The selection of a retting method for the extraction of bast fibers as response to challenges in composite reinforcement

Abstract

This article presents an evaluation of bast fiber properties conditioned by the selection of a retting method in terms of meeting requirements for the final application of the fibers as composite reinforcement.

Two different methods of fiber extraction were used in the experiment: dew retting and osmotic degumming. The fibrous material was extracted from flax (Modran variety) and hemp (Bialobrzeskie variety). In addition, retted kenaf fibers (from China) were evaluated for the comparison of fiber properties.

The properties of the retted and degummed fibers were evaluated according to relevant valid standards regarding color, linear mass, breaking tenacity, aspect ratio, microscopic images and chemical composition, which included the determination of wax and fat, cellulose, hemicellulose, lignin and pectin content and the thermogravimetric analysis coupled with Fourier transform infrared spectroscopy technique. The Attenuated Total Reflection FTIR spectroscopic technique was used for the characterization of the fibers. Also, fogging and volatile organic compound (VOC) emissions were tested in order to evaluate the suitability of the bast fiber for composite formation. The results of the study proved that osmotic degumming applied for bast fiber extraction improves significantly the fiber quality in terms of color, odor, aspect ratio and VOC emission. The aspect ratio of osmotically degummed flax fibers increased by about 46% and hemp fibers by about 22% in comparison with dew-retted fibers. VOC emission of osmotically degummed hemp decreased by about 35%, but in the case of flax fibers, increasing of VOC has been observed. For this reason, osmotically degummed fibers can be recommended as more suitable for composite reinforcement.

Keywords

bast fibers pre-treatment composites osmotic degumming aspect ratio

Bast fibers, due to their inherent properties, find wide applications in many branches of the economy, for example, the textile, construction, medical, paper and automotive industries.

The application of flax fiber in technical sectors as reinforcement for structural composites^1–6 requires the quality of the material to be relevant to end product needs and to endow them with new competitive properties. The quality of the fiber extracted from fibrous plants depends on the method of obtaining the fiber, on the number of elementary fibers in a stalk, their distribution in the stalk structure and the binding of the fibers in the bundle.

The extraction of fibers from plants involves the degumming of fibrous straw in a water environment in a biological, chemical or physical processes.^7–10 The use of the degumming process, based on physical phenomena, especially on osmosis, allows for obtaining fiber characterized by high quality, uniformity and lower linear density in comparison with traditional retting methods.^11–14

The main advantages of using natural fibers in composites are low cost, low density and high specific mechanical properties and biodegradability. However, poor thermal stability and water resistance coupled with poor interfacial adhesion to petroleum-based polymers have bottlenecked the application of natural fibers in composite materials.^15,16 Despite this, the use of natural fibers in composites is an alternative for the production of petroleum-based plastics materials. There are numerous literature reports dedicated to the modification of natural fibers aiming at the reduction of their hydrophilic properties and at the improvement of fiber adhesion to polymer matrices.^5,17–21 However, the literature does not provide information on the preliminary preparation of fibers applied before exact fiber modification, which allows for limiting natural fiber disadvantages, such as unpleasant smell and dark color. The preliminary preparation also ensures adequately low linear density, high aspect ratio (AR) and low volatile organic compound (VOC) emissions. The inherent unpleasant smell of natural fibers and their dark color are linked with the dew retting method applied for fiber extraction, where the action of fungi and bacteria is very intensive, aiming at the decomposition of the woody part of stems, pectin and other glued substances. During the retting process, butanoic acid is released giving an unacceptable smell. The low linear density of fibers ensures a more uniform fiber distribution in a composite and allows for obtaining a higher AR.

This paper presents a description of the research on the preliminary preparation of bast fibers for main modification. Flax, hemp and kenaf fibers extracted from plants with the use of two retting methods, osmotic degumming and dew retting, were evaluated in terms of meeting the requirements for use as reinforcement for composites, that is, light color, lack of unpleasant smell, low linear mass and high AR.

Materials and experimental methods

Materials

Flax (Modran variety) and hemp (Bialobrzeskie variety) straw and retted fiber from the LENKON Processing Plant (Stęszew, Poland) were used. Flax and hemp fibers were extracted from fibrous plants with the use of osmotic degumming methods. The retted kenaf fibers were obtained from the Jute and Kenaf Research Department in Changsha, China.

Methodology

Two different retting methods, osmotic degumming and dew retting, were used for the extraction of flax and hemp fiber.

Osmotic degumming

The degumming process was carried out with an experimental device operating in the periodic mode (Figure 1). The apparatus was developed and constructed by two Polish research institutes. Batches of flax straw (14 kg) and hemp straw (20 kg) were fed to the device. The degumming was run at a water temperature of 30℃ and the time of the process lasted 72 h for flax and 144 h for hemp, with the flow rate at 0.5 L/s and the technological liquid of pH of around 7. After the process, the degummed straw was rinsed with cold water under pressure. Then the excess water was removed by wringing. The degummed straw was dried in a dryer for 48 h at +/–60℃.

Figure 1.

The apparatus and installation scheme for osmotic degumming in the periodic mode.

The mechanical processing of the flax and hemp straw was run with the use of the following:

– a custom-made laboratory breaking machine, made by CZECH FLAX MACHINERY – because of the stalk thickness only hemp straw was broken;

– a custom-made laboratory turbine, made by CZECH FLAX MACHINERY.

