Characterization of Elementary Industrial Hemp (Cannabis Sativa L.) Fiber and Its Fabric

ABSTRACT In recent years, man-made fibers have been largely replaced by natural fibers as a reinforcement in polymer composites. Hemp fiber has the ability to be used as reinforcement in polymer composites due to its biodegradability, abundance, higher fiber production per acre, rapid growth rate, and high strength-to-weight ratio. Before reinforcing any fiber/fabric into the matrix to form composites, its characteristics should be investigated. The quality of the fiber has a significant impact on the properties of fiber reinforced composites. The fabric’s strength has a significant effect on the performance of a composite because it bears majority of the load. This paper describes modern and systematic methods for examining the physical and mechanical properties of industrial hemp fiber and its fabric. The hemp fiber is characterized in this study based on its constituents. Additionally, thermal stability, crystallinity, elemental analysis, and Fourier transform infrared spectroscopy are determined. The study discovered that the fiber had a high crystallinity value, a higher tensile modulus, and a breaking force comparable to some synthetic fibers. Because of the properties obtained, hemp fiber and fabric are suitable alternatives for use as reinforcing materials in polymer composites.


Introduction
Over the years, researchers have used man-made fibers with a long life that end up in landfills after use. This is a significant issue for the environment. There is a demand for biodegradable and renewable materials that decrease environmental effects without sacrificing required material properties. This resulted in the development of composites made of natural fibers. Natural fibers offer high specific strength and stiffness, relatively low density, low cost, and produce minimal pollution during processing and production, resulting in very few health concerns and hence a smaller carbon and ecological imprint (Barth and Carus 2015;Cherrett et al. 2005;Pickering, Aruan Efendy, and Le 2016). Natural fibers are used in a wide variety of industries, including furniture, automotive, aviation, construction, by species, region, soil, temperature, rainfall, and harvest time (Adesina et al. 2020;Liu et al. 2015). There are over 2,000 hemp varieties in nature, according to the reports (Chen et al. 2021). The current study focuses on the characterization of Cannabis Sativa L. hemp fibers grown in India, so that based on the properties of hemp fiber under the present study, the desired application can be recommended. An investigation has been done on the physical and mechanical performance of industrial hemp fibers and their fabric (unidirectional and bidirectional) through systematic and improved methodologies.

Material
The hemp (Cannabis Sativa L.) used in the present work was obtained from a local trader in three different forms: raw fiber, unidirectional (UD) fabric, and plain weave bidirectional (BD) fabric. Both the UD and BD mats had a weight of 300 g/m 2 .

Fiber extraction
The process of hemp fiber extraction is shown in Figure 1. The hemp plant is seen in Figure 1(a) (Barbetorte 2009). Hemp is harvested as raw fiber bundles containing contaminants such as woody stems, as seen in Figure 1(b). Scutching was used to remove these contaminants from the hemp fiber. The hemp stalks were hammered with sticks during this procedure (Figure 1(c)), separating the woody stems from the fibers. Hemp fibers were dragged through heckling combs (Figure 1(f)), which divide the locked strands and create single, long, and clean fibers, as shown in Figure 1(d). Figure 1(e) depicts the fibers generated after these two methods. They were placed in a 70°C oven to eliminate excess moisture from the fibers.

Physical property test
The density of hemp fiber was calculated according to two different standards, i.e., ASTM D3800 and ASTM D1577. The first method uses Archimedes' principle, where the difference of material weights in air and liquid medium is used to determine the density. For the current study, acetone is used as the liquid medium instead of distilled water, as the density of acetone is less than the density of water and it was found that the fibers were semi submerged in the distilled water. The second method uses linear density approach to determine the density. Initially, the diameter of fiber was determined using an optical microscope (Leica, DM2500 M) and scanning electron microscope (SEM) (JEOL JSM-6480 LV). The fiber of fixed length was cut and their weight was found out with the help of a weighing machine. Substituting the obtained weight results in Equation 1, gives the density of fiber.

SEM and EDX spectroscopy
SEM was used to study the surface roughness, imperfections, and overall geometry. Energy Dispersive X-Ray (EDX or EDS) is a technique to determine the elemental composition of a material. It works on the principle of interaction of X-ray with a material. Hemp fiber was made into fine particles, which were used for both these studies. Prior to the test, the particles were coated with platinum through a process called sputtering. These analyses were carried out on JEOL JSM-6480 LV SEM with a resolution of 3 nm and magnification of 8×-300000× fitted with EDX detector of Oxford data reference system.

