\theta\). Where K represents the Scherrer’s constant K = 0.89, θ is Bragg’s angle, λ is the wavelength and β denotes the peaks full width at half-maximum.

Fourier Transform Infrared Spectroscopy

The functional groups and its corresponding fiber components of the untreated and alkali treated ZZ fiber was analyzed using Fourier transform infrared spectroscopy using an FTIR spectrometer (Model: Invenio S, Company: Bruker, Country: United Kingdom). The experiment was done in an attenuated total reflection mode where the powdered ZZ fibers were placed over a diamond and the infrared spectrum was passed over it. The infrared spectrums were recorded from the region 4000 cm−1 to 400 cm−1 at a rate of 32 scans per minute with a resolution of 2 cm−1 signal-to-noise ratio.

Thermogravimetric Analysis

The thermal stability of ZZ fiber before and after chemical treatment was analyzed using thermogravimetric analysis. The test was carried out by using a thermogravimetric analyzer (Model: TGA/ DSC 3 þ HT/1600, Company: Mettler Toledo, Switzerland). Powdered ZZ fiber sample of ≈ 6 mg was taken in alumina crucible and placed into the furnace. The test was conducted with a heating rate of 10 °C/min from room temperature 30 ℃ to 600 °C. The furnace was maintained with a nitrogen flow rate of 20 ml/min. The kinetic activation energy was calculated using the “Boats Redfern method” (Mathematical relation for the first-order reaction) to find the kinematic parameter according to equation \(log\left[\frac{-log\left(1-\alpha \right)}{{T}^{2}}\right]=log\frac{AR}{\varphi {E}_{a}}\left[1-\frac{2RT}{{E}_{a}}\right]-\frac{{E}_{a}}{2.303RT}\) [23, 24]. Where α denotes the fraction of sample degraded at a given time t, \(\alpha =\frac{\left({w}_{0}-{w}_{t}\right)}{\left({w}_{0}-{w}_{f}\right)}\). “W0” is the initial weight of the sample, “Wt” represents the sample weight at given temperature, “Wf” is the sample’s final weight, “φ” is the linear heating rate ( ℃/min), “T” represents the absolute temperature (K), “Ea” is activation energy, “A” is the frequency factor in S−1, “R” is the gas constant 8.314 j/(mol.k). The graph is plotted between \(log\left[\frac{-log\left(1-\alpha \right)}{{T}^{2}}\right]\) and \(\frac{1000}{T}\). The slope is \(-\frac{{E}_{a}}{2.303R}\) given by the best fit curve that determines the order of decomposition intercepting \(log\frac{AR}{\varphi {E}_{a}}\left[1-\frac{2RT}{{E}_{a}}\right]\).

Differential Scanning Calorimetry

The differential scanning calorimetry (DSC) was used to determine the amount of heat flow and amount of energy required for the fiber component to undergo thermal degradation. The test was carried out in a differential scanning calorimeter (Model: DSC 3+ STARe System, Company: Mettler Toledo, Country: Switzerland). Crushed ZZ fibers of 10 to 12 mg was placed in an aluminum pan and kept in the furnace. The test was carried out from 30 to 450 ℃ with a rate of change in temperature at 10 ℃/minute. The furnace was maintained with a nitrogen flow of 50 ml/min.

Single Fiber Tensile Test

The single fiber tensile test was carried out to analyze the tensile strength of the ZZ fiber with respect to chemical treatment. The test was carried out in a universal testing machine (Model:5500R, Company: Instron, Country: USA) with a loadcell of 1kN according to ASTM D3822-07. The gauge length was set to 50 mm with a loading rate of 1 mm/min. 20 samples were tested and a Weibull distribution analysis was performed to find out the satisfactory significance level for the tensile strength of the ZZ fibers before and after chemical treatment.