After osmotic degumming, the following percentage of fibres content in the straw of fibrous plants was obtained:

– Total fibres content - flax 27.21%, hemp 35.56%,

– Long fibres content - flax 12.63%, hemp 26.40%.

The installation consists of the following elements and sets (see Figure 1):

a reactor;

a reservoir;

an ultraviolet (UV) source (C-type rays);

a filtration unit;

a controller;

an installation for steering and diagnostics;

a pump.

Dew retting

Dew retting was conducted directly on a field after mechanical pulling of the stalks and deseeding. Flax stems were spread evenly in the field and left for 5 weeks. During that time, microorganisms (mainly fungi) secreted enzymes that degrade pectin, proteins, sugars, starch, fats and waxes, tannins and minerals. After that period, the mechanical process with the use of a laboratory scutching turbine was applied to separate the fibers from the woody part of stalks and to divide the fiber bundles.

Analytical methods

The flax, hemp and kenaf fibers were evaluated according to valid standards that included testing the following parameters.

Mass of extracted fiber (%). The result is represented as the mass of fiber present in 100 g of dry mass of a straw sample (g/100 g) according to Equation (1)

M = \frac{M_{F}}{M_{S}} \cdot \frac{100 + W_{S}}{100 + W_{F}} \cdot 100 %

(1)

where M is the mass of the extracted fiber, [%], M_F is the mass of the fiber sample, [g], M_S is the mass of the straw sample, [g], W_S is straw moisture, [%] and W_W is fiber moisture, [%].

Linear mass (tex) tests were done with a gravimetric method according to the Polish Standard PN-EN ISO 1973:2011. A fiber section of 10 mm was cut off from the middle part of the flax fiber and then formed into bundles of 100 fibers. The measurement of the mass of separate bundles allowed for determination of the mean linear density of the fiber. The test was carried out under ambient conditions.

Tenacity (cN/tex) was determined according to the Polish Standard PN-P-04676:1986. The tenacity was tested by breaking fiber bundles of specific linear mass with an Automatic Tensile Tester – STATIMAT ME (the distance between clamps was 3 mm and the pulling rate 10 mm/s).

Average diameter of fibers was measured by determination of the surface area of a cross-section of technical fibers. Three-hundred measurements of the surface area of the cross-section for the divided technical fibers were done for calculating the average fiber diameter. The AR was calculated as length to the average diameter of the divided “bundle” of fibers.

Hygroscopicity at 100 (%) – the ability of a textile product for water vapor sorption from the air with relative humidity at 100%, expressed as the quotient of the mass difference of the sample kept in a desiccator at 100% air humidity and the dry mass of the sample, and the dry weight of the sample, expressed in percent points. Hygroscopicity at 65% was calculated based on the above principles, with the relative humidity of air at 65% according to the standard for evaluation of fibers and textiles.

The VOC emission (mgC/g) test was conducted according to the VW standard PV 3341 (1995). This test method is used to determine emissions of organic compounds (TVOC = total volatile organic compounds). The sum of individual substances according to a gas chromatographic analysis and detection with a flame ionization detector (FID) is used as a measure of the total emission of a material. The test is performed using a capillary column (ZB-WAX plus, 0.25 mm internal diameter, 0.25 µm film thickness and 30 m length) and the Head-Space technique after tempering at 120℃ for 5 hours. Acetone serves as the calibrating substance for total carbon emission. The correlation coefficient should be higher than 0.995.

Fogging (mg) was determined according to the Standard Volvo Group STD 420-0003. The fogging test is a method applied for the measurement of fog condensate on a glass surface within the conditions established in the standard STD 420-0003 (July 2006). The equipment used, THERMO SCIENTIFIC, complies with the requirements of ISO 6452, and the gravimetric method was used in that case. Due to the fibrous nature of the material, 10 g of sample was tested. The fibers were preconditioned for 7 days in a desiccator. The temperature of the bath was 100℃ for a period of 16 h. The post-drying period was 24 h in a desiccator before measuring.

Microscopic test – photos of longitudinal views and cross-sections of flax fibers were made with a Hitachi S-3400N scanning electron microscope (SEM) in a high vacuum mode (a secondary electron detector SE). The fibers were sprayed with a gold layer prior to the tests. Both for the longitudinal views and cross-sections, the magnifications of ×200 were selected, the working distance was 20 mm and the value of the accelerating voltage was 20 kV.

Fiber color was determined according to an adapted method from the Standard: ISO 105-A02, “Textiles - Tests for color fastness - Part A02: Grey scale for assessing change in colour,” which describes the gray scale for color changes that occur during color fastness tests.

Odor of the fibers was evaluated with the use of an organoleptic method. The test was conducted by eight experts in bast fiber processing.

Thermogravimetric study (thermogravimetric analysis – TGA) was performed with a TA Instruments Analyser Q50. A tested sample (about 20 mg) was subjected to heating within the temperature range 30–650℃ and heating rate of 15℃/min in nitrogen atmosphere at a constant gas flow rate of 90 mL/min.

Fourier transform infrared spectroscopy (FTIR) was performed with a TA Instruments iZ10 model. The spectrum of the released gases contained 32 scans per second at a resolution of 4 cm⁻¹ within the range from 600 to 4000 cm⁻¹.

FTIR with an Attenuated Total Reflection (ATR) attachment was performed with a TA Instruments iS10 model. The spectrum of the released gases contained 32 scans per second at a resolution of 4 cm⁻¹ within the range from 600 to 4000 cm⁻¹.

