INVESTIGATION ON PHYSICO-CHEMICAL, MECHANICAL AND THERMAL PROPERTIES OF EXTRACTED NOVEL PINUS ROXBURGHII FIBER

ABSTRACT The adverse environmental effects of synthetic fibers have forced researchers to use natural fibers in composite preparation. Pinus Roxburghii Needles (PRNs) are widely available in the hilly region of Uttarakhand state, India. Utilizing the fiber extracted from PRNs as reinforcement in bio-composite preparation can reduce synthetic fiber and emphasize the use of natural resources. In this study, we extract the Pinus Roxburghii Fibers (PRFs) from PRNs using the water retting method. Their physical, chemical, thermal, and morphological aspects were investigated and reported. The average diameter and density of the extracted PRFs were obtained as 145 ± 17.36 µm and 1020 ± 20.27 kg/m3, respectively. The chemical analysis revealed that the extracted PRFs contain 58.30% cellulose content. Thermal stability and maximum degradation temperature of the extracted PRFs were assessed using thermogravimetric analysis (TGA) and found to be 225°C and 355.84°C, respectively. Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM) were used to study the surface morphology and surface roughness of PRFs, respectively. Further, the FTIR analysis of the extracted PRFs confirmed the presence of different functional groups of cellulose, hemicellulose, and lignin. The crystallinity index (43.52%) and crystal size (4.03 nm) of the extracted PRFs were determined using X-ray diffraction (XRD) analysis.


Introduction
Synthetic fibers, such as glass and carbon, have been used in composite preparation for a few decades even though several environmental issues are associated with them (Ranakoti, Rakesh, and Gangil components linked to the fiber bundle, resulting in detached cellulosic fibers. Clostridium, an anaerobic bacterium found in lakes, rivers, and ponds, is the primary source of this bacterial and fungal fermentation in the water retting method (Akin et al. 2004;Lee et al. 2020). Few studies have reported the use of raw PRNs in composite fabrication. However, the extraction of the PRFs is not reported till date as per the author's knowledge. Extracting high-quality PRFs with better cellulose content is still challenging.
Therefore, the present research focused on extracting PRFs using the water retting process. After water retting, the fibers were separated using the traditional manual combing process. Further, the investigations were carried out on the extracted fibers' physical, chemical, morphological, and thermal properties and their comparison with the other plant-based natural fibers. The study was further extended to investigate the tensile strength of the single extracted fiber.

PRFs extraction
The Pinus Roxburghii tree, also known as the Chir or Himalayan long-leaf tree, typically grows up to 40-50 m in height. The PRNs used for this work were collected from Srinagar, Uttarakhand, India, in August 2021 (Figure 1(a-b)). The leaf part of the Pinus Roxburghii tree is generally known as the Pinus Roxburghii needle because of its needlelike structure. PRNs have an average length of 22 cm and a diameter of approximately 1.2 mm (Figure 1(c)). The top and bottom parts of the PRNs were trimmed to make them a uniform length of ~20 cm, as shown in Figure 1(d). The PRNs were soaked in water for 4 weeks at room temperature for microbial degradation, as shown in Figure 1(e). Once the soaking period (4 weeks) is completed, the fibers are extracted by a traditional manual combing process on retted needles with the help of a wired brush (Manimaran et al. 2016). PRFs were washed with demineralized water several times during extraction to efficiently remove unwanted materials Figure 1. Extraction process. Image of (a) Pinus Roxburghii tree (b) needle with stem (c) before fiber extraction (d) after trimming the top and the bottom portion, (e) soaked in water, (f) extracted fiber (wet stage) and (g) extracted Pinus Roxburghii fiber (dry stage). from the fiber surface (Figure 1(f)). The washed PRFs were dried under sun exposure for 7 days. The final extracted PRFs are shown in Figure 1(g).

Measurement of average diameter
The extracted PRFs are irregular in shape and longitudinally non-uniform, similar to the hemp fiber used by Kabir et al. (2013). Therefore, it is difficult to measure the exact diameter of the extracted PRFs and, hence, an average diameter is reported in this study. Four observations were taken at random lengths in the longitudinal direction for each sample to measure the average diameter, using a Radical RSMr-3 optical microscope, as shown in Figure 2. The same procedure was adopted for 30 samples of extracted PRFs, and the average value of these observed data is reported as the diameter of PRFs.

Measurement of density
Density was measured using the pycnometer setup, a standard method for determining the density of any substance (Indran and Raj 2015). In this study, toluene was used as a reference liquid having a density (ρ toluene ) of 866 kg/m 3 at 25°C.