Wettability

The wettability behavior of the ZZ fiber before and after alkali treatment was measured through contact angle analysis. The test was carried out using (Model: OCA 15LJ, Company: Data physics, Country: Germany). Considering the smaller fiber diameter, the test was performed on the ZZ bark to evaluate the effect of surface modification on the wettability nature through contact angle. The test was done by dispensing 2 μl of distilled water over the ZZ bark and the image was taken after 5 s using a powerful optical camera and the contact angle was calculated. 10 reading are recorded, and the average is reported. The work of adhesion WA and the spreading coefficient SC were calculated according to equations \({W}_{A}={\gamma }_{L}\left(1+cos\theta \right)\) and \({S}_{C}={\gamma }_{L}\left(cos\theta -1\right)\). Where θ is the contact angle, γL is the surface energy of the liquid. The WA represents the amount of energy required to separate the liquid from the solid surface, while SC provides an idea about the wetting behavior of liquid over the solid.

Morphological Analysis and Energy Dispersive X-Ray Spectroscopy

The surface morphology of the ZZ fibers before and after alkali treatment was studied using the scanning electron microscopic images (SEM-images). The images were captured by using a scanning electron microscope (Model: FEI Quanta 450, Company: FELMI-ZFE, Country: Australia). The surface roughness and the 3D roughness profile ZZ fibers were analyzed from the captured SEM images using IMAGE J software. The elemental constitution of the ZZ fibers before and after alkali treatment was analyzed using energy dispersive X-Ray spectroscopy (EDX) using (Model: FEI Quanta 450, Company: FELMI-ZFE, Country: Australia).

Results and Discussions

Physical and Chemical Analysis

The Zmioculus Zamiifolia plant and the retted fiber was presented in Fig. 2a, b. The effect of chemical treatment on the density, diameter and chemical composition of the ZZ fiber before and after chemical treatment was presented in Table 1. The diameter of the untreated ZZ fiber was 339.12 ± 10.12 µm. When the ZZ fiber was subjected to alkali treatment, the diameter was reduced by 1.18 times (286.35 ± 9.85 µm). The reduction in the diameter was attributed to the removal of impurities and breaking the OH groups dissolving the excess amorphous constituents present on the fiber surface [25]. Average diameter was reported after analyzing 25 samples, and the diameter was measured at 20 random places of each ZZ fiber. The diameter of the natural fibers is heterogeneous, and it varies depending on various factors like growth rate and geographical location of the ZZ plant. Hence, a Weibull distribution plot for the diameter was presented in Fig. 2 to know the distribution and the statistical significance level on the diameter of the fibers. It was observed that the scale value of the untreated and the alkali-treated fiber diameter lies approximately in the range of the average fiber diameter. The P-value of the analysis was also > 0.05 which signifies H0 hypothesis. There was a 95% statistical satisfactory significance level in the diameter of the composite.

Fig. 2
figure 2

a Zmioculus Zamiifolia plant, b Zmioculus Zamiifolia extracted fiber c Weibull plot for diameter (µm)

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Table 1 Comparison of physical and chemical properties of ZZ with other natural fibers
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The density of the untreated and the alkali-treated ZZ fiber are 0.891 ± 0.017 g/cc and 1.113 ± 0.015 g/cc, respectively. The alkali treatment breaks down the OH groups present on the fiber surface and disintegrates excess amorphous constituents like cellulose, hemicellulose, lignin, pectin., etc., Furthermore, the surface pores were filled with graft molecules during the treatment of alkali solution. Finally, the treatment increased the high molecular weight cellulose weight percentage when compared to amorphous constituents. This phenomenon resulted in the increase of density of the fiber by 24.91%. The increase in density had a major impact on the mechanical performance of the composite. The density of the alkali-treated ZZ fiber was higher than the density of Areca (0.7 g/cc), and pineapple fiber (0.8 g/cc) but lower than Coir (1.2 g/cc) and jute (1.3 g/cc) [26, 27].

