The quality of the bond between the natural fiber and the matrix depends mainly on the chemical characterization of the fiber, the chemical composition of the polymer matrix and its atomic structure. However, as a constituent of natural fibers, cellulose is the main coupling agent in the polymer/fiber bond. On the other hand, lignin acts as an obstacle to the diffusion of the coupling agent by preventing good adhesion. During chemical treatments, constituents such as hemicelluloses were hydrolyzed by the action of alkaline solutions. The obtained results showed high amounts of lignin (28.67 wt% and 10.13 wt%) and hemicelluloses (21.35 wt% and 20.48 wt %) for untreated Alfa and Sisal respectively compared to the untreated samples involving a reduction in the fibers diameter it can be insured by SEM observation. However, Considerable amounts of cellulose (64.9 wt% and 75.1 wt%) for Alfa and Sisal fiber were found higher in treated fibers than untreated fibers (Table 1).
Table 1
Chemical composition of treated and untreated Alfa and Sisal fibers
Chemical composition of Alfa fibers
|
|
Cellulose
(wt %)
|
Hemicellulose
(wt %)
|
Lignin
(wt %)
|
Ref
|
Untreated
|
43,08
|
21.35
|
28,67
|
Present Work
|
NaOH treated
|
64.9
|
6.15
|
25.05
|
Present Work
|
Untreated
|
45
|
24
|
24
|
[16]
|
Untreated
|
43.8–47
|
22.15–28.4
|
17.4–24
|
[17]
|
NaOH treated
|
93.8
|
3.7
|
0.8
|
[18]
|
Untreated
|
46.1
|
30.2
|
19.9
|
[18]
|
Untreated
|
38.8
|
33.5
|
20.5
|
[19]
|
Untreated
|
46–47
|
24–38
|
20–24
|
[20]
|
Untreated
|
44–48
|
27–22
|
12–18
|
[21]
|
NaOH treated
|
49.50
|
17.62
|
16.51
|
[22]
|
Untreated
|
39.53
|
27.63
|
19.53
|
[22]
|
Untreated
|
44.2
|
23.0
|
25.4
|
[13]
|
Untreated
|
45
|
24
|
24
|
[23]
|
Untreated
|
45
|
25
|
23
|
[24]
|
chemical composition of Sisal fibers
|
Untreated
|
63.7
|
20.48
|
10.13
|
Present Work
|
NaOH treated
|
75.1
|
5.51
|
8.29
|
Present Work
|
Untreated
|
65.19 ± 1.2
|
32.09 ± 1.7
|
2.72 ± 0.28
|
[25]
|
NaOH treated
|
70.35 ± 1.1
|
27.29 ± 2.6
|
2.34 ± 0.5
|
[25]
|
Untreated
|
66
|
13
|
10
|
[6]
|
Untreated
|
65–68
|
10–22
|
9.9–14
|
[26]
|
NaOH treated
|
70
|
15
|
08
|
[27]
|
Untreated
|
63.8
|
15.2
|
2.6
|
[28]
|
NaOH treated
|
73.8
|
12.5
|
6.0
|
[28]
|
A very significant increase in the cellulose content is revealed, which explains the impact of the NaOH treatment on the increase of the fiber physico-mechanical properties.
Atr-ftir Analysis
(ATR-FTIR) is a very powerful technique for qualitative analysis to study the changes in the chemical composition during different stages of treatment. Untreated and treated Alfa and Sisal fibers were studied with attenuated total reflectance-Fourier transform infrared spectroscopy. The ATR-FTIR measurement was obtained using the Thermoscientific Nicolet IS 10 instrument equipped with ATR Thermoscientific Smart ITR module at room temperature driven by computer software, with spectral resolution of 2 cm− 1 and over wave number range of 4000 − 400 cm− 1(wave lengths 2.5 to 25 µm). The fiber samples were cut and grinded into a fine powder then carefully placed in the analysis chamber.