Analysis of chemical composition
Numerous approaches have been used to determine the percentage of components in plant-based natural fibers. The present study determines % of cellulose, hemicellulose, lignin, wax, ash, and moisture. The cellulose present in the extracted PRFs was calculated using Kurschner and Hoffer's techniques. The hemicellulose constituent was determined using the NFT standard 120-008 . For lignin content evaluation, the Klason technique was used (Manimaran et al. 2018). The Conrad technique was used to calculate the wax content in the extracted PRFs (Indran, Raj, and Sreenivasan 2014). The ASTM D2654-67 and E1755-01 standards were used to estimate the moisture and ash content, respectively (Krishna, Kailasanathan, and NagarajaGanesh 2020).

Fourier-Transform Infrared Spectroscopy (FTIR) analysis
The functional group components present in the extracted PRFs were studied using an infrared spectrum obtained from FTIR equipment (Perkin Elmer/Spectrum-2). The spectrum was recorded in the wavenumber range from 4000 to 400 cm −1 with 32 scans/minute and a resolution of 2 cm −1 (Moshi et al. 2020). The crushed and oven-dried extracted PRFs (~2 mg), mixed with potassium bromide (200 mg), were used to capture the absorbance spectrum (Vinod et al. 2021).

X-ray Diffraction (XRD) analysis
The Bruker D8 equipment was used for XRD analysis with the CuKα radiation at a wavelength of 1.54 Å. During the analysis, the diffractometer was set to 40 kV and 15 mA. The powdered samples of the extracted PRFs were scanned from 10° to 80° (a step of 0.02°) with a 10°/min scanning speed (Manimaran et al. 2016). Equation 1 is used to evaluate the PRFs' crystallinity index (CI) (Segal et al. 1959).
Where, I amorphous and I crystalline are the intensities of the peaks associated with the amorphous and crystalline phases, respectively. The crystallite size (CS) is found using Equation (2) (Manimaran et al. 2018). The value for Scherer's constant (K) was taken as 0.89.

CS ¼ Kλ βcosθ
(2) Where θ is Bragg's angle, β is full width at the half-maximum and λ is the wavelength.

Thermal analysis
Thermal analysis of natural fibers is essential in deciding the maximum temperature limits during composite fabrication (Moshi et al. 2020). TGA was accomplished by utilizing EXSTAR thermal analyzer (SII 6300 model). The differential thermogravimetric (DTG) analysis is also done using the same equipment as TGA. A total of 10 mg crushed extracted PRFs were filled in an alumina crucible before keeping them in the TG analyzer machine. This process was accomplished in a Nitrogen atmosphere (100 ml/min flow rate) at a temperature range from 35°C to 600°C (Vijay et al. 2019).

Morphological study
The morphology of the PRFs was examined using NOVA Nanosem 450 FESEM equipment, which was operated at 20 kV accelerated voltage for surface analysis and 10 kV accelerated voltage for cross-section analysis. The extracted PRFs surface was gold-coated to provide a conducting surface for microscopic inspection (Manimaran et al. 2018). Energy Dispersive X-ray Spectroscopy (EDX) of type Apreo EDX APEX-Octane Elite Plus attached with the FESEM was used to examine the presence of various elements. The distribution of the elements was recorded at three distinct locations of the fiber, and its average value is considered ). In addition, the surface roughness profile of the PRFs was measured using atomic force microscopy (AFM) with the Ntegra (NT-MDT, Russia) machine and Nova (NT-MDT, Russia) image processing software having a 3-50 nm tip radius of curvature.

Single fiber tensile test
The mechanical properties such as tensile strength, Young's modulus, and elongation-at-break of single extracted PRFs were determined using Zwick/Roell INSTRON 5982 universal testing equipment (Md et al. 2020). A single PRF sample of 65 mm length and 25 mm of gauge length was prepared according to ASTM D3822-07. During the experiment, the sample of single extracted PRFs was put through at a constant speed of 18 mm/min while under a pre-load of 0.1 N and at 23°C with 65% relative humidity (Krishna, Kailasanathan, and NagarajaGanesh 2020). The same procedure was repeated for 30 samples, and the average tensile strength was noted. Young's modulus and elongation at break were also measured.

Average diameter and density measurement
Diameter calculation in PRFs is complex as fibers have irregular shapes and sizes throughout their length. However, it is assumed to have a circular shape for calculating mechanical properties (Amutha and Senthilkumar 2021). The average diameter of the extracted PRFs of 30 samples was measured as explained in section 2.2.1 and observed as 145 ± 17.36 µm. It is also essential to estimate the density of the extracted fiber. It is a decisive factor in checking the eligibility of fibers in fabricating lightweight composites. The density of the extracted PRFs was measured and estimated as 1020 ± 20.27 kg/m 3 , which is very close to other plant-based natural fibers, such as Acacia arabica bark (1028 kg/m 3 ), mulberry (1070 kg/m 3 ) and so on, as shown in Table 1. This shows that the extracted PRFs can be extensively used for lightweight applications.