The chemical constituent of the natural fibers had a major impact in controlling various properties like crystalline property, moisture absorption, and mechanical strength of the material. Cellulose content in a fiber plays a major role in improving the mechanical strength and stiffness of the fiber and thereby the composite. From the chemical analysis (Table 1), the cellulose % of alkali-treated ZZ fiber was 52.63 ± 3.57% which was 1.27 times higher than untreated ZZ fiber (42.12 ± 3.32%). This was because the amorphous constituents with lower molecular weight like hemicellulose, wax, lignin, pectin, etc. were broken down by alkali treatment. Thus, resulting in the higher weight ratio of cellulose I compared to the amorphous constituents and the untreated ZZ. The cellulose % of alkali-treated ZZ fiber was higher than the cellulose content of Mutingia Calabura bark fiber (41.5%) and phoenix dactylifera L fiber (35%) but lower than Phoenix sp (61.13%) and Shwetark stem fiber (69.65%) [6, 12, 17, 36]. The hemicellulose present in the fiber is prone to moisture attraction while the lignin content provides the structural rigidity. The lignin also tends to hold the moisture content within the fiber. However, higher lignin content destroys the fiber structure. From Table 1, it was noted that there was a reduction in the weight % of hemicellulose and lignin due to the disintegration of hydroxyl groups through alkali treatment. It was also observed that the percentage of wax was reduced to 0.24 ± 0.32% which was 5.26 times lower than the untreated ZZ fiber (1.35 ± 0.78%). The reduction in moisture content was attributed to the reduction of moisture-sensitive hemicellulose content. Higher wax and moisture content hinders the wetting behavior of matrix with fiber, thus lowering the mechanical performance of the composite. A comparative study of the physical and chemical properties of ZZ fiber with other natural fibers was presented in Table 1.

XRD

Figure 3 represents the X-ray diffractograms of the ZZ fibers before and after alkali treatment. The intensity peak observed at 2θ range 22.01° at the lattice plane (2 0 0) attributes the peak of cellulose-I [38, 39]. From the diffractograms, the diffraction intensity observed in the 2θ range 18.61° at the minimum valley between the peaks belongs to the amorphous constituents in the fiber [38,39,40,41]. Using Segal’s equation, the crystalline index of the untreated and alkali-treated ZZ is analyzed as 25.75 ± 2.34% and 37.80 ± 4.57%, respectively [39]. The alkali-treated fiber showed a higher crystalline index because of the removal of excess amorphous constituents in the fiber thus it increased the weight ratio of crystalline cellulose and was evident from the chemical analysis. The alkali treatment partially breaks down the bonding between the cellulose and hemicellulose removing the non-cellulose materials. This brings the cellulose crystals closer and makes it ordered through hydrogen bonding thereby increasing the crystalline index. It was also evident from the X-ray diffractograms at the minimum valley region (2θ = 18.71°) diffraction intensity was decreased in the alkali-treated fiber when compared to the untreated fibers confirming the reduction of the weight ratio of amorphous constituents. The higher crystalline index increases the stiffness and brittleness of the fiber which further influences the mechanical strength of the composite. The crystalline index of alkali-treated ZZ fiber was 1.68 times higher than Cortaderia Sellona (22%), 1.83 times higher than Leucas Aspera (20.23%), 1.43 times lower than Coccinia Indica stem (53.03%), and 1.59 times higher than Kigelia Africana (59%) [16, 25, 42, 43]. The crystalline size of the untreated and alkali treated fibers as calculated through Scherer’s equation as 2.54 ± 0.24 nm and 4.16 ± 0.62 nm, respectively. The crystalline size plays a major role in governing the water absorption property of the material [25]. Alkali treatment had increased the crystal size up to 63.77%. The higher crystal size reduces the water absorption of the material by reducing the window of water penetration. The crystal size of alkali-treated ZZ was 2.17 times higher than Coccinia grandis stem (1.91 nm), 1.24 times lower than Ficus religiosa (5.18 nm), and 1.22 times lower than Grewia damine stem fiber (5.09 nm) [4, 27, 44]. A comparative property of crystalline nature, thermal property, and mechanical strength of ZZ fibers with other natural fibers are presented in Table 2.

