Process Optimization
Surface Dewaxing
Microwave assisted dewaxing has been configured based on wax and impurities extraction rate as verified by using FTIR. Figure 4 shows that rate of dewaxing is directly proportional to the microwave power, and as such medium and high microwave powers are recommended for higher rate of wax extraction. The low power is also capable to extract wax but at a lower rate. Therefore, a medium power was selected as it is sufficient to improve the rate of wax removal with lower energy consumption while maintaining fiber structure and preventing damage that could occur from excessive microwave power. Overall, it is found that the optimal time which maximizes fiber dewaxing is 60 minutes.
Also, FTIR as shown in Fig. 5, indicates that the major internal chemical composition of the fiber was not altered because of dewaxing which preserved the fiber structural integrity. FTIR spectra for all experiments were almost consistent, confirming that no additional absorption bands were applied to the fiber. The characteristic peaks refer to an individual component of natural fiber such as wax, cellulose, hemicellulose, lignin, pectin and water molecules. The peak at around 1730 cm− 1 is due to carbonyl (C = O) stretching of acetyl groups in hemicellulose and ester bonds of carboxyl groups in lignin and hemicellulose (Lu and Oza 2013). The complete dewaxing is verified by the disappearance of the (C = O) after one hour of treatment time using microwave assisted heating. The same process requires 4 hours in case of conventional heating assisted dewaxing. The use of Hydroxyl group (OH) is substantial for modifying the reactivity of fiber surface for further chemical crosslinking with matrix material.
Acetylation
The acidified sodium chloride solution is used to sterilize, purify, and bleach the dewaxed fibers. Figure 6 displays the degree of acetylation as a function of time at three microwave power levels. It is noted that the maximum degree of acetylation is attained at high microwave power after 20 minutes of reaction time. However, medium heating power was adopted for this analysis as it has an acceptable degree of acetylation that would not weaken the structural integrity of the fiber as opposed to the use of high power. Acetylation decreases the hygroscopic characteristic of natural fiber and become more stable due to the replacement of OH groups in DPF by acetyl groups and thus enhance hydrophobicity. This also can be indicated by OH group intensity reduction as shown in Fig. 7. Based on the FTIR of AH option, it is also found that the spectra were not potentially changed after treatment except for intensity reduction of the peak at 2855 cm− 1 which is related to hemicellulose content (Bezazi et al. 2014). Thus, the aim of acylation to bleach fibers and remove portion of amorphous biomass is confirmed. This finding was not observed in the case of CH which indicate superiority of using microwave to bleach the fiber and achieve higher yield in a short time.
Mercerization
After applying DoE analysis, the optimal conditions to maximize the crystallinity degree were found at lower concentrations of NaOH and medium levels of microwave power for a time of 20 minutes. Results indicate that high concentrations of NaOH and high microwave power tend to degrade the crystallinity degree. This was reflected by massive damage to the cellulose structure due to the increased removal rate of non-cellulosic biomass. The FTIR spectra for final yield of the treatment method is presented in Fig. 8. Bands 1336 and 1316 cm− 1 show significant differences in their absorbances. These are the bands assigned to both crystalline celluloses (Cel I and Cel II), and after treatment those band became more intense in the treated DPF than in the untreated fibers especially in case of microwave assisted heating. In addition, the absorption peaks at 3328 cm− 1 attributed to OH stretching vibration, 2900 cm− 1 attributed to CH stretching vibration, 1430 cm− 1 attributed to CH2 and OCH plane bending vibration, 1370 cm− 1 attributed to CH deformation vibration, and 896 cm− 1 attributed to anomeric carbon vibration can be observed in the spectra of treated DPF. All these are characteristic peaks of cellulose (brown shaded in Fig. 8), indicating extraction of highly rich cellulose fiber (Zhao et al. 2015). Hence, higher strength, improved hydrophobicity is achieved compared to the untreated fiber. The differences between microwave and conventional heat assisted treatment can be depicted as the higher intensity of the cellulose backbone (1030 cm− 1) and OH group (around 3325 cm1), the lower intensity of hemicellulose peak (2855 cm− 1). Therefore, the modified DPF using microwave power is anticipated to be more compatible with the base polymer resulting in improved bio-composite.