Waxes and fats content (%) was measured according to the Polish Standard no. BN-86/7501-10. The percentage content of wax and fat substances was determined by extracting them with an organic solvent (petroleum ether) in a Soxhlet extractor and weighing the residues after vaporization of the solvent.

Cellulose content (%) in flax and hemp fiber was measured according to the Polish Standard no. PN-92/50092. The cellulose content was measured by dissolving lignins and other substances present in the fiber with a mixture of acetyl acetone and dioxane, acidified with hydrochloric acid and by measuring the weight of the residues.

Hemicelluloses content (%) in flax and hemp fiber was determined according to the Polish Standard BN-77/7529-02. The hemicellulose content was measured by dissolving the hemicellulose present in the fiber with 1% solution of sodium hydroxide, filtering off the residue after dissolution, drying it and weighing. Then the percentage content was calculated from the mass loss of the sample.

α-Cellulose and hemicelluloses content (%) in retted kenaf fiber – the determination method consists of extracting the holocellulose present in the fiber by delignification with sodium chlorite in the presence of glacial acetic acid, followed by extracting α-cellulose from the obtained holocellulose with a Mercer reagent. The content of hemicellulose is the difference between holocellulose and α-cellulose.

Lignin content (%) was determined according to the Polish Standard BN-86/7501-11. The lignin content was measured by dissolving cellulose, hemicellulose and pectins with a mixture of concentrated sulfuric and ortho phosphoric acids, followed by draining off the remaining insoluble lignin.

Pectin content (%) tests were conducted by a gravimetric method according to the method developed at the Institute of Natural Fibres and Medicinal Plants (INF&MP). The percent share of pectins was determined by dissolving them in ammonium citrate and then precipitated from the solution with calcium chloride and by measuring the weight of the calcium pectinate precipitated from the solution.

The standard deviation (SD) of the obtained results was determined for each tested fiber.

Results and discussion

The properties of dew-retted and osmotically degummed flax and hemp fibers, as well as retted kenaf fibers, were evaluated according to the methodology described above to check which one fulfils the requirements for composite application to the largest extent. The requirements concerned a high AR, low linear mass, high purity, minimal odor, bright color and low VOC emission.

Color and odor of the fibers

Real views of the fibers extracted from the straw with the use of osmotic degumming and dew retting are presented in Figure 2.

Figure 2.

Views of: (a) dew-retted flax fiber; (b) flax fiber after osmotic degumming; (c) dew-retted hemp; (d) hemp after osmotic degumming; (e) retted kenaf.

The osmotically degummed flax and hemp fibers are characterized by a light color in comparison with the fibers obtained with the use of the traditional dew retting method. The color difference between dew-retted and osmotically degummed flax and hemp fibers was evaluated according to an adapted ISO 105-A02 method with the use of the gray scale.

The differences were evaluated on at least level “1” for both types of fibers and it is the largest color difference available to determine with the use of the gray scale.

The fiber odor was evaluated with the use of an organoleptic method. The tests proved that osmotically degummed fibers are characterized by an absence of unpleasant odor, which usually accompanies dew-retted fibers. The unpleasant odor of dew-retted fibers results from the activity of bacteria and fungi, which decompose the woody part of straw during the retting process. During osmotic degumming this phenomenon does not occur.

Evaluation of linear mass, tenacity and hygroscopicity of the fiber

Evaluation of the properties of the fibers coming from dew retting and osmotic degumming was conducted based on relevant standards; the test results are presented on Figures 3 –5 and Table 1.

Figure 3.

Linear mass of the flax and hemp fibers after dew retting and osmotic degumming and retted kenaf fiber.

Table 1.

Tenacity of the flax and hemp fibers after dew retting and osmotic degumming and retted kenaf fiber

Fiber description	Tenacity (SD), cN/tex	Breaking force (SD), N	Elongation (SD), %
Dew-retted flax	49.57 (4.95)	101.85 (5.08)	51.74 (0.85)
Osmotically degummed flax	43.18 (5.45)	89.67 (9.29)	47.11 (3.38)
Dew-retted hemp	52.19 (2.88)	108.05 (11.71)	51.49 (3.21)
Osmotically degummed hemp	43.20 (7.95)	74.82 (13.33)	43.19 (4.39)
Retted kenaf	41.58 (5.03)	79.53 (11.40)	44.56 (3.37)

The osmotic degumming method applied for fiber extraction resulted in better division of fiber bundles into smaller fiber complexes, which is shown in Figure 3. Thanks to this, the linear mass of the osmotically degummed flax and hemp fibers is lower in comparison with that of traditional fibers. Water flowing through stalks during the osmotic degumming process is able to remove pectin and substances gluing the fibers and the woody part in a more effective way in comparison with the dew retting process. The linear mass of the retted kenaf is similar to that of the dew-retted hemp fibers.

The test results for mechanical properties of the flax, hemp and kenaf fibers indicated that cleaner and better divided fibers after osmotic degumming showed lower tenacity (Table 1). Glued elementary fibers in the form of thicker technical fibers show higher tenacity due to the highest number of elementary fibers in the cross-section and due to the action of bonding substances as fiber reinforcement. Nevertheless, the tenacity of the osmotically degummed flax and hemp fibers are at the same level as retted kenaf.

In terms of fiber ability to moisture sorption, the osmotic degumming method did not cause significant differences in fiber hygroscopicity (at 65% relative air humidity) in comparison with dew-retted fibers, which is shown in Figure 4. At 100% relative air humidity, the hygroscopicity of all types of the tested fibers (except for kenaf) reached similar levels; the kenaf fibers showed the highest ability for moisture sorption (Figure 5).