FTIR analysis
Fourier Transform Infrared (FTIR) spectroscopy helps in providing the knowledge about the absence or presence of specific functional groups and molecular fragments. It was used to examine the composition of hemp fiber which uses the standard KBr pellet technique and a spectrometer. FTIR measurements were carried out on Shimadzu IR Prestige-21 with a wavelength range of 400 cm −1 to 4700 cm −1 . The sample was tested with a resolution of 2 cm −1 . The fibers were made into small particles and then ground along with KBr powder. This mixture was then made into a pellet with the help of a manual press and die. FTIR can also be used to predict the crystallinity index (CI) of hemp fiber. It is found that variations in cellulose crystallinity can cause deviations in intensities of some IR spectra bands. The baseline corrected absorption peaks at 898 cm −1 to 1435 cm −1 correspond to the sensitive frequencies of amorphous and crystalline cellulose structures, respectively. Relative cellulose crystallinity can be measured using the intensities of both these peaks. CI FTIR is calculated using Equation 2 (Fan, Dai, and Huang 2012). (2)

Crystallinity with XRD
The crystallinity of hemp fiber was determined with the help of X-ray diffraction (XRD) technique. A Bruker AXS D8 Advance X-Ray diffractor, employing CoKα (λ = 1.78901) radiation. The diffraction intensity used was in the range of 5° to 79.5°of 2θ (Bragg angle) with a scanning speed of 5°/min. CI XRD was calculated using Segal empirical equation (Segal et al. 1959) represented in Equation 3.
where I 002 is the maximum intensity of the 002 lattice diffraction plane at a 2θ angle between 22° and 23° which represents both amorphous and crystalline materials. The intensity of diffraction of amorphous materials in cellulosic fibers is represented by I am which is taken at a 2θ angle between 18°and 19° where the intensity is at a minimum. Peak deconvolution method was also used to determine the crystallinity index of the sample. In this method, the crystalline and amorphous peaks were separated from the diffraction curve using a curvefitting process. Gaussian function was used to deconvolute the XRD curve. The ratio of area of all the crystalline peaks to the total area gives the crystallinity of the fiber.

Mechanical property test for fiber
The tensile modulus of hemp fiber was determined in accordance with ASTM C1557-20. The single fiber used in testing was extracted randomly from the bundle with sufficient length to allow for convenient gripping and handling. For easy facilitation of gripping of fiber by the machine, a mounting tab was used as illustrated in Figure 2(a). The fiber to be tested was placed in the middle of the mounting tab, and the end of the fiber was bonded to the mounting tab using an epoxy adhesive. Three different gage lengths were considered for testing, namely, 25.4 (illustrated in Figure 2(b)), 50.8 and 76.2 mm. The fabricated sample was carefully placed inside the grippers of INSTRON 8862, which has a maximum load capacity of 100kN fitted with a Dynacell™ fatigue-rated load cell. After the sample was held tightly between the grippers, the middle-gage section of the mounting tab is cut away (as shown in Figure 2(c)), so that the fiber would bear the load. The load was given at a rate of 0.05 mm/ min. A minimum of 3 samples for each gage length was tested.

Mechanical property test for fabric
A fabric's breaking strength is defined as the highest load it can withstand without tearing when subjected to uniaxial tensile loading. The breaking strength and elongation of the fabrics were determined by two distinct procedures, namely the grab test and the strip method, according to the method described in ASTM D5034-21 and ASTM D2035-11, respectively. For the grab test, a specimen of 150 mm long and 100 mm wide was cut from both fabrics, where fibers in the unidirectional fabric are aligned in the length direction parallel to the force application and testing direction. Test method G-E was used which denotes grab test carried out with a constant rate of extension. For strip test, a specimen of 150 mm long and 62 mm wide was cut from the fabrics. To achieve a testing width of 50 mm, equal number of yarns were raveled from each side. Test method 2 R-E was employed, which refers to 50 mm raveled strip test with a constant rate of extension. The sample was loaded into the machine such that the gauge length would be 75 mm. The loading rate was kept constant at 300 mm/min for both grab and strip test.