Fig. 3
figure 3

X-ray diffraction analysis of ZZ fibers

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Table 2 Comparison of crystalline, thermal, and mechanical property of ZZ with other natural fibers
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FTIR

Table 3 Functional group and corresponding fiber component of the ZZ fiber
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The FTIR spectrum of the untreated and alkali-treated ZZ fibers were shown in Fig. 4 and the corresponding functional groups were presented in Table 3. From the infrared spectrum, the first strong broad U-shaped intensity peak was observed in the range of 3297 cm−1 and 3292 cm−1 for both the untreated and alkali-treated ZZ fiber, respectively. It attributes to the stretching of hydrogen-bonded OH stretching and indicated the presence of cellulose I [46]. The next intensity peak observed at ranges 2917 cm−1, 2912 cm−1, 2847 cm−1, and 2849 cm−1 was may be due to the asymmetric and symmetric stretching vibrations of acetyl groups in C–H. This phenomenon confirmed the presence of cellulose, lignin, and carboxylic acid [45]. The next peak at 1740 cm−1 was due to the stretching of C=O, which was a characteristic feature of hemicellulose. The absence of a peak in alkali treated ZZ fiber was because of the removal of excess amorphous constituents in the plant fiber. Next peaks at 1598 cm−1 and 1605 cm−1 corresponds to carbonyl group and vibration in C=C (ester, aromatic ring, and ketones) signified the presence of hemicellulose and lignin. The reduction of peak height in the alkali treated ZZ fibers confirmed the removal of some parts of hemicellulose and lignin which was evident from the EDX results that showed a reduction in carbon weight % and increase in oxygen weight %. From the infrared spectrograms Fig. 4, the alkali treated sample showed smaller peak at 1605 cm−1 when compared to the untreated ZZ fiber which was because of the removal of excess hemicellulose and lignin [14]. Next consecutive peaks observed at the range 1383 cm−1 and 1411 cm−1 attributed to the presence of lignin and hemicellulose through the stretching of CO in the acetyl groups [17]. The final peaks were observed in the range 1006 cm−1 and 1011 cm−1 corresponds to the vibration and bending of C–O and C–H denoting the presence of aromatic ring in polysaccharides [22].