Chemical composition of PRFs
Cellulose is the main component of natural fibers as it significantly impacts their mechanical properties, whereas others act as a matrix or binder (Moshi et al. 2020). The chemical composition study reveals that the extracted PRFs have a cellulose content of 58.30%, which is comparable to various other plant-based natural fibers, such as Pigeon pea (55.03%), Artisdita hystrix (59.54%), and Cardiospermum halicacabum (59.82%), as mentioned in Table 2. The higher proportion of hemicellulose is unfavorable for fiber strength due to the disintegration of cellulose microfibrils . The extracted PRFs have 19 wt% of hemicellulose, which is closest to already known fibers such as Heteropogon Contortus (19.34%), Phoenix dactylifera L. (leaf sheath) (24%) and Seagrass (38%), as shown in Table 2. High lignin content could affect the fiber-matrix compatibility resulting in lesser mechanical properties and weak bonding with matrix materials (Porras, Maranon, and Ashcroft 2015). The extracted PRFs achieved optimum lignin content (20.2%) which is comparable to previously extracted plant-based natural fibers (Table 2). If a natural fiber has a lower weight percentage of wax content, the bond between the matrix and the fiber will be stronger while fabricating composite structures. The extracted PRFs have 0.94 wt% of wax content, which is lower than the wax contents of Cardiospermum halicacabum (1.3 wt%), flax (1.5 wt%), and Pigeon pea (2.38 wt%). Furthermore, the results revealed that the extracted PRFs have 9.48% moisture and 1.60% ash.

FTIR spectroscopic analysis
Figure 3(a) shows the FTIR spectrum of the extracted PRFs comprising the primary (4000-1500 cm −1 ) and fingerprint areas (1500-400 cm −1 ). The prominent peak at 3419 cm −1 is the absorbance owing to hydrogen-bonded OH (alcohol group) stretching vibrations, indicating the presence of cellulose molecules. This result agrees with the observations made by Kathiresan et al. (2016). The second sharp peak at 2923 cm −1 resulted from C-H stretching vibrations, confirming the existence of cellulose and hemicellulose (Vijay et al. 2019). However, the absorbance at 1630 cm −1 is related to the OH bending of moisture, indicating water retention in PRFs (Hyness et al. 2018). The absorbance peaks at 1512 cm −1 , 1406 cm −1 , and 1383 cm −1 are owing to C = C stretching, C -H bending and -C=O bending in the aromatic ring of the lignin and hemicellulose (Indran, Raj, and Sreenivasan 2014;Kulandaivel, Muralikannan, and Kalyana Sundaram 2018). The peak at 1160 cm −1 is cellulose's C-O-C asymmetric stretching vibration's absorbance (Saravanakumar et al. 2014). The peak at 1033  Vijay et al. (2019) cm −1 results from symmetric C-O stretching (lignin) and 617 cm −1 is associated with C-OH bending in cellulose (Kulandaivel, Muralikannan, and Kalyana Sundaram 2018).   cellulose in the PRFs, respectively. These two peaks also show the presence of cellulose I, having a monoclinic crystal structure. A similar result was also reported by Raja et al. (2021) with Cymbopogon flexuosus fiber. The CI of PRFs was found to be 43.52%, which is comparable to various other plant-based natural fibers like Ficus religiosa fiber (42.92%), Phaseolus vulgaris (43.01%), Artisdita hystrix leaf (44.85%), etc. as shown in Table 1. PRFs were found to have a CS value of 4.03 nm, which is lower than that of Ficus religiosa fiber (5.18 nm) and Albizia julibrissin (6.79 nm).

TGA and DTG analysis
The TGA curve of the PRFs shows a three-step degradation behavior, as shown in Figure 4. The water evaporation from PRFs results in initial mass loss from 35°C to 120°C (9.39%) whose maximum loss is at 67.46°C, as shown in Figure 4 (Babu et al. 2022). The second stage of degradation is the most considerable mass loss of 50.42% due to the cellulose and hemicellulose degradation present inside the PRFs. This change occurs from 225°C to 376°C, whose maximum weight loss is at 355.84°C, as shown in the DTG curve (Jeyabalaji et al. 2021). The mass loss of 14.90% after 376°C is due to the degradation of fiber's lignin and wax content (Moshi et al. 2020).