Figure 4.

Hygroscopicity of the flax and hemp fibers after dew retting and osmotic degumming and retted kenaf fiber tested at 65% relative air humidity.

Figure 5.

Hygroscopicity of the flax and hemp fibers after dew retting and osmotic degumming and retted kenaf fiber tested at 100% relative air humidity.

Aspect ratio of fiber

An important parameter for short fibers dedicated to composite reinforcement is the AR (s), defined as the ratio between the length (l) and the diameter (d) of the fibers, according to Equation (2)

s = \frac{l}{d}

(2)

The applied methods of fiber extraction proved that osmotic degumming resulted in the fiber bundles being divided better – into smaller complexes – in comparison with the dew retting method, which is shown in Figure 6. Thanks to this, the linear mass of flax and hemp fibers, and related with this fiber diameter, is lower in comparison with the linear mass and diameter of the traditional ones, which has a strong effect on the AR values. The AR calculated for the fibers cut to 4 mm segments is shown in Figure 7. It can be seen that the AR is higher for the osmotically degummed fibers in comparison with the dew-retted fibers. The high SD of the AR of the osmotically degummed fibers is related to the method of fiber degumming. Osmotic degumming caused better dividing of technical fiber, and for this reason a higher number of elementary fibers were obtained with the parallel existence of smaller technical fibers. Nevertheless, the average linear mass of the osmotically degummed fibers was lower in comparison with that of the dew-retted fibers.

Figure 6.

Comparison of microscope images of cross-sections of: (a) dew-retted flax fiber; (b) flax fiber after osmotic degumming; (c) dew-retted hemp; (d) hemp after osmotic degumming; (e) retted kenaf.

Figure 7.

The aspect ratio of fibers cut to 4 mm.

The value of the AR depends on the length of fibers with constant diameter. If longer fibers are used, the AR is higher. The AR calculated for the fibers in two different lengths, 4 mm and 5 cm, is presented in Table 2.

Table 2.

Aspect ratio of the tested fibers

Fiber samples	Aspect ratio (SD), 4 mm	Aspect ratio (SD), 5 cm
Dew-retted flax	40.65 (19.26)	571.65 (240.80)
Osmotic degummed flax	59.56 (48.06)	1047.39 (600.73)
Dew-retted hemp	36.21 (20.72)	539.98 (259.02)
Osmotic degummed hemp	44.01 (35.20)	740.68 (439.96)
Retted kenaf	38.73 (10.89)	514.18 (136.12)

It can be seen that the AR for osmotically degummed fibers is higher than for the dew-retted fibers. The AR of all types of fibers cut to 5 cm is higher than 100, which means that the fibers meet the requirements for composite reinforcement.

Evaluation of the chemical composition of the fiber

The main fiber component in the chemical composition of fibrous plants is cellulose. The substances that occur along with cellulose are waxes and fats, hemicellulose, lignin and pectins. The percentage shares of specific compounds in the tested fibers are presented in Table 3.

Table 3.

Chemical composition of the retted kenaf fiber, dew-retted and osmotically degummed flax and hemp fiber

Fiber description	Content (SD)
Fiber description	Waxes and fats, %	Lignin, %	Pectin, %	Cellulose, %	α-Cellulose, %	Hemicellulose, %
Dew-retted flax	1.53 (0.06)	7.42 (0.12)	1.63 (0.22)	72.76 (0.54)	–	19.08 (0.03)
Osmotically degummed flax	1.10 (0.16)	3.69 (0.52)	0.92 (0.14)	79.75 (0.72)	–	14.39 (0.07)
Dew-retted hemp	1.84 (0.38)	8.15 (0.63)	1.81 (0.01)	72.85 (2.89)	–	20.46 (0.08)
Osmotically degummed hemp	0.25 (0.08)	4.25 (0.03)	1.07 (0.17)	71.26 (0.45)	–	15.99 (0.13)
Retted kenaf	0.12 (0.02)	10.04 (0.02)	0.62 (0.05)	–	63.61 (0.07)	32.93 (0.07)

The results of the study showed that the osmotic degumming process removed non-cellulosic substances from the tested fibers in a much more effective way in comparison with dew retting process – the osmotically degummed fibers contained lower amounts of lignin, pectin and waxes in their chemical structure in comparison with the dew-retted fibers.

VOC emission

The VOC limit is 20 mgC/g. According to Renault (Volvo Group), the TVOC requirements for selected injected parts to be used in the interior of cabin trucks were established as 20 µgC/g of a sample for the footrest part and 40 µgC/g of a sample for the bracket part.

The results of the tests are shown in Figure 8.

Figure 8.

Volatile organic compound (VOC) emissions determined for the tested fibers.

The results of the test indicated that the TVOC is lower for the hemp fibers extracted with the use of osmotic degumming in comparison with that of the dew-retted hemp fibers.

The TVOC values of the degummed hemp fibers were at the same level for the retted flax and retted kenaf fibers.

Fogging

The results of the fogging test are shown in Figure 9.

Figure 9.

Determination of fogging for the tested fibers.

Concerning the fogging results, it can be said that the osmotic degumming increases the fogging values in comparison with the dew-retted flax and hemp fibers, which is shown in Figure 9. Nevertheless, the osmotically degummed hemp fibers showed lower fogging values than osmotically degummed and retted flax fibers. The value of fogging for the retted kenaf fiber is similar to that of osmotically degummed flax fibers.