Results And Discussion
A comparison between the FTIR spectra (Fig. 3) of the treated and untreated two fibers (Alfa and Sisal) shows a large absorption band, observed around 3320 cm− 1, attributed to the (-OH) group. After treatment, the band appeared weaker in intensity as a result of the arrangement of more hydroxyl groups, induced by the crack of bonds that creates from lignin and hemicelluloses with cellulose [15, 29, 30]. Both peaks observed at 2923 cm− 1 and 2861 cm− 1 are associated to (CH2 and CH) groups of cellulose and hemicelluloses. Their decrease is due to the removal of hemicelluloses [15, 29, 31, 32]. The peak at wave number 1727 cm− 1,which corresponds to the carbonyl groups (C = O), can be attributed to the vibration of the acetyl group of hemicellulose or to the carboxylic group (of the ferulicring and p-coumaric acid of lignin and/or hemicelluloses). Such a peak appears only in the case of untreated fibers [15, 32, 33] while the band at 1545 cm− 1 indicates the presence of (C = C) groups of. The 1454 cm− 1 peak reflects the stretching vibration of CH2 bonds which occurs for treated sisal because of the formation of cellulose while that around 1420 cm− 1 corresponds to the (CH2) groups of cellulose. Alcohol group of cellulose (OH deformation) are characterized by peaks located at 1389 cm− 1. Peaks at 1370 cm− 1 contributed to the C-H asymmetric deformation [21, 34], and that located at 1319 cm− 1 is assigned to the CH2 wagging of lignin [33]. The intensity at this peak decreases after treatment for both types of fibers; indicating that the lignin component in fibers is removed by alkalization. The 1238 cm− 1 peak, attributed to the stretching frequency of the COO groups, indicates the presence of hemicelluloses and appears only for untreated fibers and not for alkali treated [29, 21, 32].. This is due to the removal of the hemicelluloses by chemical treatment (sodium hydroxide reacts with hemicelluloses constituents which are removed from the natural fibers). Besides, the peaks at 1155 cm− 1 and 1099 cm− 1 are a characteristic of the stretching frequency of the C-O-C groups which indicates the presence of the cellulose for treated fiber [20]. The occurrence of the peak at 1022 cm− 1 is attributed to the stretching vibration of C-O bonds of cellulose [15, 34, 35], while the 896 cm− 1 and 609 cm− 1 peaks are associated to (C-O-C) groups related respectively to the β-glycosidic linkages between glucose in the cellulose and the deformation vibration of the C–OH bond [21, 32, 36].
Now, from Fig. 3 it is remarked that the spectra of fibers have the same appearance and all samples show closely similar spectra despite the decrease or disappearance of the intensity of the bands, characterizing the non-cellular constituents (lignin, hemicelluloses and wax), with the alteration of the treatment concentration; making fibers cleaner and increasing the interfacial adhesion between the fiber and the matrix. On the other hand, it is observed that some peaks appear and others increase in the treated fibers; indicating the presence of cellulose. This component endows it with rigidity for reinforcing composite material [37] in agreement with previous findings. Consequently, the present FTIR results are in good accord with the literature; suggesting that the alkali treatments explored in this work have been successful.
Thermal Analyses
It is important to evaluate the thermal behavior of natural fibers in order to determine the best conditions for the implementation of composite materials reinforced by these fibres. TGA is an essential analysis to determine the thermal properties of fibers throughout their life cycle and for evaluating their thermal stability to avoid the degradation of their physico-mechanical properties. The analysis was performed using the NETZSCH STA 409PC/PG instrument in a nitrogen atmosphere at a heating rate of 10°C/min. The spectrum was recorded in a temperature range of 20 to 600°C. The samples analyzed weighed between 3 and 6 mg.
Thermogravimetric analysis result
Figure 4a represents the TGA curves of the various samples of treated and untreated Alfa and Sisal fibers. The degradation curves are similar due to their identical chemical compositions.
Different stages of thermal degradation are observed. A first mass loss was observed below 100°C. It is evaluated at 9.73wt%, 9.09 wt%, 9.79 wt % and 13.25 wt %, for the untreated and treated Alfa fibers; and untreated and treated Sisal fibers respectively. This loss of mass represents the evaporation of water present in the fibers [37–39]. At this stage, the fibers have thermal stability. Mass loss in this first stage is lower for treated fibers than untreated fibers, indicating less moisture in the case of treated fibers. Note that the fibers have thermal stability up to temperatures 192 C°, 216 C°, 156 C° and 218 C°.