Morphological analysis
Figure 5 reveals the morphology of the extracted PRFs obtained from FESEM. The void space present in the PRFs confirms their hydrophilic nature, as shown in Figure 5(a). The wax and lignin present on the PRFs surface make it shiny, as shown in Figure 5(b). The extracted PRF was observed to be nearly circular in shape, as shown in Figure 5(c). The PRFs' surface has a parallel set of micro-fibrils, causing roughness along the width of the fiber. This enhances the mechanical interlocking between the fiber and matrix during the composite fabrication (Madhu et al. 2019). Figure 6(a-b)shows the 2D and 3D AFM images of PRFs, respectively. This analysis is done to understand the topological behavior of PRFs. The PRFs have an average roughness (R a ) of 118.473 nm, which is higher than the Senna Auriculata fiber (R a = 75.99 nm) (Nagaraja et al. 2019) and Eichhornia Crassipes fibers (R a = 101.84 nm) (Palai, Sarangi, and Mohapatra 2021). However, it is lower than Cornhusk fiber having R a = 189.74 nm (Kambli et al. 2016). The higher Ra value indicates less impurity present on the fiber's surface. The 10-point height and root mean square (RMS) values of PRFs are 899.40 nm and 144.18 nm, respectively, while the peak-to-peak height value is 1514.52 nm. The ratio of RMS and R a values for PRFs is found to be 1.22, similar to the values obtained for Areca palm leaf (1.19) (Shanmugasundaram, Rajendran, and  Ramkumar 2018) and Acalypha Indica L (1.25) (Jeyabalaji et al. 2021). This property makes PRFs suitable for tribological applications as they are close to their acceptance value of 1.25 to 1.33 (Krishna, Kailasanathan, and NagarajaGanesh 2020).
Surface kurtosis is another crucial characteristic that shows how widely the spikes are spread concerning the mean position. The surface kurtosis of PRFs is −0.38. The lower kurtosis value  indicates that the roughness of the PRFs is sufficient for the proper mechanical interlocking during the composite fabrication (Jeyabalaji et al. 2021).

EDX analysis
EDX attached with FESEM is used for the elemental analysis of PRFs. The EDX spectrometric graph shows the presence of carbon (C), oxygen (O), and calcium (Ca) in the fiber. The EDX spectrum validates the existence of the main constituents of carbon and oxygen in PRFs, whose weight percentage was 65.20% and 34.40%, respectively, as shown in Figure 7. The impurity present is much less in PRFs as only 0.4 wt% of Ca over fiber's surface. A similar result was reported by Md et al. (2020).

Single fiber tensile test
It was observed from the SEM images ( Figure 5(c)) that the cross-section area of the extracted single PRFs is nearly circular. Therefore, the tensile strength of the fiber is calculated considering that the cross-section of the fiber is circular (Amutha and Senthilkumar 2021;Fiore, Scalici, and Valenza 2014;Kabir et al. 2013). An aggregate of 30 extracted PRFs samples were tested for tensile strength, out of which the maximum strength of 99.17 MPa was obtained. The average tensile strength of 30 samples was obtained as 51.48 ± 19.75 MPa with 3.6% elongation-at-break. The value of average tensile strength is close to that of other plant-based natural fibers such as Tridax procumbens (25.75 ± 2.45 MPa), Manicaria saccifera palm (72.09 ± 15.73 MPa), and so on, as shown in Table 1. The tensile strength of the extracted PRFs is comparable with other natural plant-based fibers, which can be further improved with surface modifications. Stress-strain curves for all 30 specimens are shown in Figure 8. The extracted PRFs have an average Young's modulus of 1.44 ± 0.67 GPa, which is near to Tridax procumbens fiber (0.94 ± 0.09 GPa), Artisdita hystrix (1.57 ± 0.04 GPa), and Pigeon pea (2.1 GPa) (Table 1).

Conclusions
The present study reveals that fiber can be extracted from the naturally available Pinus Roxburghii needle in the Himalayan region using the water retting method and traditional combing process. The extracted PRFs have shown excellent physico-chemical, mechanical, thermal, and morphological properties; therefore, they may extensively be used for making biocomposites. Based on the findings, the following conclusions are made: • The density of extracted PRFs (1020 ± 20.27 kg/m 3 ) is much less than that of synthetic fibers, making them suitable for lightweight composite fabrication. • The EDX analysis indicated that the main elements of extracted PRFs were carbon and oxygen, confirming the fiber to be organic. • The chemical composition analysis shows that the extracted PRFs are cellulosic in nature and have nearly 58.30% of cellulose content. • The crystallinity index and crystal size of the extracted PRFs were obtained as 43.52% and 4.03 nm, respectively. • The FESEM image reveals that the cross-section of the extracted PRF is nearly circular and the nature of the fiber is hydrophilic. • The mechanical properties of the extracted PRFs such as average single fiber tensile strength, Young's modulus, and elongation-at-break were obtained as 51.48 ± 19.75 MPa, 1.44 ± 0.67 GPa, and 3.6%, respectively.