Microscopic analysis of fiber

Images of longitudinal and cross-section views of the dew-retted and osmotically degummed flax, hemp and retted kenaf fiber are shown in Figure 10.

Figure 10.

Longitudinal and cross-section views of: (a) dew-retted flax fiber; (b) flax fiber after osmotic degumming; (c) dew-retted hemp; (d) hemp after osmotic degumming; (e) retted kenaf.

The studies of the morphological structure of the dew-retted and osmotically degummed fibers indicate clearly considerable changes in the degree of dividing the technical fibers into smaller fiber complexes.

For all the tested fibers the surface is smooth and undamaged.

IR-ATR analysis of fiber

In order to identify the compounds present in the fiber, spectrophotometric analysis was used coupled with a total internal reflection method with the use of an ATR attachment. The spectra of the tested fibers in the infrared spectrum are presented in Figure 11.

Figure 11.

The Fourier transform infrared Attenuated Total Reflection spectra of the tested fibers.

The chemical analysis, presented earlier, allows for concluding that the main fiber component is cellulose and next are waxes and fats, hemicellulose, lignin and pectins. The percentage share of specific compounds is presented in Table 4.

Table 4.

The characteristics of the main absorbance Fourier transform infrared spectra of flax fiber

Bond	Vibration type	Wavenumber [cm⁻¹]	Remarks
O-H	Stretching	3100–3600	Cellulose, hemicellulose, lignin, pectin
C-H₃	Stretching	2954	Lignins, waxes and fats
C-H, C-H₂	Stretching	2847–2850; 2915–2920	Cellulose, hemicellulose, lignin, pectins, waxes and fats
C=O	Stretching	1730–1735	Carboxylic acids, aldehydes, esters (pectin, lignin, waxes and fats)
O-H	Stretching	1623–1649	Adsorbed water
C=C aromatic	Symmetrical Stretching	1591; 1540–1546; 1503–1506	Lignin
O-H	Bending	1461	Adsorbed water
C-H₃ and C-H₂	Deforming	1461; 1472	Lignin and cellulose, hemicellulose, pectins, waxes and fats
COO	Stretching	1416–1427	Acids (pectin)
C-H₃	Symmetrical Deformation	1369–1373	Lignins
O-H	Bending	1337–1342	Cellulose, hemicellulose, lignin, pectin
CH₂	Scissoring (bending)	1315	Cellulose, hemicellulose
C-H	Bending	1277	Peak characteristic for lignin
C-O	Stretching	1236–1245	Hemicellulose or pectin
C-H	Bending	1201–1205	Flax, hemp
C-O-C	Bending	1158–1160; 1025–1029	Cellulose, hemicellulose, pectin, waxes and fats
C-O	Stretching	910–1125	Cellulose, hemicellulose, pectin
β-1,4-Glycosidic bond		895–898	Cellulose, hemicellulose

Studies of the fibers in infrared showed spectra representing chemical components of the fiber, that is, cellulose, pectin, lignin, hemicellulose, waxes and fats. These are the spectra representing vibrations of the functional groups, such as O-H, C=O, C=C, COO, C-H, CH₂, CH₃ and COC (Table 4).

The characteristics of the main absorbance spectra of the flax and hemp fibers, dew-retted and osmotically degummed, and of the retted kenaf fibers, are shown in Figure 11 and Table 4.

The spectra of 1623–1649 cm⁻¹ and 1461 cm⁻¹ can be distinguished in the structure of all the fibers, which indicates stretching vibrations for the OH group for the water adsorbed in the fiber.²² One can also distinguish stretching bands for the OH group within the 3100–3600 cm⁻¹ absorbance range that comes from cellulose and hemicellulose present in the fiber.

Cellulose is a polysaccharide made of glucose molecules bonded with β-1,4-glycoside bonds, which represent the wave numbers of 895–898 cm⁻¹.²³ Also, stretching and bending bonds CH and CH₂ (2847–2850; 2915–2920; 1461; 1472 and 1315 cm⁻¹), bending bonds C-O-C (1158–1160; 1025–1029 cm⁻¹) and stretching bonds C-O (910–1125 cm⁻¹) are visible in the FTIR spectra²³ (Figure 11).

Hemicellulose, like cellulose, also belongs to the polysaccharides and their derivatives, bonded with β-glycoside bonds. As above, hemicellulose shows absorbance within the range of the bonds: OH (3100–3600 cm⁻¹); CH₂ and CH (2847–2850; 2915–2920 cm⁻¹); C-O-C (1158–1160; 1025–1029 cm⁻¹); and C-O (1158–1160; 1025–1029 cm⁻¹) derived from ethers. Apart from the bonds mentioned above, also a stretching C-O bond is visible coming from the acetyl group at the wave length of 1236–1245 cm⁻¹; this bond may be both linked with hemicellulose and pectin.²²

Pectin belongs to polysaccharides and is mainly made of polygalacturonic acid. Pectin shows absorbance within the range of the following bonds: OH (3100–3600 cm⁻¹); CH (2847–2850; 2915–2920 cm⁻¹); C=O (1730–1735 cm⁻¹) derived from carboxylic group; COOH (1416–1427 cm⁻¹); and C-O-C (1158–1160; 1025–1029 cm⁻¹) derived from ethers. A stretching COO bond is characteristic for pectin (1416–1427 cm⁻¹). Moreover, another stretching C=O bond can be distinguished (1730–1735 cm⁻¹), which may come from both pectin, lignin and waxes and fats.²²

