Effect of fique fibers and its processing by-products on morphology, thermal and mechanical properties of epoxy based biocomposites

Mechanical properties

Figure 5 shows the influence of the addition of fique to the EP mechanical properties (data available in the supplementary information). These results are also summarized and presented in Table 1.

Average Tensile and flexural stress vs deformation of neat EP and their EP-fique biocomposites.

Tensile tests show that fique powder and fibers incorporation generate significant increases (p < 0.05) in tensile modulus (TM) values between 2600 and 5700% for EP-FP and EP-UF 0° (UF oriented parallel to the applied load) compared with the neat EP matrix. This stiffening effect is caused by the higher mechanical properties of fique and a decrease in the EP mobility due to fique fibers observed in DSC tests (“Thermal Properties” section).

Tensile strength (TS) of EP-FP and EP-UF 90° (UF oriented perpendicularly to the applied load) was not significantly different from EP (p ≥ 0.05). However, TS values of EP-NWF and EP-UF 0° increased 71 and 277% respectively, compared with EP. These results show that fique powder and the unidirectional fique mats oriented at 90° acts as a filler, while non-woven and unidirectional Fique mats oriented at 0° reinforced the EP matrix.

Previous studies performed on PP-Kenaf¹⁸ and HDPE-henequen¹⁹ biocomposites suggested that biocomposites with fibers oriented parallel to the applied load (0°) were able to contribute the same applied load and possessed a much longer fiber structure due to minimal fiber breakage. Thus, aiding in the strengthening of the biocomposite structure due to a homogeneous distribution of the load. On the other hand, 90° orientation biocomposites supports less tensile loads and resulted in a higher fibers breakage^18,19. Therefore, the mechanical performance of EP-fique biocomposites is highly dependent on the structure and the fibers orientation angle.

Figure 6, shows that failure of the EP-UF 90° biocomposite material occurs and propagates along the fique fibers at 90° orientation angles. This also explains why the 0° aligned EP-UF biocomposite were able to maintain good tensile strength compared to the EP-FP and EP-UF 90° samples. Also, natural fibers with a higher cellulose content and a higher aspect ratio (L/D) may contribute to higher reinforcing efficacy, because the contact between the reinforcing elements and the matrix occur over a larger surface.

Fracture modes observed in EP-FP, EP-UF 0°, EP-UF 90°composites after tensile test.

At the same time, fique incorporation decreases the deformation at break (ε_b) of the EP matrix and could be related to the weak interfacial bonding between fique and EP as well as the stiffening effect of the interface discontinuities that affect the biocomposites deformation capacity (see “Morphology ” section). For the EP-FP samples the interfacial area between the matrix and fique particles increases due to higher superficial area, as a result εb values decreases from 90 to 2.8%. Regarding EP-fique mats biocomposites, εb decreases between 97 and 80% for EP-UF 90° and EP-UF 0° respectively in comparison with EP matrix (p < 0.05). This result could be related to a greater stress transfer between EP and fique fibers and fiber slippage within the matrix for EP-UF 0° orientation. Flexural test results show that flexural modulus values (FM) increase around 340 and 1100% for biocomposites EP-90° and EP-UF 0° respectively, compared with neat EP. The flexural strength (FS) increases around 430 and 820% for EP-90° and EP-UF 0° respectively in comparison with neat EP.

In general terms, the addition of fique fibers improved the mechanical properties of the epoxy matrix and generates a stiffening and reinforcing effect that could prove relevant for product applications such as automotive parts where rigidity and resistance are essential factors. The results showed that long fique fibers oriented at 0° generated the best mechanical properties among the analyzed samples, therefore the discussion about the fique fibers orientation is also crucial since it has effects on the performance and quality of the biocomposites.

Figure 6 shows the specimens and the fracture mechanisms of the EP-FP and EP-UF biocomposites after the tension tests. For the EP-FP and EP-UF 90° biocomposites, it should be noted that the fracture is generated by the simultaneous failure of the matrix and the dispersed fique particles or long fibers oriented at 90° within the matrix. In the case of EP-UF 0° samples, matrix failure occurs first, followed by the aligned fique fibers.

This difference in the fracture mechanisms between composites formulated with particles and oriented fibers has already been observed by other researchers^9,20. These studies concluded that the fracture plane is obtained for the area with the minimum resistance of the interface between the matrix and the fibers or in the area with a lack of fibers or adhesion with the matrix.

Tensile test results show that unidirectional fique fibers oriented at 0° act as a reinforcement of the EP matrix, increasing the tensile strength almost 300% compared to the fique powder and fique fibers oriented at 90°, which did not generate significant differences in the tensile strength. The stress generated during the tension test causes shear stresses (τ) between the fiber and the matrix that generate an interface debonding. At the edge of the interface, the stress transfer from the matrix to the fibers depends on τ at the axial interface. For EP-FP and EP-UF 90° biocomposites, the interface between the particles, the oriented fibers and the matrix is easy to unbound during the test, but in the case of EP-UF 0° biocomposites the stress (τ) generated at the edge of the interface is higher due to the greater adhesion of EP and fique fibers, as well as a higher surface of contact between the reinforcing elements and the matrix. Thus, explaining the observed fracture mechanisms and the reinforcement observed in the biocomposites with unidirectional fique fibers.