A second mass loss is then highlighted indicating the maximum thermal degradation process. For the treated samples, an increase in decomposition temperatures compared to untreated fibers was observed. As previously described, this behavior suggests that celluloses with higher crystallinity have higher thermal stability. In addition, it is interesting to note that at 400°C, the mass loss for untreated samples is higher for those treated with NaOH because of its lower purity [39, 40], which is attributed to the degradation of lignin and other remaining fiber components [41]; then the loss of peas continues until 600°C. The heating up to 600° C revealed that the mass loss and the residual weight of the untreated samples were higher than those of alkali treated and corresponds to 27.56 wt%, 24.48 wt%, 24.35 wt%, and 13.06 wt % for untreated Alfa fibers, treated Alfa fiber, untreated Sisal fiber and treated Sisal fiber respectively.
This result is due to the greater quantity of cellulose present in the treated fibers. The results showed that with the NaOH treatment, the fiber becomes more heat resistant. These observations indicate that the improved thermal stability is mainly affected with the chemical treatment, where hemicellulose and lignin are eliminated [39–42]. It is important to note that the treated fibers show a stable behavior at 180°C, which corresponds to the crosslinking temperature of composites, which allows its use as a reinforcing material for the composites materials.
Figure 4b represent the curves of the different types of materials, the thermogravimetric analysis at 600◦C revealed that the mass loss and the residual mass of the samples with 40% reinforcement were higher than those with 30% reinforcement. The residual mass corresponding to each type of material is represented in Table 2. It is deduced that the increase in the volume fraction increases the thermal resistance of the materials.
Differential thermogravimetric analysis
DTGA fibers curves (Fig. 5a) indicates the first peaks below 100 C° corresponding to the evaporation of water for the treated and untreated Alfa and Sisal fibers. A series of second wider peaks at 330C°, 348C°, 300C°, 333C° for untreated and treated Alfa fibers, and untreated and treated Sisal fibers respectively.
This loss of mass is associated with chemical decomposition of the hemicellulose and glygosidic bonds of cellulose. Note that the thermal decomposition of untreated samples is more advanced indicating their low thermal stability compared to other treated samples that show a higher final decomposition temperature due to its high molecular arrangement because of its components that were eliminated by the treatment. In this second stage of major degradation, untreated fibers degrade slightly more but speedily than the treated fibers; this leads to say that thermal decomposition requires high energy [38].
The differential thermal analysis for the different types of materials (Fig. 5b indicates a first peak below 100 C° related to water evaporation. As with the fibers, a second wider peak appears indicating chemical decomposition. Note that the decomposition temperature of composites reinforced at 40% is higher than at 30%. The increase in the reinforcement ratio therefore increases the cellulose content in the materials, which gives it a high molecular arrangement. The maximum decomposition temperatures corresponding to the peaks are reported in Table 2 [20, 38, 39, 42]
Table 2
Characteristics of different types of fibers and composites from TGA and DTGA analysis
specimen
|
Initial deterioration (C°)
|
Mass loss (%)
|
Max temperature of stability (C°)
|
Mass loss (%)
|
Max temperature of decomposition (C°)
|
Mass loss (%)
|
Final degradation (C°)
|
Mass loss (%)
|
Residu
|
Untreated Alfa
|
60.93
|
93.89
|
192.59
|
90.27
|
330.93
|
51.95
|
399.33
|
34.87
|
27.56
|
Treated Alfa
|
53.62
|
95.22
|
216.12
|
90.91
|
348.62
|
46.59
|
451.12
|
29.05
|
24.48
|
Untreated Sisal
|
61.08
|
93.11
|
156.08
|
90.21
|
311.08
|
75.23
|
498.58
|
30.86
|
24.35
|
Treated Sisal
|
68.11
|
89.39
|
218.11
|
86.75
|
333.11
|
54.77
|
495.61
|
21.56
|
13.06
|
Composites
|
Pure Epoxy
|
81.78
|
98.17
|
292.25
|
76.26
|
358.02
|
48.93
|
496.68
|
12.67
|
11.44
|
30%Alfa/
Epoxy
|
87.84
|
97.95
|
272.00
|
91.26
|
357.84
|
53.11
|
478.23
|
16.34
|
13.56
|
40%Alfa/
Epoxy
|
72.96
|
98.88
|
277.96
|
93.54
|
377.96
|
52.51
|
477.96
|
21.52
|
18.79n
|
30%Sisal/Epoxy
|
92.13
|
95.11
|
243.83
|
85.43
|
373.61
|
46.04
|
519.93
|
20.05
|
18.15
|
40%Sisal/
Epoxy
|
64.06
|
84.24
|
247.39
|
90.24
|
352.11
|
55.24
|
496.61
|
26.07
|
23.13
|
30%Alfa-Sisal/Epoxy
|
86.38
|
89.87
|
239.08
|
87.88
|
356.87
|
50.13
|
490.39
|
21.38
|
18.19
|
40%Alfa-Sisal/Epoxy
|
67.50
|
96.10
|
247.50
|
87.44
|
375.50
|
56.90
|
482.44
|
26.49
|
24.07
|
Differential scanning calorimetry (DSC)
DSC measurement is used to determine the heat flow associated with transformations that do not cause mass variation and to observe and quantify the enthalpy changes of a material as a function of temperature or time. For all types of fibers, the curves (Fig. 6a) show an endothermic peak around 49.04 C°, 49.06 C°, 48.58C°, 43.11 C° for untreated Alfa fibers, treated Alfa, untreated Sisal fibers and treated Sisal fibers respectively, indicating the start of water evaporation.