Thermal stability analysis
Differential scanning calorimetry (DSC) is an analytical technique to study the thermal stability and glass transition temperature (T g ) of a material. It calculates the heat flow rate difference between a sample and an inert reference as a function of temperature and time. DSC curves were obtained on a DSC 822E calorimeter (Mettler Toledo, US) in accordance with ASTM D3418. The fibers (~10 mg) were reduced to particles and placed inside an aluminum crucible. Dynamic scans were obtained within a nitrogen atmosphere at a heating rate of 10°C/min in the temperature range from 25°C to 250°C. For effective heat transfer and removal of volatiles, nitrogen was utilized. Using both SEM and optical microscope images, the diameter of fibers was measured on multiple samples from 56 distinct locations. The diameter distribution and the fitted curve of normal distribution are shown in Figure 3(c). The diameter of hemp fiber was observed to be around 106 µm to 142 µm, with an average diameter of 123.73 µm and a standard deviation of 7.82 µm. The average linear density of hemp fiber was found out to be 1.5545 × 10 −5 kg/m. Substituting diameter and linear density in Equation 1, the average density of hemp fiber was found to be 1308.425 ± 168.387 kg/m 3 . The average density determined using Archimedes' principle was found to be 1196.628 kg/m 3 .

SEM and EDS study
The surface of hemp fiber observed under SEM was found to be rough and clean. Fiber bundles and fibrils were arranged in the axis of fiber. Micro-graphs of hemp fiber are represented in Figure 4(a). EDS revealed the elemental analysis and composition of hemp fiber illustrated in Figure 4(b) and Table 1, respectively. Signals of C (carbon) and O (oxygen) indicate that the fiber is organic in nature. Ca (calcium) was also observed in the elemental analysis, which is essential for plant metabolism and various growth-related physiological processes. Traces of other elements like Al (aluminum), and Si (silicon), minute traces of Mg (magnesium), K (potassium), and Fe (iron) were observed that might have come from the soil. Figure 5 depicts the Fourier transform infrared spectra of hemp fiber. Table 2 lists the major functional groups that correlate to a given wavelength, as well as the sources of those functional groups. The table   W avenumb er (cm -1 )  shows that there are five major components in hemp fiber, which are cellulose, hemicellulose, lignin, pectin, and waxes, which can be separated into two categories. In the FTIR spectrum, a weak absorption was seen around 1527 cm −1 and 1246 cm −1 , which corresponds to C=O and G ring stretching and C=C aromatic symmetrical stretching, respectively, indicating the presence of lignin. After extraction, the absorbance around 1527 cm −1 correlates to the presence of water, which is represented by the OH bending that suggests the presence of water. This absorbance is in the range of 1729 cm −1 , which can be attributed to the presence of C=O (carbonyl) in the waxes and pectins studied. Changing the crystallinity of cellulose makes particular bands in the infrared spectrum more sensitive, which can be used to calculate the crystallinity index (CI FTIR ) of cellulose. The absorbance peak ratios at 1435 cm −1 and 898 cm −1 , 1381 cm −1 to 2920 cm1, 1381 cm −1 to 673 cm −1 , and 1381 cm −1 to 2920 cm −1 are the most commonly utilized to determine CI (Fan, Dai, and Huang 2012). The ratios of 1381 cm −1 to 2920 cm −1 and 1381 cm −1 to 673 cm −1 are too low in this current work, making them inappropriate for evaluation, whereas the ratio of 1435 cm −1 to 898 cm −1 is 64.4%. As a result, the ratio of 1435 cm −1 to 898 cm −1 is better appropriate for CI evaluation, as indicated by a value of 64.4% obtained from Equation 2.

Crystallinity of fiber
XRD photograph of hemp fiber is shown in Figure 6. The major crystalline peak observed at 2θ = 22.4° with an intensity of around 2700 in the x-ray diffractogram of hemp fiber corresponds to the cellulose crystallographic plane (002, Bragg reflection). At 2θ = 18.3°, a trough was observed between 002 and 110 peaks which corresponds to amorphous intensity (I am ). The CI XRD of hemp fiber is 85.25% which was calculated from Equation 3. High CI indicates better mechanical properties of hemp fiber. Other crystallographic planes observed in the diffractogram, namely ( � 110), (110), and (004) are obtained at well-defined peaks at 2θ = 12.95°, 2θ = 14.7° and 2θ = 36.1°. The deconvoluted crystalline and amorphous peaks can be seen in Figure 6. The deconvolution resulted in a curve that fit the original data with an R-square value of 0.992. The total area and area of all the crystalline peaks were determined, which produced a crystallinity of 49.37%. The huge difference in the crystallinity obtained by Segal's empirical and deconvolution methods can be attributed to the broad peaks of amorphous cellulose. These broad peaks lead to overlapping peaks which cannot be deconvoluted into narrow diffraction peaks leading to lower crystallinity values (Park et al. 2010). It was observed that there is a difference in the CI of hemp fiber measured through XRD and FTIR. The difference in cellulose crystallinity results can have many reasons. Even though the FTIR spectrum contains peaks corresponding to amorphous and crystalline regions, it is the most apt method for determining the crystallinity of natural fibers. There are several drawbacks which cannot be eliminated in the determination of crystallinity using XRD diffractogram in the case of natural fibers. Peak overlapping is destined to occur around 2θ = 18° for monoclinic cellulose due to the smaller size of cellulose crystallites, which gives lower or higher values of I am . Cellulose obtained from different species has different crystalline sizes and apparent cell parameters, which influence the crystalline peak I 002 , thereby affecting the relative intensities of peaks. Based on the factors and results obtained, for natural lignocellulosic fiber, the FTIR method is more suitable for CI evaluation.

