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.

摘 要

合成纤维的不利环境影响迫使研究人员在复合材料制备中使用天然纤维. 在印度北阿坎德邦的丘陵地区，刺针（PRN）随处可见. 利用从PRN中提取的纤维作为生物复合材料制备中的增强材料，可以减少合成纤维，并强调利用自然资源. 本研究采用水浸法从刺梨松中提取刺梨松纤维. 对它们的物理、化学、热和形态进行了研究和报道. 提取的PRF的平均直径和密度分别为145 ± 17.36 µm和1020 ± 20.27 kg/m3. 化学分析表明，提取的PRF含有58.30%的纤维素含量. 使用热重分析（TGA）评估提取的PRF的热稳定性和最大降解温度，发现分别为225°C和355.84°C. 利用场发射扫描电子显微镜（FESEM）和原子力显微镜（AFM）分别研究了PRF的表面形貌和表面粗糙度. 此外，提取的PRF的FTIR分析证实了纤维素、半纤维素和木质素的不同官能团的存在. 使用X射线衍射（XRD）分析测定提取的PRF的结晶度指数（43.52%）和晶体尺寸（4.03 nm）.

KEYWORDS:

• Pinus Roxburghii Fiber
• chemical composition
• TGA
• AFM
• FTIR
• XRD

关键词:

• 关键词
• 刺梨松纤维
• 化学成分
• TGA
• AFM
• FTIR
• XRD

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 2021). Alternatively, natural fibers have no impact on the environment as they are renewable and biodegradable resources (Kulandaivel, Muralikannan, and Kalyana Sundaram 2018). The environment-friendly nature of natural fiber has resulted in its enormous demand in composite fabrication. In addition, natural fibers have more advantages, such as lightweight, low density, ease in processing, non-abrasive, non-hazardous, and mainly low cost compared to synthetic fibers (Shanmugasundaram and Rajendran 2016). Natural fibers can be obtained from various sources, including plants, minerals, and animals. Asbestos or basalt is the primary component of mineral-based natural fibers, whereas protein is the primary component of animal-based natural fibers. Asbestos is currently regarded as hazardous to human health, and its use has been banned in most countries, whereas animal-origin fibers are becoming less available and more expensive (Pickering, Efendy, and Le 2016; Vinod et al. 2021).

Plant selection for natural fiber is essential for fabricating high-strength composites as the fiber’s property depends on the cellulose content, cross-section, and chemical composition, which varies from plant to plant (Faruk et al. 2012). Plant-based cellulosic fibers such as bamboo, cotton, flax, jute, sisal, kenaf and ramie fibers, have been used in composite preparation due to their excellent mechanical properties and biodegradability (Pickering, Efendy, and Le 2016; Tahir et al. 2011). However, bamboo takes more than three to four years to achieve its full height and strength, whereas flax, hemp, sisal, jute, and cotton fiber need proper harvesting, resulting in enhanced cost (Lee et al. 2020; Ramesh 2018). However, the mechanical strength of kenaf fiber-based composites is less because of the poor compatibility of kenaf fiber with the polymer matrix (Radzuan et al. 2019). Moreover, these fibers have the issue of their maturity time and cost. Therefore, the need of the hour is to find readily available and low-cost plant-based materials, which can be used to fabricate cost-effective polymer composites.

Pinus Roxburghii Needles (PRNs) are abundant in India’s Himalayan areas, which continuously grow and shed from the Pinus Roxburghii tree. Every summer, PRNs catch fire in the forest, resulting in a massive loss of flora and fauna and eventually causing environmental pollution and energy loss (Thakur, Singha, and Thakur 2013). As per the literature, the annual gross availability of PRNs in Uttarakhand, India, is 1.37 to 2.37 million tonnes (Kala and Subbarao 2018). However, utilizing these PRNs as fibers would be beneficial in making an environment-friendly composite from readily available needles, reducing the massive forest loss.

Singha and Thakur (2008) fabricated pine needles reinforced resorcinol formaldehyde composite and revealed that the pine needles reinforced composite’s mechanical properties were better than the matrix alone. Kumar et al. (2022) used PRNs leaves in asbestos-free brake friction material by sun drying and alkali treatment. The composition of 10% PRNs has shown excellent mechanical properties for the brake material. Gairola et al. (2019) prepared the hybrid composite reinforcing PRNs and pistachio shells in the epoxy matrix. The impact strength of these hybrid composites was found to be 22.33 kJ/m². Thus, PRNs are an excellent option for cellulosic reinforcement in composite fabrication. Besides the composite preparation, pine needles in various forms have different applications such as food packing, bedding material for domestic animals, energy feedstock, etc. (Kadam, Singh, and Gaikwad 2021; Kala and Subbarao 2018). Various research has been done on utilizing raw PRNs as reinforcement in composite fabrication. However, these needles have various impurities and contain a high amount of lignin, which may reduce the strength of the composite (Faruk et al. 2012). Therefore, research was extended to extract the fibers from the PRNs to fabricate the composites.