For alfa fibers a second endothermic peak appears around 413.58 C° 428.11 C°. It is attributed to the decomposition of cellulose. No other exothermic or endothermic peaks were observed between the first and second peaks, indicating that both fibers remained stable and retained their mass. The enthalpy of the different samples can be calculated by integrating over time (in seconds), i.e. the graph area corresponds to the enthalpy of the sample transformation. The different values taken from the curves are presented in Table 3, [38, 40, 20, 42].
For all types of materials, the curves (Fig. 5b represent an endothermic peak at the start of heating indicating the beginning of water evaporation. The different values obtained from the curves are presented in Table 3. According to these values, it is noted that the quantity of enthalpy of materials reinforced at 40% is greater than that of materials reinforced at 30%. The increase in enthalpies indicates an increase in the total crystallinity of the composites. [20, 38, 39, 42].
Table 3
Onset decomposition temperatures, peak temperatures and heat of decomposition of different types of fibers and composites
Specimen
|
Onset decomposition temperature (C°)
|
Peak temperature (C°)
|
Heat of decomposition J/g
|
Untreated Alfa
|
49.04
|
/
|
131.28
|
Treated Alfa
|
49.06
|
/
|
185.20
|
Untreated Sisal
|
48.58
|
413.58
|
171.52
|
Treated Sisal
|
43.11
|
428.11
|
108.45
|
Composites
|
Pure Epoxy
|
48.69
|
372.53
|
100.44
|
30%Alfa/Epoxy
|
60.46
|
370.46
|
21.71
|
40%Alfa/Epoxy
|
58.10
|
405.33
|
42.51
|
30%Sisal/Epoxy
|
49.49
|
/
|
40.10
|
40%Sisal/Epoxy
|
49.50
|
534.32
|
57.23
|
30%Alfa-Sisal
/Epoxy
|
50.00
|
375.00
|
53.37
|
40%Alfa-Sisal
/Epoxy
|
51.08
|
523.96
|
73.72
|
Sem Observation
A scanning electron microscopy (SEM) observation was performed to observe the morphology and cell walls of untreated and treated fibers (Fig. 7).
8.1 Results and discussion
The images showed that long untreated Alfa fibers (Fig. 7a) and Sisal fibers (Fig. 7c) are composed of bundles linked together by lignocellulosic constituents that act as the link element which protects the plant from external effects. They are covered with impurities made of cells and other constituents, such as waxes and pectin. The fibers walls are composed of microfibers or fibrils of very small dimensions. After chemical treatments, the roughness of the fibers is attenuated and cleaned. As shown in (Fig. 7b) and (Fig. 7d), the roughness of the fibers has reduced after treatment, a smoother surface morphology appeared compared to untreated fibers and the microfibrils are more visible because the treatment dissolved the cement link between them, same remark was observed, see [10, 43, 44]. The chemical alkali treatment, attacked the hemicellulose contents of the fibers so they are cleaned, the impurities are eliminated so the diameter is reduced.
Fiber Static Tensile Test
Prior to testing, the diameter of each selected fiber is measured. In general, fibers have complex morphology which depends on environmental conditions, age, season, and location. Each fiber is considered cylindrical, with an average diameter of three measurements taken at three different locations of the fiber; at both ends and in the middle, it has been determined by means of an OPTIKA optical microscope equipped with a camera and controlled by the Action-vision software. Tensile mechanical characterization tests of treated and untreated sisal and alfa fibers were performed according to ASTM D3822-07, on the Zwick/Roell Z010 universal test machine with 5 kN load at a constant speed of 2mm/min and a gauge length of 80mm. 20 samples of each type of untreated and treated fibers were used, (Fig. 8).