Fig. 4
figure 4

Fourier transform infrared spectroscopy

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Thermogravimetric Analysis

The TGA and DTG curves of the untreated and alkali-treated ZZ fibers were presented in Fig. 5a, b, the thermal degradation range with weight loss % was presented in Table 4. From the thermograms Fig. 5a, it was observed that the initial degradation (Initial) occurred between the range 60 ℃ to 122 ℃, which was due to the degradation of structurally integrated water molecules present in the fiber. The initial degradation of water molecules for the untreated and alkali-treated ZZ fibers occurred up to 113.19º ± 1.12 ℃ with weight loss of 8.95 ± 0.92% and up to 121.84º ± 1.37 ℃ with weight loss 6.48 ± 0.74%, respectively. It was also evident from the chemical analysis that the alkali treatment reduced the moisture content in the fiber by removing a considerable amount of moisture sensitive hemicellulose from the ZZ fiber. Moreover, the reduction in hydroxyl groups will not be able to absorb more moisture. The second stage degradation or TOnset degradation occurred in the range 210 ℃ to 362 ℃ which corresponds to the degradation of low thermally stable hemicellulose and high thermally stable cellulose [47, 48]. From the DTG curve Fig. 5b, a prominent peak was observed at the range 310.32° ± 2.64 ℃ and 326.82° ± 2.53 ℃ for the untreated and alkali treated ZZ fibers respectively, represented the degradation and depolymerization of cellulose and hemicellulose. In the second stage (TOnset) stage, the untreated ZZ fiber showed a weight loss of 45.16 ± 2.34% which was 1.07 times higher than the alkali-treated ZZ fiber (41.87 ± 1.47%). The higher weight loss in untreated ZZ was due to the presence of less thermally stable amorphous constituents as evident from chemical analysis. While the alkali-treated ZZ showed a lower weight loss due to the presence of a high thermally stable cellulose weight ratio and lower amorphous constituents when compared to the untreated ZZ [22]. The maximum peak temperature of the DTG curve was considered as the thermal stability of the fiber. The thermal stability of the alkali-treated ZZ fiber was 326.82 ℃ which was 1.05 times higher than that of untreated fiber (310.32 ℃) and was evident from the DTG curves Fig. 5b. The cellulose and lignin started to disintegrate at a higher temperature due to their long polymer chain and complex structure. The final stage of degradation occurred in the range 365 ℃ to 600 ℃ which was attributed to the degradation of lignin wax and other fiber constituents [37, 42]. At this stage, the untreated and alkali-treated ZZ showed a weight loss % of 14.42 ± 1.89% and 10.89 ± 1.23%, respectively. The weight of alkali-treated ZZ fiber was 1.32 times lower than untreated ZZ fiber, this phenomenon was because the alkali treatment had broken down the hydroxyl groups dissolving wax, impurities, thus improving the thermal stability. The char residue of untreated ZZ fiber was 30.63 ± 1.96% while the alkali treatment ZZ fiber showed a char residue of 35.42 ± 1.23% which was 1.16 times higher than the untreated ZZ fiber. A comparative study of thermal stability of ZZ fiber with other natural fibers was presented in Table 2.

Fig. 5
figure 5

a Thermogravimetric analysis, b TGA-first derivative, c Kinetic activation energy (Coats Redfern method)

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Table 4 Thermal properties of the raw and chemical treated ZZ
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The thermal stability of the untreated and alkali-treated ZZ fiber can also be determined through the activation energy. The kinetic activation energy of the untreated and alkali-treated ZZ fibers was calculated according to the “Coats Redfern method” and presented in Fig. 5c. According to the “Coats Redfern equation” the kinetic activation energy (Ea) of the untreated and alkali-treated ZZ fiber was calculated as 66.12 ± 1.27 kJ/mol and 71.86 ± 1.34 kJ/mol, respectively. The values of Ea lies within the range (60 kJ/mol to 170 kJ/mol), which is the range of biomaterials wood decomposition range [27]. It was observed an 8.68% of more Ea was required for the alkali-treated ZZ to activate the degradation-transformation process of atoms and molecules from one stage to another when compared to untreated ZZ. This phenomenon was due to the fact of removal less thermally stable amorphous materials like hemicellulose, lignin wax, and impurities from the fibers; resulted in the ordered structure of atoms and molecules; increased crystal size and crystallinity. Thus, increased the thermal stability and required more kinetic activation energy to start and end the thermal degradation process [1]. The results represented that there was no phase transition occurred in the temperature range 305 ℃ to 315 ℃ and 321 ℃ to 231 ℃ for the untreated and alkali-treated ZZ fibers, respectively. The kinetic activation energy of alkali-treated ZZ fiber was almost similar to Prosopis Juliflora bark (76.72 kJ/mol) [49], 1.12 times higher than Luffa vine fiber (63.9 kJ/mol) [9], 1.14 times lower than Coccinia grandis stem (82.3 kJ/mol) [27], and 1.07 times lower than Thespesia populnea (77.32 kJ/mol) [50].