Various extraction methods, such as mechanical, chemical, water retting, and steam explosions, have been used to extract plant-based natural fibers (Md et al. 2020). All these methods have their benefits and drawbacks. Tahir et al. (2011) compared the tensile strength of the Kenaf fiber extracted using mechanical and chemical retting methods. They reported that the chemical retting was much cleaner, but tensile properties were poor. The traditional approach to extract the natural fiber is water retting. The water retting method is environmentally friendly and cost-effective. This method requires two to four weeks to break the lignin of the fiber. The water retting method uses anaerobic bacteria fermentation and fungal colonization on fiber bundles to create enzymes that hydrolyze fiber-binding components. Retting is a biological process in which enzymatic activity removes non-cellulosic 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.

Materials and method

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 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).

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).

Analysis of physical properties

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.

Figure 2. Optical microscopic image of a single extracted PRF showing diameter measurement.

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/m³ 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 (Raja et al. 2021). 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).

(1) $\mathit{CI}=\left\{1-\left(\frac{{\mathit{I}}_{\mathit{amorphus}}}{I_{\mathit{crystalline}}}\right)\right\}$(1)

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)(2) $\mathit{CS}=\frac{\mathit{K\lambda }}{\mathit{\beta cos\theta }}$(2) (Manimaran et al. 2018). The value for Scherer’s constant (K) was taken as 0.89.

(2) $\mathit{CS}=\frac{\mathit{K\lambda }}{\mathit{\beta cos\theta }}$(2)

Where $\mathit{\theta }$ is Bragg’s angle, $\mathit{\beta }$ is full width at the half-maximum and $\mathit{\lambda }$ 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 (Raja et al. 2021). 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.

Results and discussion

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/m³, which is very close to other plant-based natural fibers, such as Acacia arabica bark (1028 kg/m³), mulberry (1070 kg/m³) and so on, as shown in Table 1. This shows that the extracted PRFs can be extensively used for lightweight applications.

Table 1. Comparison of various properties of PRFs with other natural fibers.

Fiber	Diameter (µm)	Density (kg/m^3)	Tensile Strength (MPa)	Young’s Modulus in tension (GPa)	% Elongation at break (Tensile loading)	Crystallinity index (%)	Crystalline size (nm)	References
Pinus Roxburghii fiber	145 ± 17.36	1020 ± 20.27	51.48 ± 19.75	1.44 ± 0.67	3.6	43.52	4.03	Present study
Acacia arabica bark	-	1028	-	-	-	51.72	15	Manimaran et al. (2016)
Albizia julibrissin	162.01	1331	483.40 ± 18	6.76 ± 0.49	9.35 ± 0.78	27.77	6.79	Md et al. (2020)
Artisdita hystrix leaf	-	540	440 ± 13.4	1.57 ± 0.04	-	44.85	-	Kathiresan et al. (2016)
Arundo donax	-	1168	248	9.4	3.24	-	-	Fiore, Scalici, and Valenza (2014)
Cardiospermum halicababum	315.4	1141	20.7	-	-	32.21	29.77	Vinod et al. (2021)
Cissus quadrangularis (stem)	770 – 870	1220	2300 – 5479	56 – 234	3.75–11.14	47.15	31.55	Indran and Raj (2015)
Cymbopogon flexuosus	118	1270	431.19 ± 23.96	53.889 ± 17.17	8 ± 0.1	46.02	13.96	Raja et al. (2021)
Elephant grass	70 – 400	817.53	185	7.4	2.50	-	-	Rao et al. (2007)
Ficus religiosa fiber	25.62	1246	433.32 ± 44	5.42 ± 2.6	8.74 ± 1.8	42.92	5.18	Moshi et al. (2020)
Flax	-	1500	345 - 1035	27.6	2.7–3.2	-	-	Bledzki and Gassan (1999)
Heteropogon contortus	-	602	476 ± 11.6	48 ± 2.8	1.63 ± 0.06	-	-	Hyness et al. (2018)
Jute	-	1300	393 – 773	26	1.5–1.8	-	-	Bledzki and Gassan (1999)
Manicaria saccifera palm	-	840–1070	72.09 ± 15.73	2.20 ± 0.44	-	-	-	Porras, Maranon, and Ashcroft (2015)
Mulberry	255–480	1070	463.84 ± 32.69	10.45	4.73 ± 1.06	-	-	Shanmugasundaram and Rajendran (2016)
Phaseolus vulgaris	352	934	436 ± 10.1	8.98 ± 1.237	1.48 ± 0.03	43.01	4.07	Babu et al. (2022)
Pigeon pea	-	1738	131	2.1	2.31	65.89	11.6	Kulandaivel, Muralikannan, and Kalyana Sundaram (2018)
Red banana peduncle	150–250	990	440 ± 13.4	12.41 ± 5	1.57 ± 0.04	62.1	12.1	Manimaran et al. (2018)
Sansevieria roxb. leaves	106.33	950	213.6	16.02	1.12	95.55	5.25	Krishna, Kailasanathan, and NagarajaGanesh (2020)
Sisal	50–300	1450	227 – 700	9–20	3-14	-	-	Rao et al. (2007)
Tridax procumbens	233.1 ± 9.9	1160 ± 120	25.75 ± 2.45	0.94 ± 0.09	2.77 ± 0.27	34.46	25.04	Vijay et al. (2019)

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 (Raja et al. 2021). 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.