Thermal properties

The cooling curves show the absence of crystallization exotherms, which indicates that the EP and the EP-fique biocomposites do not present crystallization during cooling (Fig. 7a.) The second heating cycle (Fig. 7b) shows the glass transition temperature (Tg) of the EP at 22 °C. The addition of fique powder and fique fibers increases this temperature to 29 and 57 °C respectively. This increase in Tg values indicates that the presence of fique affects the mobility of the EP chains. However, this effect is greater for EP-UF biocomposites. It may be related to a greater reinforcing effect of the fique fibers compared to fique powder, which restricts the movement of the EP macromolecule chains. Similar results were recently reported by Hidalgo and Correa for EP-NWF mat biocomposites³.

(a) Cooling and (b) Second Heating for neat EP and their fique biocomposites.

Thermogravimetry (TG) and Derivative thermogravimetry over temperature (DTG) curves were used to determine the thermal stability of fique powder, fique fibers, EP and their fique biocomposites. The results are shown in Figs. 8 and 9. The thermal results from these tests are also summarized in Table 2.

(a) TGA and (b) DTG of fique Fibers and fique powder.

TGA of neat Epoxy resin and EP-fique Biocomposites.

For fique fibers and powder, three main sectors of mass loss were observed. The first zone located between 60 and 100° C is related to moisture evaporation, that was present in the surface of the sample. The other two were located between 250–350 °C and 350–600 °C and are related to hemicellulose and cellulose degradation respectively. The onset degradation temperatures (To) of these regions are higher for fique fibers compared to fique powder. This is related to the fique powder coming from the outer zone of the fique fibers and being produced on an industrial scale during the detangled process of the fibers. This fique powder may contain waxes and cellular content, which could reduce its thermal stability. The DTG curves show a first peak related to the maximum weight loss rate temperature (Tmax) of hemicellulose located at 256 °C for fique powder and 298 °C for fique fibers. The second peak is related to the Tmax of cellulose is higher for fique fibers.

The height of the peaks observed in the DTG curves are also related with the concentrations of several materials, as polymeric blends²¹ and lignocellulosic residues²². From the DTG curves, it is observed that the height of the peak related to cellulose decomposition is higher for fique fibers compared to fique powder, which could be related to a higher cellulose content in fique fibers.

Figure 9a shows that EP decomposition and their fique biocomposites occur in a two-step process Thus indicating that these materials have a similar thermal degradation behaviour. The first degradation step occurs between 90 and 200 °C, attributed to the decomposition of small molecules of the EP. The second degradation step, observed in the range of 250–500° C, shows the decomposition of themain polymeric chain^3,23. For both degradation steps, To values of fique biocomposites were higher than To values of neat EP. The result has already been observed and reported for EP-NWF mat biocomposites³.

Tmax values of fique biocomposites were higher than those observed for neat EP (Fig. 8b). They show that fique powder and fique fibers improve the thermal stability of EP. These results have been reported for several biocomposites materials based on thermoplastic ^5,24,25 and thermosetting matrices ^3,11 and can be considered as an advantage in the performance and service temperature of these materials.

Morphology

SEM micrographs of the EP-fique biocomposites are shown in Fig. 10. In these images it can be observed that the fique type used (powder or fiber) and the fique mat arrangement generate changes in the biocomposites morphology.

SEM micrographs of the cross-sectional area of EP-fique biocomposites.

For EP-FP biocomposites, semicircular and elongated fique powder particles were observed within EP matrix with diameters around 100 µm. The fique mat used and fiber orientation, generates changes in the direction and dispersion of the fibers within the matrix. For EP-UF 0° and EP-UF 90° biocomposites, the fibers are aligned perpendicular and parallel to the fracture plane respectively, while in the EP-NWF composites, the fibers are in a two-dimensional random arrangement. In addition, the interfacial spaces observed (yellow circles in Fig. 10) indicate a weak interfacial bonding between fique and Epoxy matrix and could be related to the observed decreasing in deformation at break (“Mechanical properties” section).

Product application

The design process of the car parts was carried out using the SolidWorks surfaces module It allows the generation of sketches of the main lines and coatings with surfaces of complex geometries that would be impossible to generate otherwise. One of the benefits of the surface module is that a 3D modeling process can be carried out, in which the main perspectives of an object are used. In turn each is arranged in its corresponding plane, to generate a lineal system in space, whose coordinates are affected by each corresponding plane. In so doing, the side, top, and front views could be sketched separately and stitched to generate complex sketches. For this study, a complete scaled car was designed, and two parts (door and hood) were manufactured using in EP-UF 0° biocomposites (Fig. 11). This selection was due to higher modulus and resistance achieved during tensile and flexural tests. The fique fibers oriented parallel to the applied load during the mechanical tests and the stiffening effect of the fibers allowed to obtain a rigid biocomposite material with tensile and flexural strength (36.6 and 21.2 MPa, respectively) comparable to other automotive commercial products reported in the literature¹⁸. Also, the resin film infusion processing could generate a considerable pressure within the mold, which then allows for a homogeneous compaction of fique fibers and EP during the curing. In this way, it is possible to obtain scaled car parts with uniform thicknesses and with good homogeneity, which can be scaled for car parts production.

Automotive parts (door and hood) produced from EP-UF 0° biocomposites (scale 1:5).