DSC

The differential scanning calorimetry results showed the heat flow and enthalpy (ΔH) during the thermal degradation of untreated and chemically treated ZZ fiber and were presented in Fig. 6 and Table 5. From the DSC curves, the first slopes of endothermic curve was observed in the temperature range of 100 ℃ showing endothermic peaks at 88.54° ± 2.34 ℃ and 91.89° ± 2.62 ℃, respectively for both untreated and alkali-treated ZZ fibers which corresponded to the evaporation of moisture content [50]. The respective enthalpy values calculated through integrating the peaks were 187.56 ± 3.45 j/g and 141.8 ± 4.76 J/g, respectively. The presence of higher moisture in the untreated ZZ resulted in the requirement of 1.52 times more energy when compared to the treated ZZ fiber to volatilize the structurally integrated water molecules. This was in accordance with the results of TGA analysis. The second peak for the untreated and treated ZZ observed at 167.17° ± 3.12 ℃ and 177.17° ± 3.54 ℃, respectively with corresponding enthalpy values 6.18 ± 1.65 J/g and 1.50 ± 0.89 J/g was may be due to the softer material structure showing distorted behavior of heat flow [46]. Later significant exothermic shoulder peaks were observed for untreated and alkali-treated ZZ fibers at 286.12° ± 2.54 ℃ and 298.15° ± 2.78 ℃, respectively with corresponding enthalpy (ΔH) values 16.72 ± 2.34 J/g and 11.97 ± 1.27 J/g. This phenomenon was due to the degradation of the amorphous hemicellulose present in the ZZ fibers [51]. The next sharp exothermic peak observed in the range 321 ℃ to 366 ℃ corresponded to the degradation of cellulose present in the ZZ fiber which was in accordance with the TGA & DTG results Fig. 5a, b. It was noted that the peaks of untreated and alkali-treated ZZ fibers were centered at 339.83º ± 1.76 ℃ and 344.75º ± 2.59 ℃, respectively with corresponding enthalpy (ΔH) values 32.66 ± 3.78 J/g and 48.84 ± 3.54 J/g. These temperature peaks were associated with the degradation range of thermally stable cellulose [15]. The higher ΔH values in cellulose degradation for the alkali treated ZZ was due to the higher cellulose weight ratio and higher crystalline index. It was achieved by breaking considerable OH groups and bringing the molecules of cellulose crystals closer. Beyond this temperature after 400 ℃, gasification and liquefaction occurred with the degradation of lignin and other fiber components. From the results of the thermal tolerance, it was observed that the alkali-treated ZZ fibers could withstand high temperatures and were suitable for the fabrication of polymer composites.

Fig. 6
figure 6

Differential scanning calorimetry

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Table 5 Comparison of temperature peaks and heat flow enthalpy
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Single Fiber Tension

Figure 7a showed the tensile strength of the ZZ fibers before and after alkali treatment. From the stress–strain curves, the maximum tensile strength of alkali-treated ZZ fiber was 48.25 ± 5.49 MPa which was 1.4 times higher than untreated ZZ fiber (34.92 ± 5.47 MPa). The increment in the tensile strength was because of the elimination of cementing constituents like hemicellulose, wax, lignin, and other amorphous constituents which resulted in the formation of new hydrogen bond between the cellulose fibrils [50]. The tensile strength of alkali-treated ZZ fiber was higher than Tridax procumbens (33.62 MPa), Cordia dichotoma (16.9 MPa) but lower than coir (90 MPa), and Grewia damine (375.6 MPa) [4, 11, 52]. From Fig. 7a, it was noted that the strain of alkali-treated ZZ fiber was (0.20%) which was 1.25 times lower when compared to the untreated ZZ fiber (0.25%). The strain of the reinforcing fiber played an important role in controlling the elongation % of the composite. The lower strain in alkali-treated ZZ fiber was because of the increased stiffness caused by the elimination of amorphous hemicellulose and lignin from the ZZ fiber. The elastic modulus of untreated ZZ fiber was 0.136 ± 0.052 GPa which was 1.73 times lower than the alkali-treated ZZ fibers (0.235 ± 0.039 GPa). The increase in the tensile modulus was due to the increase in stiffness and reduction of strain [11]. This phenomenon made the fiber brittle but increased the toughness of the composite. The tensile modulus of the alkali treated ZZ fibers were similar to the tensile modulus of fibers extracted from the aerial roots of banyan tree (0.82 GPa) [53]. These tensile values were closer to other natural fibers like Artisdita hystrix (1.57 ± 0.04 GPa), Piassava (1.07–4.59 GPa), and Coconut tree leaf (2.3 ± 0.9 GPa) [54]. In Fig. 7b, Weibull distribution was performed to analyze the significant satisfactory level of the tensile strength of the untreated and alkali treated ZZ fibers. Results of 20 samples were considered for the test, it was noted that the scale value of the results were approximately equal to the tensile strength of the ZZ fibers. The P-value of the results were greater than 0.05, which denoted H0 hypothesis, there was no significant difference between the specimens of the tested sample [4, 11, 51].