Table 2. Percentage of chemical constituents present in PRFs and other natural fibers.

Fiber	Weight % of						References
	Cellulose	Hemicellulose	Lignin	Wax	Moisture	Ash
Pinus Roxburghii fiber	58.3	19.0	20.2	0.94	9.48	1.60	This Study
Acacia arabica bark	68.10	9.36	16.86	0.49	-	-	Manimaran et al. (2016)
Albizia julibrissin	55.83	10.74	16.03	0.53	8.44	10.84	Md et al. (2020)
Areca palm leaf stalk	57.49 ± 0.66	18.34 ± 0.24	7.26 ± 0.12	0.71 ± 0.024	9.35 ± 0.15	-	Shanmugasundaram, Rajendran, and Ramkumar (2018)
Artisdita hystrix leaf	59.54	11.35	8.42	-	-	-	Kathiresan et al. (2016)
Cardiospermum halicababum	59.82	16.75	9.3	1.3	1.9	4.5	Vinod et al. (2021)
Cissus quadrangularis root	77.17	11.02	10.45	.014	7.3	-	Indran, Raj, and Sreenivasan (2014)
Cissus quadrangularis stem	82.73	7.96	11.27	0.18	6.6	-	Indran and Raj (2015)
Cymbopogon flexuosus	68.13	11.56	23.9	0.53	9.17	4.81	Raja et al. (2021)
Ficus religiosa fiber	55.58	13.86	10.13	0.72	9.33	4.86	Moshi et al. (2020)
Flax	64.1	16.7	2.0	1.5	-	-	Bledzki and Gassan (1999)
Heteropogon contortus	64.87	19.34	13.56	0.22	7.4	-	Hyness et al. (2018)
Jute	64.4	12	11.8	0.5	-	-	Bledzki and Gassan (1999)
Kenaf	45–57	8-13	21.5	0.8	6-12	-	Indran and Raj (2015)
Manicaria saccifera palm	74.1	12.0	31.1	-	-	-	Porras, Maranon, and Ashcroft (2015)
Mulberry	75.06	11.27	5.22	0.86	4.25	1.87	Shanmugasundaram and Rajendran (2016)
Phoenix dactylifera L. (leaf sheath)	43.50	24	18	-	6.8	7.73	Alotaibi et al. (2019)
Phoenix dactylifera L. (stalk)	44	26	11.45	-	9.6	1.85	Alotaibi et al. (2019)
Phoenix dactylifera L. (trunk)	35	15.40	20.13	-	15.6	12.6	Alotaibi et al. (2019)
Pigeon pea	55.03	-	18.32	2.38	8.13	4.67	Kulandaivel, Muralikannan, and Kalyana Sundaram (2018)
Sansevieria roxburghiana	78.63	2.01	9.86	0.16	7.85	1.49	Krishna, Kailasanathan, and NagarajaGanesh (2020)
Sea grass	57	38	5	-	-	-	Davies et al. (2007)
Tridax procumbens	32	6.8	3	0.71	11.2	-	Vijay et al. (2019)

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 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).

Figure 3. (a) FTIR spectra of PRFs (b) XRD diffractograms of PRFs.

XRD analysis

Figure 3(b)reveals the diffractogram of extracted PRFs, showing the two prominent peaks at 15.08° and 22.26°, representing the amorphous (110) and crystalline parts (200) of hemicellulose, lignin, and 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).

Figure 4. TG and DTG analysis curve for PRFs.

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).

Figure 5. FESEM images of the PRFs (a) Showing void space (b) Showing wax content (c) Circular cross-section.

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).

Figure 6. AFM analysis of PRFs: (a) 2D image and (b) 3D image.

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).

Figure 7. EDX analysis of PRFs.

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).

Figure 8. Stress-strain diagram of PRFs.

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/m³) 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.

Highlights

• Novel Pinus Roxburghii needle fibers were extracted by water retting process.

• The physical and chemical composition and mechanical behavior of the extracted Pinus Roxburghii needle fibers were investigated.

• The extracted Pinus Roxburghii needle fibers may be a potential reinforcement in the development of polymer composites

Acknowledgement

The authors gratefully acknowledge the Institute Instrument Center, IIT Roorkee; and South India Textile Research Association, Tamilnadu, for providing support for this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work is financially supported by Design Innovation Center (Grant number 15-9/2017-PN-1), Ministry of Education, Government of India, for providing support for this work.