Results And Discussion
It can be seen that the fibers of Alfa have a better elastic behavior than those of Sisal. By comparing the values of the Young's modulus of Alfa untreated and treated (19,93 GPa vs 37,70 GPa) and 27,3GPa vs 31,90 GPa for Sisal fibers (Table 4), we conclude that the chemical treatments used increase significantly the mechanical properties of the fibers. This tendency can be easily explained by considering that the waxes, gums and cement materials which provide the cohesion between the cellulose microfibrils are partially removed by the chemical treatments which result in an increase of the cellulose rate in the material [45]. For a comparison, Arthanarieswaran et al. [6] and Khaldi et al. [11] found respectively the following tension Young's moduli 3,50 GPa for Banana fiber, 28,43 GPa for Alfa fiber and 25,01GPa for sisal fiber. Benyanina et al. [23] found 12,7GPa for Alfa fiber and 26,50 GPa for jute fiber.
Table 4
Mechanical characteristics in traction of untreated and treated Alfa and Sisal fibers
Specimen
|
Diameter (µm)
|
σ max (MPa)
|
ℰ-Fmax (%)
|
Young modulus (MPa)
|
Untreated Alfa fiber
|
218,66 ± 57,12
|
6882,94 ± 245,19
|
2,07 ± 0,68
|
18577,31 ± 411,46
|
Treated Alfa fiber
|
106,95 ± 32,30
|
9220,54 ± 276,81
|
2,49 ± 0,79
|
41248,30 ± 627,92
|
Untreated Sisal fiber
|
102,51 ± 25,36
|
2762,21 ± 122,00
|
13,30 ± 3,75
|
14685,81 ± 394,78
|
Untreated Sisal fiber
|
122,08 ± 13,40
|
5539,62 ± 133,45
|
3,03 ± 0,68
|
17673,82 ± 639,27
|
The determination of the standard deviation made it possible to evaluate the dispersion of the results relative to the mean. For the Young's modulus, the standard deviation of the treated fibers is greater than the untreated fibers, the calibration of the moduli obtained for the 20 samples are therefore close to the average.
Preparation Of The Alfa/epoxy Resin Composites
Weaving of Material
Using treated long Alfa and Sisal fibers (Fig. 9a and b), strands of same length and equal weight were elaborated; giving rise to 480g/m2 and 2mm thick Alfa Sisal and hybrid Alfa/Sisal hand woven satin cloth types. Hybrid Alfa /Sisal cloth is composed of 50% of each type of fiber (Fig. 9c-e).
Elaboration of composite materials by VARTM method
The composite material was obtained by VARTM (Fig. 9), in an aluminum mould of 300 mm long, 170 mm wide and 4 mm thickness at room temperature. First, a mold release agent was used so as to facilitate the release of each type of composite. Each cloth of reinforcement (Fig. 9c-e) is put into the mould. Prior to starting injection, care was taken to insure that the pump-mold circuit is hermetic: the resin supply line is blocked by the vise-grip long nose (1). The vacuum pump dial indicates (-1000mbar), which must be maintained during operation and shutdown. This allowed the hydrophilic fibers to be dehumidified and air to be evacuated so as to ensure good fiber/ resin adhesion. The resin injection starts and then stopped first by the resin outlet vise-grip long nose (2), once the volume of the mould is completely filled, then by the second resin inlet vise-grip long nose (1). The excess resin reservoir prevents resin residues from reaching the vacuum pump and damaging it. The demolding process is done after 12 hours, when the complete cross-linking of the resin is achieved. The obtained material is put in an oven (MEMERT UN110) at 80°C during 8 hours, and then allowed to cool in it to room temperature to ensure structure homogenization required by ISO 527 standard mechanical characterization tests.
Mechanical characterization of composites
Composites Tensile Test
The mechanical properties of the samples were measured by tensile tests. They were tested under uni-axial loading as in ISO-527-4 standard on a universal machine of the Zwick/Roell Z010 type, with a speed of 2mm/min and capacity charge of 10 kN (Fig. 10). Three samples of pure epoxy resin and five samples of each reinforced composite material have been tested. The Young's modulus, maximum stress and strain of each sample were determined (Table 5).
