Research Article, J Fashion Technol Textile Eng Vol: 8 Issue: 4
Study of the Mechanical Properties of Hemp Fibers Treated with Supercritical CO2 Fluid
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.
The aim of this study is to analyze the efficiency of supercritical CO2 (scCO2) fluid technology for the pretreatment of hemp fiber materials. In this investigation, although the limited conditions studied here, the test results show that we have obtained a promising quality of hemp fibers after this specific pretreatment in presence of swelling agent. The scCO2 treatment appeared to remove non cellulosic compounds from the fiber, thereby resulting in improvement in its appearance, and decrease in its linear density. We should continue this investigation through further studies, since it is an environmentally benign process.
Keywords: Supercritical fluid; Hemp fiber; Mechanical properties; Fineness, Tensile properties
For the last two decades, natural cellulose-based fibers are increasingly gaining attention as their application is diversified into textile  composite and engineering end uses such as building materials . 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 .
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 . 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 . 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 .
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
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.
|Samples||Time (min)||Temperature (°C)||Pressure (MPa)|
|Sample 1 (S1)||60||100||25|
|Sample 2 (S2)||60||120||25|
|Sample 3 (S3)||60||150||25|
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 .
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 .
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 .
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 . 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.
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, Ɛmax (%) using equation 2.
Ɛmax(%) 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.
Where: Fmax 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.
W is work of rupture in mJ; F1 is the first data point in N; F2 is the second data point in N; and Fn 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 . 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 . 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.
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 . 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 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 . 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 . 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.
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.
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.
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 . The peak at 2873 cm-1 represents C–H stretching vibration of methyl and methylene groups in cellulose and hemicellulose . 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 .
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.
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.
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