In summary, the current technique consists of three steps that should be applied in the specified sequence to achieve an optimal performance, including configured and optimized input variables. Each step is taken to accomplish a particular objective in preparation for the next step; surface dewaxing is used to extract both wax and impurities to expose the wall cell to react with imposed functional groups; acetylation is conducted to bleach and sanitize the fiber to improve its processability in the mercerization step and removes the amorphous cellulose. Mercerization is used to further remove/alleviate amorphous biomass and regulate the penetration level of NaOH-based ionic solution to react with OH group in hemicellulose and lignin to change fiber surface characteristics and increase the content of crystalline cellulose. However, considering the processing time and yield it is found that AH treatment produces better bleached and purified DPF in a shorter time (~ 1.5 hr) with a reduction of (~ 80%) compared to the CH treatment. Figure 9 portrays a flow-chart illustrating the multi-step chemical treatment method of DPF using CH and AH routes.
Biomass Composition
Biomass composition (%) of both untreated and treated fibers after each step using both CH and AH procedures are shown in Fig. 10. The untreated DPF contains mainly cellulose, lignin and hemicellulose and small quantities of wax, impurities, and pectin. After dewaxing, it is found that mostly wax, impurities, and pectin are removed from DPF in both CH and AH with slightly larger removal in case of AH as indicated by the higher percentage of remaining biomass after dewaxing. Due to acetylation, cellulose content is further increased because of the partial extraction of amorphous biomass which is mainly lignin. The most crucial step is mercerization in which NaOH is used to remove the optimal percentage of amorphous biomass without affecting fiber integrity. After biomass composition analysis, it is shown that cellulose content is increased and DPF became rich in cellulose because of reduction of the extractives and amorphous biomass removal. Moreover, removal of the amorphous material is more evident in the case of AH due to the ability of alkali solution to penetrate inside fiber subsurface.
Morphological
Scanning Electron Microscope (SEM) micrographs were obtained and interpreted to evaluate the effect of the treatment steps on fiber morphology and internal structure. The untreated surface in Fig. 11 clearly shows a cover of wax layer and other impurities. After dewaxing, roughness of this outer layer is reduced due to wax and impurities removal as confirmed by FTIR. A major removal of impurities and less rough surface are observed in the case of AH compared to CH case, which indicates a higher efficiency of the microwave power to clean the fiber surface. On the other hand, dewaxing did not result in any fiber damage or deterioration, as it only took place at the fiber surface. The unaltered cross section due to dewaxing as confirmed by SEM images (Fig. 11) reveals that the fiber structure is well preserved.
Figure 12 demonstrates the surface morphology and cross section of the acetylated fibers. Compared to the untreated and dewaxed fiber, it is noted that AH-acetylation eliminates the deposits residing at fiber surface and therefore makes a smoother surface. The microwave power stimulated acetylation to bleach surface and extract further amorphous biomass including pectin and lignin. Whereas CH-acetylation showed no major effect to the dewaxed surface. Similarly, the magnified micrographs of cross section show a minor change in case of AH to the fiber interior subsurface where the elementary fiber become stuffed and squeezed which might be due to biomass removal. This action was not clearly observed in the case of CH, as the microwave power forced the functional groups to penetrate the fiber interior structure.
Figure 13 shows images of the fiber surface and cross section of treated DPF in comparison to untreated DPF after implementation of mercerization, which is the final step of the developed process. It is evident from these micrographs that the surface topography of DPF looks rougher and cleaner in this step of treatment compared to the previous steps. This is because mercerization eliminates a substantial amount of amorphous biomass mainly lignin and hemicellulose. In addition, the fiber backbone structure is preserved, and the fiber surface becomes fibrillated supporting compatibility with synthetic polymer thereby improving fiber/polymer interfacial adhesion. It is worthwhile to mention that the treatment process carefully controls the removal of hemicellulose and lignin to prevent fiber pulping (pure dissimilated cellulose) and maintain fiber structural integrity. In this regard, SEM micrographs confirmed the potency of developed method to enhance the surface topography and preserves the fiber integrity. SEM of cross-section morphology reveals that circular bundles which form structural strength/integrity of fiber were potentially affected by the treatment method. As clearly shown in the corresponding micrographs in Fig. 13, the holes are collapsed and closed due to full removal of amorphous biomass. As lignin was removed, the middle lamella became more packed which cause a gradual elimination of micro-voids as indicated in the square in the magnified images.