Composites Three-points bending test
Three-points bending tests were performed to determine the mechanical properties. In this case, a given sample is placed on two supports separated by a distance 16 times the thickness of the sample. Midway of the supports, a 10 kN force is applied with constant loading punch speed of 2 mm/min. Four specimens conform to ISO 178 (80*10*3 mm) standards have been tested on a Zwik/Roell Z010 machine type (Fig. 10). The three-points bending Young's modulus, maximum bending stress and strain of each sample were determined (Table 6).
11.3 Composites compression test.
A series of compression tests of the elaborated composites were carried out in order to evaluate their mechanical properties. The specimens were cut according to the EN ISO 14126 standard (10*120*4) with the length between the gauges of 10 mm, on a Zwick/Roell Z010 machine, equipped with a 10 kN force sensor. The loading was incremented at a displacement speed of 2 mm/min. The axial displacement in compression was measured and stopped after the displacement exceeded 4 mm. Four tests were performed for each specimen (Table 7).
Results And Discussion
The static tensile test results (Fig. 11a) of the composites were compared to those of the pure epoxy resin which showed a significant increase in Young's moduli of about 333% and 160% respectively for 40wt% and 30wt% woven Alfa fibers /Epoxy compared to the pure epoxy. In addition, 40wt% and 30wt% woven Sisal fibers /Epoxy induced an increase of 75% and 8% respectively. Finally, 40wt% and 30wt% Hybrid woven Alfa-Sisal fibers /Epoxy led to 57% and 17% increase respectively. Mechanical properties of hybrid materials depend on the properties and volume fraction of their constituents. In Table 5 is given the average value of 5 samples of the maximum stress and strain for each type of material.
Table 5
Mechanical characteristics in traction of woven's Alfa, Sisal and hybrid Alfa/Sisal fibers reinforced epoxy
Specimen
|
Breaking strain (%)
|
Tensile strength
(MPa)
|
Young modulus
(MPa)
|
Improvement rate
|
Pure resin epoxy
|
3,94 ± 0,05
|
57,55 ± 1,12
|
2846,43 ± 7,06
|
//
|
40 wt% woven Alfa fibers /Epoxy
|
2,38 ± 0,45
|
79,08 ± 3,91
|
12345,17 ± 990,54
|
333%
|
30 wt% woven Alfa fibers/Epoxy
|
1,73 ± 0,11
|
44,65 ± 2,98
|
7413,22 ± 500,36
|
160%
|
40 wt% woven Sisal fibers /Epoxy
|
4,52 ± 0,58
|
84,14 ± 5,96
|
4993,51 ± 276,32
|
75%
|
30 wt% woven Sisal fibers /Epoxy
|
3,37 ± 0,23
|
27,44 ± 6,02
|
3078,56 ± 200,79
|
8%
|
40 wt% Hybrid woven Alfa-Sisal fibers /Epoxy
|
3,80 ± 0,37
|
53,56 ± 4,00
|
4467,87 ± 513,96
|
57%
|
30 wt% Hybrid woven Alfa-Sisal Fibers /Epoxy
|
4,29 ± 1,26
|
49,47 ± 9,01
|
3343,96 ± 618,63
|
17%
|
The fracture mechanism in traction of the composite was analyzed by a scanning electron microscopy. From the obtained images (Fig. 11d), it is found that the reinforcement is well incorporated in the middle of the matrix and the transverse fibers have not been broken and adhered well to the matrix. One also notices some cracks in the matrix and the fracture of the transverse fibers leaving some empty holes.
Three-points bending characterization
It is noted that the best results are obtained for materials with 40wt% volume fraction of reinforcement for the both types of fibers. The bending strength and the strain can be calculated as follows [46].
$$\sigma =\frac{3\bullet F\bullet d}{2\bullet b\bullet {h}^{2}}$$
$$\epsilon =\frac{6\bullet h\bullet D}{{d}^{2}}$$
Where σ is the stress of the maximal deflexion (MPa); F is the breaking force (N); d is the distance between supports (mm); b is the width of the specimen (mm); h is the thickness of the specimen (mm); D is the maximal deflexion (mm) and ℰ is the strain of the maximal deflexion. The obtained results are presented in Table 6.
