Study of the Mechanical Properties of Hemp Fibers Treated with Supercritical CO2 Fluid

¹Université de Haute-Alsace, Mulhouse, France

²Université de Strasbourg, France

*Corresponding Author: Lotfi Harrabi
Université de Haute-Alsace, Mulhouse, France
Tel: 0762031377
E-mail: lotfi_harrabi@yahoo.fr

Received: August 31, 2020 Accepted: September 16, 2020 Published: September 24, 2020

Citation: Harrabi L, Almuhamed R, Drean JY (2020) Study of the Mechanical Properties of Hemp Fibers Treated with Supercritical CO2 Fluid. J Fashion Technol Textile Eng 8:4.

Abstract

Keywords: Supercritical fluid; Hemp fiber; Mechanical properties; Fineness, Tensile properties

Introduction

For the last two decades, natural cellulose-based fibers are increasingly gaining attention as their application is diversified into textile [1] composite and engineering end uses such as building materials [2]. They have started to be considered as alternatives to conventional man-made fibers in various applications such as transportation, construction and packaging. Some of them like hemp, flax, kenaf, jute, and sisal, which have been utilized as fiber sources since historical times, have become the focus of research attention once again [3].

Generally, Lignocellulosic agro-based materials show a composite structure which is constituted by an organic matrix reinforced by bundles of cellulose microfibers. The organic matrix is mainly composed of hemicelluloses, pectin, lignin, waxes, and proteins [4]. In our case, hemp fibers contain 70.2%-74.4% cellulose, 3.7%- 5.7% lignin, 17.9%-22.4% hemicellulose, 0.9% pectin, and 0.8% wax [5]. They are among the most suitable fibers for use in various textile applications. However, their extraction is a very complicated process. Traditional treatments such as dew retting, tank retting and chemical methods have been used to separate hemp fiber bundles and remove non-cellulosic materials. Unfortunately, these traditional treatments have many disadvantages like environmental pollution, fiber degradation and long-term treatment [6].

Other possibilities for refining coarse fibre bundles are the steam explosion process [7] the ultrasonic technique [8] and the enzymatic treatment [9]. All of them are still in exploratory stage.

In this study, the scCO₂ fluid technique was employed in the pretreatment of hemp fibers because it is an environmentally benign process. It is found here that the non-cellulosic compounds around the fibers could be removed apparently by scCO₂ under appropriate conditions and in presence of swelling agent [10,11].

Materials and Test Methods

Materials

Hemp fibers were sourced by CDE (Chanvriers De l’Est) in Eastern France. They have been treated at 3 different conditions in this study using scCO₂ as shown in Table 1.

Table 1: Experimental conditions.

Test Methods

Pretreatment in scCO₂: All experiments were performed on a batch-type supercritical extractor. The pretreatment of the hemp fibers was conducted in a 10 cm3 sample cartridge [12,13]. After heating the sample to a specific temperature, the scCO₂ was injected into the stainless-steel vessel via the high-pressure syringe pump to the desired pressure. The testing sample was then treated for a certain time at a constant temperature and pressure. In order to enhance the swelling effect of scCO₂ for the hemp fibers, a certain amount of swelling agent was added into the scCO₂ treating system. After the supercritical process, the sample was taken out after decompression and dried.

Measurments: After scCO₂ treatment, hemp fibre samples were cut into 55 mm in length to be used as specimens in tensile tests. Based on the textile standards, the specimens were conditioned at 20 ± 2°C and 65% ± 2% relative humidity for 48 hours before tests were started [14].

Fiber fineness: Fibre fineness is defined as linear density in terms of textile technology. The NF EN 13392 test method (13392 2001) has been used to calculate the fiber linear density as shown in the equation (1). Each fiber was first weighed using an electronic microweighting balance [15].

T (Tex)=m(g)/L(m).10³ (1)

With T: Fiber linear density (Tex)

m: Fiber weight (gram)

L: Fiber length (meter)

The fiber specimens were conditioned at 20 ± 2°C and 65% ± 2% relative humidity for 48 hours before every test [16].