Mechanical properties of fiber
The stress-strain graph of hemp fibers of various gage lengths are illustrated in Figure 7. It can be observed that the stress-strain graph of hemp fibers is linear in nature. It can be seen that there is a sudden decrease in the strain value, indicating the brittle nature of hemp fiber. The obtained tensile modulus, strength, and maximum strain values are reported in Table 3. It was observed that the average strain for the hemp fiber is 1.08% with a standard deviation of 0.31%. It can also be seen that the tensile strength of hemp fiber varies from 302 to 941 MPa with an average value of 640 MPa and a standard deviation of 205 MPa. The tensile modulus of hemp fiber was found to be varying from 46.726 GPa to 77.397 GPa, with a mean value of 59.053 GPa and a standard deviation of 8.448 GPa. The tests were conducted under the assumption that the diameter of each fiber was equal to the average diameter. This led to a higher standard deviation in the results. Therefore, it can be concluded that the hemp fiber reinforced composites will have high mechanical strength, which is comparable to glass fiber reinforced composites (Chokshi et al. 2022).

Breaking strength of fabric
Figure 8(a) and Figure 8(b) depicts the force vs. elongation curves for BD and UD fabric, respectively. In both cases, the strip method demonstrated more elongation before ripping but with a higher breaking force than the grab test. For a given force, the strip method produced more elongation in the case of BD fabric than the grab test. In the case of UD fabric, the grab test produced greater elongation than the strip method. The breaking force and apparent elongation at the breaking force of both fabrics for the grab test and strip method are shown in Figure 8(c) and Figure 8(d), respectively. The figures show that UD fabric needed more breaking force than BD fabric, whereas elongation at breaking force was greater for BD fabric than UD fabric. These fabrics have a breaking force comparable to glass fabric.

Differential scanning calorimetry analysis
DSC analysis was used to investigate the thermal behavior of hemp fiber. The obtained DSC curve of hemp fiber is presented in Figure 9. In the temperature range of 50-100°C, the DSC curve displayed an endothermic peak. The evaporation of free water in the intercellular region of the fiber may have caused this endothermic region where heat was absorbed. Typically, linked water attached to cellulose in fiber was difficult to remove than free water. To remove linked water, higher temperatures are  required which can be observed as a kink in the curve at approximately 100°C. The trough of the peak, which was observed at 62.16°C, can be the starting point where the material starts to degrade. This trough represents T g , where the phase transition of the fiber from a glassy/crystalline state to a leathery state takes place.

Conclusion
Extraction of fiber from raw fiber bundles was done by using methods like scutching and heckling. The physical and mechanical properties of the hemp fiber and fabric were studied. The diameter distribution profile was found to follow normal distribution with an average of 123.73 ± 7.82 µm. The density of hemp fibers was found to be 1196.628 kg/m 3 according to Archimedes' principle. The surface was found to be rough from SEM analysis. EDS showed the presence of C and O, indicating fiber as an organic material. FTIR showed the presence of lignin, cellulose, hemicellulose, pectin, and waxes in hemp fiber. CI FTIR was found to be 64.4%, whereas CI XRD was found to be 85.25% and 49.37% when calculated according to Segal and deconvolution method, respectively. The stress-strain graph of hemp fiber indicates the higher tensile modulus of 58.6 ± 8.81 GPa and high breaking strength of fabrics, which is comparable to synthetic glass fiber. The mean strain was found to be around 1%. High CI, comparable tensile modulus to glass fiber and high maximum strain rate make the hemp fiber and its fabric a potential reinforcement for polymer composites.

Highlights
• Physical and mechanical properties of elementary hemp fiber and fabric are determined using improved and systematic methods • Tensile strength and tensile modulus are determined using an improved method following ASTM C1557-20 • Density of hemp fiber is less and tensile modulus similar to glass fiber • Breaking strength of hemp fabric comparable to synthetic fabrics • High crystallinity of hemp makes it a potential reinforcement for polymer composites

Disclosure statement
No potential conflict of interest was reported by the authors.