Lignin is a polymer, monomers of which are organic compounds derived from phenolic alcohols. Lignin shows absorbance within the range of the following bonds: OH (3100–3600 cm⁻¹); CH₃ (2954 cm⁻¹); C=O (1730–1735 cm⁻¹); and C=C (1591; 1540–1546; 1506 cm⁻¹). Among the aforementioned chemical bonds, special attention should be paid to the C-H₃ stretching bond (2954 cm⁻¹) and deforming bond (1369–1373 cm⁻¹), C-H bending bond (1277 cm⁻¹) and C=C stretching bond (1591; 1540–1546; 1506 cm⁻¹) that come from aromatic compounds.^22,23

Waxes and fats. Waxes belong to esters of higher monocarboxylic fatty acids and higher monohydroxide alcohols. Fats belong to lipids, glycerol esters and fatty acids. Waxes and fats show absorbance within the range of the following bonds: CH₃ (2954 cm⁻¹); CH_2, and CH (2847–2850; 2915–2920 cm⁻¹); and also C=O (1730–1735 cm⁻¹) bonds from the COOC (1158–1160; 1025–1029 cm⁻¹) ester group.

The presence of the same absorption spectra of a few compounds results in overlapping, what makes it impossible to identify individual substances without doubts.

For all the tested fibers, the FTIR spectra are similar; they differ only in the intensity of absorbance of chemical bonds of the relevant chemical compounds, that is, lignin, pectin, cellulose, hemicellulose, waxes and fats (Figure 12).

Figure 12.

Changes in the Fourier transform infrared Attenuated Total Reflection spectra of fibers.

The FTIR spectra clearly illustrate the differences in emerging signals within the range of wavelengths at 1593, 1546 and 1503–1506 cm⁻¹, which are linked with the aromatic group of the stretching C=C bond. For retted kenaf fiber, the highest signal intensity is observed for the wavelength at 1954 and 1503 cm⁻¹, whereas for the dew-retted flax and hemp the highest intensity is observed at 1546 cm⁻¹.

Also there are clear differences for wavelengths at 1471 and 1462 cm⁻¹, which are linked with the signal from the OH bond for the water adsorbed, and for CH₃ and CH₂ bonds, where the intensity is the highest for the dew-retted flax and hemp fibers.

Moreover, the signals at wavelengths of 2847–2850 and 2915–2920 cm⁻¹ are distinct for the dew-retted flax and hemp fibers and for the osmotically degummed flax fibers. For the hemp fibers and retted kenaf fibers, the signals became blurred and looked as one broad peak.

In the remaining cases, the analysis of the spectrum intensity showed that the retted kenaf fiber is characterized with the highest intensity of the spectra within the absorbance range of 800–1450, 1730–1735 and 3200–3500 cm⁻¹, and the same stands for the retted hemp fiber in the absorbance range of 1460–1650 and 2850–2960 cm⁻¹.

TGA-FTIR analysis of fiber

A comparison of the thermogravimetric curves TGA/derivative thermogravimetry (DTG) for the tested fibers is shown in Figure 13. The thermal properties and mass loss of the flax, hemp and kenaf fibers are shown in Tables 5 and 6.

Figure 13.

The thermogravimetric analysis- derivative thermogravimetry curves of the retted kenaf fiber, dew-retted and osmotically degummed flax and hemp fiber.

Table 5.

Thermal properties of retted kenaf fiber, dew-retted and osmotically degummed flax and hemp fiber

Fiber description	Initial T_onset (℃)	T_onset (℃)	DTG peak (℃)	% Residues at 600℃	T₁₀%/℃	T₆₀%/℃
Dew-retted flax	254.31	344.72	368.40	19.45	281.90	375.30
Osmotically degummed flax	259.05	348.40	369.49	17.12	292.09	374.55
Dew-retted hemp	269.04	351.27	372.95	17.44	282.88	377.64
Osmotically degummed hemp	260.61	345.31	365.92	20.20	287.28	371.84
Retted kenaf	277.02	355.11	374.32	14.07	287.70	374.36

DTG: derivative thermogravimetry.

Table 6.

Mass loss of the tested fiber according to the stages of thermal decomposition

Fiber description	Stage of thermal decomposition
Fiber description	1st (%)	2nd (%)	3rd (%)	4th (%)
Dew-retted flax	4.66	5.40	61.97	7.38
Osmotically degummed flax	2.98	5.12	64.99	6.86
Dew-retted hemp	4.17	6.29	63.31	6.94
Osmotically degummed hemp	3.62	6.48	61.30	6.43
Retted kenaf	4.24	11.98	63.56	4.63

The TGA curve and Table 5 show that the initial thermal stability grows depending on the fiber type in the following order: flax → hemp → kenaf. However, while for flax using osmotic degumming improves the thermal stability, for the retted kenaf the effect is opposite. Also, for all tested five types of fibers, four stages of thermal degradation are observed.

During the first stage, at about 100℃, water evaporation occurs. The mass loss observed amounts to 3–4.6% (Figure 13).