Tensile properties: The tensile properties of hemp fibers were determined according to single fiber tensile test as described by ASTM D3822 standard (ASTMD3822 2020) using MTS tensile Tester. For each test, the 55 mm long specimen was placed on a cardboard which had a window (35 mm) to define the clamping length of test specimen (Figure 1). Both the lower and upper ends (10 mm each) of fiber were glued to the cardboard [17]. Thus, the fiber was fixed across the cardboard window. The cardboard was fixed in the tensile testing machine, and was cut along the discontinuous lines as shown in Figure 1, so that the applied load could be transmitted through the fiber only. The load-elongation curve was then recorded for each specimen during the test.

Figure 1: Set-up for tensile testing of single fiber.

Parameters generated from the measurements: Specific tensile strength, strain and the work of rupture were generated from the load-elongation curves (Saville 1999).

Strain: Elongations at break were read from the load-elongation curves and converted to maximum strain, Ɛ_max (%) using equation 2.

(2)

Where:

Ɛ_max(%) is maximum strain in %; l is elongation at break in mm; l₀ is initial length in mm (35 mm).

Specific tensile strength: The specific tensile strength was expressed as tenacity-strength related to fineness (load per unit fineness). Maximum loads were read from the load-elongation curves and specific tensile strength was determined using equation 3.

(3)

Where: F_max is the maximum load in cN.

Work of fracture: The work of fracture is determined as the area under the load elongation curve. It indicates the total energy required to break the fiber. Higher work of rupture means that the fiber is more durable. It is calculated using numerical integration as shown in equation 4.

(4)

Where:

W is work of rupture in mJ; F₁ is the first data point in N; F₂ is the second data point in N; and F_n is the nth data point in N; Δl is elongation interval in mm.

Surface morphology analysis: Scanning electron microscopy (SEM) test was conducted to examine the surface morphology of untreated and treated hemp fibers [18]. The HITACHI model was used in this study for image acquisition at a voltage of 20 kV.

Fourier transform infrared spectroscopy analysis: The purpose of FTIR test was to observe the change of functional groups, which will help to confirm the removal of impurities of hemp fibers after scCO₂ treatment. The FTIR test was conducted using Bruker IFS 66/S spectrometer [19]. Hemp fibers were ground into fine particles for Fourrier transform infrared spectroscopy (FTIR) analysis.

Results and Discussions

Typical stress-strain curve

Stress–strain curves for treated hemp fibers are displayed in Figure 2. They are obtained by simple tensile test. Their forms are relatively similar.

Figure 2: Example of load-elongation curves for hemp treated fibers.

Fineness

The mean value of the linear density of untreated hemp fibers was 16.50 Tex with a coefficient of variation of 21%. It seems that it follows a decreasing trend while increasing the temperature of the scCO₂ treatment process (Figure 3). For higher temperature 150°C, the linear density was 5.3 Tex with a coefficient of variation of 29.7%. That means that hemp fibers are finer after scCO₂ treatment, which is very interesting result in terms of fiber quality [20]. SEM analysis (Figure 4) shows less impurities at the surface of treated fibers, which explain the efficiency of the treatment. The decrease of the weight per unit of length and consequently the linear density (Tex) of the treated fibers seems to be the result of the cleaning of the impurities such as lignin present around the hemp fibers.

Figure 3: Hemp fiber fineness.

Figure 4: Strain behaviour.

Strain

Figure 4 shows strain values of untreated and treated hemp fibers. They vary from 0.79% for the untreated fibers to 2.5 % for the treated ones. These small values are attributed in general to the brittle aspect of this kind of fibers [21]. It seems that the strain follows an increasing trend with temperature. At higher temperature of the process, it seems that there are less impurities in the hemp fibers, which let them single and more extensible. These impurities such as lignin are like a matrix around fibers. For that, we think that untreated hemp fibers are stronger and less extensible.

Specific tensile strength

The specific tensile strength is displayed in Figure 5 for untreated and treated hemp fibers. It seems that it follows a decreasing trend while increasing the temperature of the treatment process (Figure 5). Its mean value was 44.23 cN/Tex with a coefficient of variation of 14.41% for the untreated hemp fibers. For high temperature 150°C, the specific tensile strength was 36 cN/Tex, 18.6% lower than the untreated hemp fiber [22]. This could be explained by the fact that higher temperature promotes the thermal expansion of hemp fibers, resulting in the increase of their free volume. Therefore, the CO₂ molecules can more easily diffuse into the hemp fibers and extract the impurities. It seems that untreated hemp fibers are stronger mainly because of the presence of the impurities such as lignin compound around the fiber.