Table 6
3-points bending characterization of woven’s Alfa, Sisal and hybrid Alfa/Sisal fibers reinforced epoxy
woven fibers /Epoxy
|
D max (mm)
|
F max (N)
|
Bending strength 𝜹max (MPa)
|
ℰmax (%)
|
Bending modulus (MPa)
|
Improvement rate
|
Pure Epoxy
|
17,40 ± 3,22
|
99,80 ± 16,07
|
59,88 ± 9,64
|
0,10 ± 0,02
|
3165,83 ± 424,52
|
//
|
40 wt% Alfa fibers/Epoxy
|
4,60 ± 0,84
|
127,49 ± 16,68
|
76,49 ± 10,01
|
0,03 ± 0,00
|
6712,16 ± 809,69
|
113%
|
30wt% Alfa fibers/Epoxy
|
3,85 ± 0,31
|
75,60 ± 9,24
|
45,36 ± 5,54
|
0,02 ± 0,00
|
5034,39 ± 49,36
|
60%
|
40wt%Sisal fibers/Epoxy
|
10,89 ± 4,73
|
132,06 ± 20,08
|
79,24 ± 12,05
|
0,06 ± 0,03
|
5638,00 ± 436,61
|
79%
|
30wt%Sisal fibers/Epoxy
|
7,80 ± 1,24
|
119,07 ± 12,14
|
71,44 ± 7,28
|
0,05 ± 0,01
|
5346,50 ± 606,11
|
69%
|
40wt%Hybrid Alfa-Sisal /Epoxy
|
8,03 ± 2,07
|
88,99 ± 12,52
|
53,39 ± 7,51
|
0,05 ± 0,01
|
6471,84 ± 508,67
|
105%
|
30wt%Hybrid Alfa-Sisal /Epoxy
|
6,19 ± 1,19
|
103,05 ± 17,69
|
61,83 ± 10,61
|
0,04 ± 0,01
|
6451,53 ± 660,47
|
104%
|
For all types of materials considered, the best results of 3-points bending were compared to those of pure epoxy resin and showed a significant increase of Young's modulus of approximately 113% and 60%, respectively for 40wt% and 30wt% woven Alfa fibers /Epoxy. In addition, 40wt% and 30wt% Hybrid woven Alfa-Sisal fibers /Epoxy generated an increase of 105% and 104% respectively. Moreover, 40wt% and 30wt% woven Sisal fibers/Epoxy gave 79% and 69% increase respectively. Hybrid materials have properties which depend on those of their constituent and their volume fraction (Fig. 11b).
Besides, as indicated in (Table 6), the bending modulus obtained for each tested material appears to be much higher than the value of 3100MPa corresponding to Flax yarn long/SPI [7] and Polyester/Alfa [46] respectively.
Compression characterization
Different behaviors in compression of the different types of materials considered are noticed. For the composite material reinforced with alfa fibers represents a typical curve for a rigid material breaking at small deformations of 1.77 mm and a greater force of 2736.27 kN without presenting a yield point as for plastic materials. As for the other materials, they rather represent a curve typical of a rubber-like plastic material which breaks at higher deformations Table 7, Fig. 11c.
Tableau 7 Compression characterization of woven’s Alfa, Sisal and hybrid Alfa/Sisal fibers reinforced epoxy.
Materials
|
Displacement (mm)
|
Force (kN)
|
Modulus (MPa)
|
Improvement rate
|
Pure Epoxy
|
3,99 ± 00
|
1426,12 ± 120,10
|
3867,75 ± 198,39
|
//
|
40% Woven Alfa
fibers/Epoxy
|
2,02 ± 1,13
|
2883,46 ± 90,91
|
7016,82 ± 965,36
|
81%
|
30% Woven Alfa
fibers/Epoxy
|
3,034 ± 1,23
|
2322,09 ± 50,29
|
4590,18 ± 608,46
|
19%
|
40% Woven Sisal
fibers/Epoxy
|
3,26 ± 1,07
|
1694,61 ± 124,83
|
5854,74 ± 236,97
|
51%
|
30% Woven Sisal
fibers/Epoxy
|
3,99 ± 00
|
1783,07 ± 565,16
|
5568,22 ± 659,87
|
44%
|
40% Woven hybrid
fibers/Epoxy
|
2,49 ± 0,56
|
1067,81 ± 224,38
|
5709,46 ± 450,22
|
48%
|
30% Woven hybrid
fibers/Epoxy
|
3,34 ± 0,39
|
1089,62 ± 179,11
|
4584,79 ± 131,63
|
19%
|
The following histograms (Fig. 12) show the comparison of different composites of Young moduli and strength in tension, 3-points bending and compression tests.