During the second stage, at temperatures between 185℃ and 300℃ (Figure 13), as reported by Benítez-Guerrero et al.,²⁴ hemicellulose and pectin degrade. This stage is the most visible for the retted kenaf and the least visible for the dew-retted flax (see Figure 13). The mass loss was 5.1% for the dew-retted flax and 11.98% for the retted kenaf. This relation was later confirmed by the results of hemicellulose content in the fiber (Table 3). Zheng et al.²⁵ showed that this stage is so little visible for flax that it overlaps with the main cellulose degradation stage; therefore, determining the precise moment of the curve peak is very difficult.

At the third stage of thermal degradation, the temperature is about 370℃ (Figure 13). As reported by Poletto et al.,²⁶ the cellulose molecule is a very long polymer of glucose units, and its crystalline regions improve the thermal stability of lignocellulosic fibers. The highest thermal stability, that is, 374.32℃, was observed for the long retted kenaf and the lowest (365.92℃) for the hemp after osmotic degumming. Depending on the fiber type, an increase in thermal stability was observed in the following order: dew-retted flax → osmotically degummed hemp → osmotically degummed flax → dew-retted hemp → retted kenaf. In the third stage, the mass loss is the highest and reaches the value of 61.3–65.0% (Table 6). At this stage, cellulose degradation occurs by the breaking of glycosidic linkages of the glucose chain.²⁴ It was shown that the mass loss was consistent with the analysis of the cellulose content, presented in Table 3. It was also observed that the mass loss increases depending on the type of the fiber in the following order: flax → hemp → kenaf.

During the fourth stage the temperature varies between 410℃ and 600℃ (Figure 13). The decomposition process is attributed to the degradation of lignin. According to the literature, the process is slow and starts at 250℃.^24,26 Mass loss was observed at 4.6–7.4% (Table 6) in the following order kenaf → hemp → flax, and also the method of degumming increases the mass loss toward osmotic degumming → dew retting. However, according to Kim et al.,²⁷ the temperature at which the lignin degradation starts is 250℃.

For all stages of the thermal degradation, mass loss was determined on the basis of the DTG peak.

The released gases were immediately analyzed with the use of FTIR. The FTIR spectra for the gaseous products of the thermal decomposition of the flax, hemp and kenaf are shown in Figure 14.

Figure 14.

The Fourier transform infrared spectra for the gaseous products of thermal decomposition of: (a) dew-retted flax fiber; (b) flax fiber after osmotic degumming; (c) dew-retted hemp; (d) hemp after osmotic degumming; (e) retted kenaf.

The FTIR tests of the gases released during thermal degradation of the tested fibers allowed for determination of the following compounds: water, carbon dioxide, carbon monoxide, acetic acid, formic acid and formaldehyde. The released gases depend on four stages of thermal decomposition as follows.

– In the first stage, that is, water evaporation, a band at 3737 cm⁻¹ is observed, which is associated with stretching vibrations of the OH bond, characteristic for water present within the fiber (Figure 15).

– In the second stage, that is, decomposition for hemicelluloses, bands are observed at 3737 (OH), 3590 (–OH), 2976 (-CH₃), 2910 (-CH), 2355; 2311 and 671 (CO₂), 2182 (CO), 1795 and 1771 cm⁻¹ (C=O) (Figure 16). The composition of the retted kenaf fiber leads mostly to the formation of acetic acid, then of carbon dioxide and water. However, for the flax and hemp fibers the decomposition produces mostly carbon dioxide and water.

– In the third stage, that is, cellulose degradation, bands are observed at 3737 (OH), 3590 (–OH), 2976 (-CH₃), 2910 (-CH), 2810 and 2728 (C-HO); 2355; 2311 and 671 (CO₂), 2182 (CO), 1795; 1770 and 1746 (C=O), 1177; 1121 and 1067 cm⁻¹ (C-O) (Figure 17). The cellulose decomposition leads to the production of formaldehyde, acetic and formic acid, carbon dioxide and water. The decomposition of retted kenaf fiber leads mostly to the formation of organic compounds, next carbon dioxide, whereas for the flax and hemp fibers carbon dioxide is released in higher amounts than other organic compounds. The intensity of the released organic gases decreases depending on the fiber type: retted kenaf → flax after osmosis → dew-retted hemp → dew-retted flax → hemp after osmosis.

– In the fourth stage, that is, lignin decomposition, the following bands are observed: 3589 (–OH), 3030–3150 (Ar-H), 3017; 2937; 2863; 1600–1870; 1420–1580; 1305 and 950–1250 (-CH), 2355; 2311 and 671 (CO₂), 2182 (CO), 1172; 1110 and 1034 cm⁻¹ (C-O) (Figure 18). The lignin decomposition leads to production of phenol, methane, methanol, water, carbon mono and dioxide.²⁸ However, during pyrolysis of the tested fibers, the highest (↑) and the lowest (↓) amounts of released compounds are observed for the following:

– water: the dew-retted flax ↑ and dew-retted hemp ↓;

– carbon dioxide: the dew-retted flax ↑ and osmotically degummed flax ↓;

– carbon monoxide: the osmotically degummed hemp ↑ and osmotically degummed flax ↓;

– phenol: the osmotically degummed flax ↑ and retted kenaf ↓;

– methanol: the retted kenaf ↑ and osmotically degummed flax ↓;

– methane: the osmotically degummed hemp ↑ and retted kenaf ↓.

Figure 15.

The FTIR spectra for the gaseous products of the first thermal decomposition stage of the tested fiber.

Figure 16.

The Fourier transform infrared spectra for the gaseous products of the second thermal decomposition stage of the tested fiber.

Figure 17.