Figure 5: Tenacity behaviour.

Work of rupture

Figure 6 shows the work of fracture for raw and degummed hemp fibers. The effect of temperature on this work seemed to follow a decreasing trend (Figure 6) with the highest W being 0.27 mJ/ Tex with a coefficient of variation of 21.92% for 100°C of the scCO₂ treatment (S1). This trend was similar to that of the tenacity and was the reverse of the trend observed for strain.

Figure 6: Work of fracture.

Morphology analysis

The scanning electron microscopic (SEM) images of untreated and treated hemp fibers are displayed in Figure 7. As shown, the surface of untreated hemp fiber seemed to have impurities present on the surface. After treatment, fibers become apparently smoother and looked cleaner. The scCO₂ treatment was able to remove some of these impurities. The sample S3, which was treated at 150°C, has less impurities at the surface compared to the others[23].

Figure 7: SEM images of hemp fibers before and after scCO₂ treatment.

FTIR analysis

Figure 8 shows FTIR spectra for untreated and treated hemp fibers. The peak at 3325 cm^-1 represents the O-H stretching vibration and hydrogen bond of hydroxyl groups assigned to intramolecular or intermolecular hydrogen bonding and free OH hydroxyl [24]. The peak at 2873 cm^-1 represents C–H stretching vibration of methyl and methylene groups in cellulose and hemicellulose [24]. The peak around 1732 cm^-1 is attributed to the presence of the carboxylic ester in pectin and wax. The reduction of its intensity is especially pronounced for the sample S3. This could be explained by the partial removal of these non-cellulosic components. The peak around 1610 cm⁻¹ represents the C=O stretching in lignin [25-27]. The weakening of its intensity is due potentially to the partial degradation of lignin. The peak at 1425 cm^-1 is associated to the aromatic ring of lignin, its decrease after treatment confirms the partial degradation of this component. The peak around 896 cm^-1, which represents the β-D glucosidic bond in cellulose remained unaffected after treatment [28].

Figure 8: FTIR spectra of untreated and treated hemp fibers.

Conclusion

This study deals with the application of scCO₂ fluid technology for the pretreatment of hemp fibers. It was found that the scCO₂ fluid could remove the impurities of the hemp fibers in presence of some swelling agent, especially at higher temperature. In comparison with the conventionally method where a large amount of strong alkaline solutions is employed, the scCO₂ fluid is a better way to eliminate non cellulosic compounds from the hemp fibers. We should continue this investigation in order to determine the best configuration in terms of temperature, pression and time for hemp pretreatment fibers.

Acknowledgement

The authors gratefully acknowledge Alsace Region for the financial support. The authors would like also to thank all the members of CHAFILTEX consortium for their support.