Fig. 7
figure 7

a Single fiber tensile strength, b Weibull analysis

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Contact Angle Analysis

The influence of chemical treatment on the wettability behavior of the ZZ fibers was analyzed through contact angle analysis. The knowledge wettability of the plant cellulose fibers was very essential to analyze the interfacial adhesion and interaction between the matrix and fiber. Wettability of a reinforcing fiber plays a important role in controlling the mechanical property of the composite [17]. The contact angle images of ZZ fibers before and after chemical treatment was presented in Fig. 8a, b. From the images, it was noticed that the untreated ZZ fiber showed a contact angle of 77.2° which was 1.09 times higher than that of alkali treated ZZ fiber (70.6°). The higher contact angle for the untreated ZZ fiber was a result of low wettability and was because of the presence of impurities and non-polar materials like lignin, wax, etc., restricting the liquid to spread over the fiber surface [11]. While the alkali treated ZZ fiber showed a lower contact angle as a characteristic feature of good wettability. This phenomenon was because the alkali treatment had broken down the hydroxyl groups present on the fiber surface removing the impurities and non-polar materials [55]. The removal of non-polar materials exposed more reaction sites thus enabling the liquid to spread over the fiber surface faster and lowers the contact angle. The work of adhesion and spreading coefficient are calculated using the contact angle values and is presented in the Table 6. From the calculated data, it is observed that the alkali treated ZZ fiber shows the highest work of adhesion and high spreading coefficient when compared to the untreated fiber. The removal of non-polar materials from the fiber surface made the liquid to spread easier and faster when compared to the untreated fiber.

Fig. 8
figure 8

a Untreated ZZ contact angle, b Alkali treated contact angle, c 3D surface plot untreated ZZ, d 3D Surface plot alkali treated ZZ

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Table 6 Surface roughness
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The 3D surface profile of the ZZ fibers before and after chemical treatment was presented in Fig. 8c, d. The alkali treatment influences the surface texture of the fibers, which had a major impact on wettability behavior and interfacial adhesion between the fiber and polymer matrices. The 3D surface profile was plotted, and the surface roughness was calculated from the SEM images using Image J software. The surface roughness values of untreated and alkali treated ZZ fiber were presented in Table 4.

From the roughness values of Table 4, it was seen that the average roughness of alkali treated ZZ fiber was increased by 1.31 times when compared to the untreated ZZ fibers. This was because the alkali treatment disintegrates the impurities and amorphous constituents present on the fiber surface [17]. The rough texture of the fiber surface exposes more reaction sites and increases contact area. Thus, enabling the fiber to have good interfacial bonding and additionally the rough surface improved the adhesion by forming mechanical interlocks between the fiber and matrix.