The Fourier transform infrared spectra for the gaseous products of the third thermal decomposition stage of the tested fiber.

Figure 18.

The Fourier transform infrared spectra for the gaseous products of the fourth thermal decomposition stage of the tested fiber.

All the stages concern the main components of the fiber, that is, cellulose, hemicellulose and lignin, which is linked with the share of these compounds as determined in the chemical analysis (Table 4). The list of the compounds and their functional groups is given in Table 7.

Table 7.

List of the detected and identified compounds and their functional groups released during the thermal decomposition of the retted kenaf fiber, dew-retted and osmotically degummed flax and hemp fiber

Compound identified	Molecular formula	Functional group	Wave number (cm⁻¹)
Water	H₂O	OH	3737
Carbon dioxide	CO₂	CO₂	2355; 2311; 671
Carbon monoxide	CO	CO	2182
		OH	3590
		C=O	1795; 1770
		C-O	1177
Acetic acid	CH₃COOH	-CH₃	2976
		OH	3590
		C=O	1795; 1770
		C-O	1121; 1067
Formic acid	CHOOH	-CH	2910
		C-HO	2810; 2728
Formaldehyde	CHOH	C=O	1770; 1746
			3030–3150; 1600–1870;
Phenol	C₆H₅OH	Ar-H, -CH	1420–1580; 950–1250
Methane	CH₄	-CH	3017; 1305
		OH	3589
		C-H	2937; 2863
Methanol	CH₃OH	C-O	1172; 1110; 1034

Conclusion

The objective of the work was evaluation of the osmotically degummed and dew-retted flax and hemp as well as the retted kenaf fibers in terms of their suitability for using them to reinforce composites. The conclusions formulated based on the presented study are as follows.

The osmotic degumming applied for bast fiber extraction improves significantly the fiber quality in terms of color, AR and VOC emission.

The osmotically degummed bast fibers are characterized by light color and lack of unpleasant smell in comparison with the dew-retted fibers. Microorganisms acting during the dew retting process are responsible for odor and dark fiber color. There is no bacteria and/or fungi activity in case of the osmotic degumming process.

The linear mass of the fibers extracted from straw with the osmotic degumming method was lower by about 33% for flax and by 32% for hemp in comparison with the traditionally extracted fibers, which resulted in the higher AR – by about 22% for flax and by 46% for hemp. This means that the osmotically degummed fiber bundles were better divided into elementary fibers and they could be distributed in the polylactic acid (PLA) matrices in a more uniform way. On the other hand, the SD of the AR calculated for the osmotically degummed fibers was very high. It was related to the fact that the osmotic degumming produced high numbers of elementary fibers with very low diameter, but some number of glued fibers with a higher diameter were still present. The linear mass of the retted kenaf fiber was high and similar to that of the dew-retted hemp fibers, which resulted from the specificity of kenaf plants. The AR of the retted kenaf fibers is also close to the AR of the dew-retted hemp fibers.

There were no significant differences observed between hygroscopicity of the bast fibers extracted with the use of the dew retting and osmotic degumming.

The osmotically degummed flax and hemp fibers were characterized by lower content of waxes, lignin, pectin and hemicellulose in their chemical composition. This proved that the osmotic degumming method used for the fiber extraction was more effective in the removal of non-cellulosic substances from the fibers in comparison with the dew retting. The retted kenaf fibers contained the highest amount of hemicellulose and lignin, as well as a lower amount of waxes and pectin, in comparison with the flax and hemp, which resulted from the different types of fibrous plants.

The lowest VOC emission was observed for the dew-retted flax, osmotically degummed hemp and retted kenaf fibers. The TVOC were lower by about 35% for hemp fibers extracted with the use of the osmotic degumming in comparison with the dew-retted hemp fibers. The highest fogging was found in the case of the retted kenaf and osmotically degummed flax fibers. The osmotically degummed hemp fibers showed lower fogging values than the osmotically degummed and retted flax fibers. The work on a detailed explanation of this phenomenon will be continued. The FTIR analysis confirmed typical chemical bonding for lignocellulosic fibers. Some differences in records of the FTIR spectra observed for each type of the tested fibers resulted from specificity of those fibers; however, the greatest differences occurred between the retted kenaf and other tested fibers, which is related to the chemical composition of those fibers.

The fiber thermal decomposition process progressed in four stages: moisture evaporation, hemicellulose decomposition, cellulose degradation and lignin degradation. The main gaseous products of the fiber decomposition included CO₂, CO, H₂O and some organic compounds, such as acetic acid, formic acid and formaldehyde.

The mass loss of the tested fiber analyzed for the four stages of the thermal decomposition was lower for the osmotically degummed fibers in comparison with the dew-retted fibers in most cases. This means that the osmotically degummed fibers showed better thermal stability than the dew-retted fibers and retted kenaf fibers. The percentage of residues after the thermal decomposition at 600℃ of osmotically degummed hemp is the highest in comparison with other tested fibers; nevertheless, the initial temperature of the thermal decomposition of hemp after osmosis is lower.

The results of the study on the natural bast fibers for preliminary processes applied for preparing the fibers for the main modification proved that the application of the osmotic degumming allowed for obtaining fibers that meet the requirements for composite reinforcement.

Based on the study, it can be concluded that the osmotically degummed fibers should be recommended as more suitable for the composite reinforcement application in comparison with the dew-retted fibers.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was carried out within the NATURTRUCK project (FP7/2007-2013, Grant agreement No. 605658).

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