References

1. 13392, AFNOR NF EN (2001) "Textiles - Monofilaments - Détermination de la masse linéique." In.
2. ASTMD3822 (2020) "Standard Test Method for Tensile Properties of Single Textile Fibers." In.
3. Amandine C, Freour S, Jacquemin F, Casari P (2013) Characterization and modeling of the moisture diffusion behavior of natural fibers. J Appl Polym Sci 130: 297-306.
4. Morad C, Grimi N, Bals O, Ziegler-Devin I, Brosse N (2019) Steam explosion process for the selective extraction of hemicelluloses polymers from spruce sawdust. Ind Crops Prod 141: 111757.
5. Dreyer J, Mussig J, Koschke N, Ibenthal W, Harig H (2002) Comparison of Enzymatically Separated Hemp and Nettle Fibre to Chemically Separated and Steam Exploded Hemp Fibre. J Ind Hemp 7: 43-59.
6. Oudiani El, Chaabouni AY, Msahli S, Sakli F (2012) Mercerization of Agave americana L. fibers. J Text Inst 103: 565-74.
7. Garcia-Jaldon, Dupeyre CD, Vignon MR (1998) Fibres from semi-retted hemp bundles by steam explosion treatment. Biomass Bioenergy 251-60.
8. Aofei G, Sun Z, Satyavolu J (2019) Impact of chemical treatment on the physiochemical and mechanical properties of kenaf fibers. Ind Crops Prod 141: 111726.
9. Marek J, Antonov V, Bjelkova M, Prokop S, Fischer H (2008) Enzymatic Bioprocessing - New Tool of Extensive Natural Fibre Source Utilization.
10. Silvija K, Gravitis J, Putnina A, Stikute A (2011) The effect of steam explosion treatment on technical hemp fibres." In 8th International Scientific and Practical Conference on Environment. Technology and Resources, 230-37.
11. Navdeep K, Dipayan D (2017) Alkali treatment on nettle fibers. J Text Inst 108: 1461-67.
12. Liu M, Silva D, Fernando D, Meyer AS, Madsen B, et al. (2016) Controlled retting of hemp fibres: Effect of hydrothermal pre-treatment and enzymatic retting on the mechanical properties of unidirectional hemp/epoxy composites. Compos Part A Appl Sci Manuf 88: 253-262.
13. Msahli S, Mounir J, Fouzi S, Jean-Yves D (2015) Study of the Mechanical Properties of Fibers Extracted from Tunisian Agave americana L. J Nat Fibers 12: 552-560.
14. Mwaikambo, Leonard, Martin Ansell (2002) Chemical modification of hemp, sisal, jute, and kapok fibers by alkalization. ‎J Appl Polym Sci 84: 2222-34.
15. Ouajai, Sirisart, Robert A Shanks (2009) Preparation, structure and mechanical properties of all-hemp cellulose biocomposites. Compos Sci Technol69: 2119-2126.
16. Pakarinen A, Zhang J, Brock T, Maijala P, Viikari L (2012) Enzymatic accessibility of fiber hemp is enhanced by enzymatic or chemical removal of pectin. Bioresour Technol 107: 275-281.
17. Reacutequileacute S, Duigou A Le, Bourmaud A, Baley C (2019) Interfacial properties of hemp fiber/epoxy system measured by microdroplet test: Effect of relative humidity. Compos Sci Technol 181: 305-311.
18. Reddy N, Yang Y (2005) Properties and potential applications of natural cellulose fibers from cornhusks. Green Chemistry 7: 190-95.
19. Renouard S, Hano C, Doussot J, Blondeau JP, Laine E (2014) Characterization of ultrasonic impact on coir, flax and hemp fibers. Materials Letters 129: 137-41.
20. Prosenjit S, Manna S, Chowdhury SR, Sen R, Roy D (2010) Enhancement of tensile strength of lignocellulosic jute fibers by alkali-steam treatment. Bioresour Technol 101: 3182-87.
21. Saville, B.P. 1999. Physical Testing of Textiles (Elsevier Science).
22. Anete S, Kukle S, Zelca Z (2019) Properties of hemp fibres reinforced PLA composites." In 59th International Scientific Conference of Riga Technical University (RTU) Section of Materials Science and Applied Chemistry, MSAC 2018, October 26, 2018 - October 26, 2018, 205-09. Riga, Latvia: Trans Tech Publications Ltd.
23. Stankovi, Sneana B, Novakovi M, Popovi DM, Popari GB (2019) Novel engineering approach to optimization of thermal comfort properties of hemp containing textiles. J Text Inst 110: 1271-79.
24. Eva T, Stevulova N, Sicakova A, Schwarzova I (2014) Impact of ultrasonic treatment of hemp hurds on the mechanical properties of composites. In 14th International Multidisciplinary Scientific Geoconference and EXPO, SGEM 2014, June 17, 2014 - June 26, 2014, 193-98. Albena, Bulgaria: International Multidisciplinary Scientific Geoconference.
25. Vignon MR, Dupeyre D, Garcia-Jaldon C (1996) Morphological characterization of steam-exploded hemp fibers and their utilization in polypropylene-based composites. Biores Tech 58: 203-15.
26. Wu Junnan, Xinmin Hao, Xinxing Feng, Jianyong Chen (2007) Effect of steam explosion treatment on the structure and properties of china-hemp fibers." In 2007 International Conference on Advanced Fibers and Polymer Materials, ICAFPM 2007, October 15, 2007 - October 17, 2007, 795-98. Shanghai, China: Chemical Industry Press.
27. Yılmaz, Deniz N (2013) Effects of enzymatic treatments on the mechanical properties of corn husk fibers. J Text Inst 104: 396-406.
28. Yulu, Guo, Shulin Z (2010) The Application of Ultrasonic in Degumming for Hemp. Applied Physics Research 2.