Morphology

The surface morphology of the untreated and alkali treated ZZ fibers was studied using SEM images and presented in Fig. 9a, b. The study on the morphological alterations that occur on fiber surfaces due to the influence of chemical treatment was very significant in understanding the surface roughness of the ZZ fiber. Additionally, it supports to study of the fiber-matrix interaction when used as reinforcements in the composite. From Fig. 9a, it was observed that the untreated ZZ fiber surface had microcracks and voids [22]. Furthermore, it was also observed there was a presence of impurities like wax, oil, lignin, and pectin on the fiber surface. However, wax and oil acts as a protective layer to the plant fibers. The presence of these impurities had a major impact in the performance of composite by hindering the wettability nature and lowering the interfacial adhesion between the fiber and matrix. From Fig. 9b, it was observed that the surface topology of alkali-treated ZZ fiber was cleaner and rougher compared to that of untreated ZZ fiber and was evident from the 3D surface plot Fig. 9b [54]. From the SEM image of alkali-treated ZZ fiber, it was observed that there was absence of microcracks, voids, and other surface impurities. This phenomenon was because of the disintegration of fibers surface primary walls thus filling the voids and microcracks with graft molecules during alkali treatment [53]. This states that the chemical treatment improved the surface topology of the fibers thus enabling the fibers to have good interaction with the matrix by forming good interfacial adhesion.

Fig. 9
figure 9

a Untreated ZZ, b alkali treated ZZ

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EDX

Energy-dispersive X-ray spectroscopy is a quantitative and qualitative analysis that is carried out to estimate the elemental constituents present in the fiber before and after chemical treatment [16]. The ZZ fibers are comprised of chemical constituents like cellulose, hemicellulose, lignin, and wax which is the fundamental property of natural fiber. The basic elemental constituents of the chemical composition of ZZ fibers comprises of organic and inorganic components namely carbon, oxygen, sodium, aluminum, chlorine, and calcium [46]. The EDX spectrum of the untreated and alkali-treated fiber was presented in Fig. 10. The elemental atomic and weight % of the ZZ fiber before and after the alkali treatment was presented in Table 7. It was observed from the alkali treated fiber that there was a decrement in the carbon weight % and increment in the oxygen weight % up to 1.19 time and 1.34 time, respectively. This phenomenon was due to the removal of a considerable amount of pectin and hemicellulose during the chemical treatment. This was evident from the FTIR analysis where the reduction in peak height was observed at the wavenumber range 1598 cm−1 [56]. It was noted that alkali-treated ZZ fiber showed trace amounts of Na which was because the OH bond breaks to form ONa as evident from Fig. 1.

Fig. 10
figure 10

a Untreated ZZ, b Alkali treated ZZ

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Table 7 Weight % and atomic % of untreated and alkali treated ZZ
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Conclusion

From the results, it was concluded that the chemical treatment is very effective in breaking down the OH groups on the fiber surface and it will influence fiber diameter and will further improve the weight ratio of cellulose by reducing the amorphous constituents. In the current research the diameter of the alkali treated fiber is reduced to 18.42%. The XRD analysis concluded that the increase in cellulose ratio increased the CI and CS by 46.79% and 63.77%, respectively. This improves the stiffness and reduces the water absorption property. Further the higher weight % of cellulose and lower weight % of amorphous constituents will improve its thermal stability which was confirmed by the TGA analysis. The mechanical test concluded that the alkali treatment significantly increased the tensile strength. The contact angle, SEM and 3D plots concluded that the 5% alkali treatment improved the wettability nature and surface roughness of ZZ fibers. From the experimental results, it was finally concluded that, the alkali treatment causes significant changes structurally and chemically, thus improving the properties ZZ fibers. Hence, it can be used as a novel resource of alternative cellulose material as reinforcements in polymer composites and as alternative resource of natural fiber.

Future Scope

The newly identified fiber has a potential to be used as reinforcements in polymer matrices. However, it is found that the alkali treatment increased its physical and mechanical properties. In future it is very necessary to find the optimum concentration for treatment and alternative surface modification technique to find the nest principle. Furthermore, the fibers can also used to reinforce various polymer matrices and its mechanical performance ban be evaluated.