Crystallinity Degree
Figure 14 displays XRD patterns of untreated and treated DPF taken after completion of entire treatment process. Results show that both CH and AH procedures did not adversely influence the physical structure of DPF, since both XRD patterns show identical peaks at 21° and 18° for I002 and Iam, respectively. Clearly, the crystallinity degree has been increased owing to the treatment process which partially remove the amorphous biomass, mostly hemicellulose and lignin and promote the crystalline cellulosic content. In specific, crystallinity degree is increased by 32% when microwave is used as a heating source, whereas conventional heating improves crystallinity by 19%. Such a difference is due to the higher removal rate of non-cellulosic biomass in AH technique.
Hydrophobicity
Referring to Table 2 and comparing to untreated DPF, it is obvious that treated DPF has achieved a significant drop in water absorption especially in the AH case. This is due to the removal of biomass that absorbs water and/or moisture at a higher rate compared to cellulose (Balla et al. 2019). Even though CH-based process was able to cut water absorption in half, the AH treated fiber is often known to be the optimal option to minimize water absorption as the cellulose content is further increased. The use of higher hydrophobic fiber improves the fiber/polymer compatibility that will ultimately enhance the bio-composite strength and durability.
Table 2
Summary of average values of physical and mechanical propertied of DPF before and after treatment.
| Water Absorption (%) | SUT* (MPa) | E** (GPa) | δEL***(%) |
UT | 10 | 80 | 7 | 0.016 |
CH | 5 | 297 | 13 | 0.033 |
AH | 3 | 444 | 24 | 0.027 |
*SUT: Tensile strength; **E: Elastic modulus; ***δEL: Elongation at fracture |
Tensile Properties
Among mechanical properties of interest are the tensile strength, modulus, and elongation at break of DPF, which were substantially increased in case of treated fibers owing to the treatment effect. Figure 15 exhibits stress-strain diagrams for the untreated and treated fibers. The elastic modulus, tensile strength and elongation at break can be extracted from the plot. It is evident from the figure that fiber processed by AH option yielded the maximum strength, but with a reduced ductility. Maximum ductility is achieved in the CH derived fiber case, but its strength is roughly 33% less than the AH-derived fiber. The trade-off between strength and ductility is a typical material behavior which is observed in this work. Clearly, the AH-based fiber is more brittle compared to the CH-based fiber as modified microstructure contribute to less ductility and high strength.
Table 2 summarizes the average values of physical and mechanical properties of untreated and treated fibers. The results presented in the table confirm that the multi-step chemical treatment process with microwave-assisted heating results in major increase in elastic modulus and tensile strength. The tensile strength improved by around 5.5-fold compared to the untreated fibers when AH is used and by 3.7-fold compared to untreated fiber when CH is used. Similarly, the elastic modulus of treated fiber increased by 3.4 factor in relative to untreated fiber when AH is used and 86% increase in the case of CH option. A substantial improvement in strength and modulus is highly valuable for the manufacture of polymer-based bio-composites. Thanks to optimized content of ingredients (cellulose, lignin, hemicellulose) and the efficient elimination of nonbeneficial components (wax, impurities), as verified by SEM images (Sect. 3.3). The elimination of such unfavorable surface contaminants further decreases the discontinuity and structural defects of the fiber. The successful completion of the current treatment process with the aid of heating, the fibrils are rearranged along the main axis, resulting in an improved resistance to deformation along the fiber axis. The experimental findings suggest that the inclusion of hemicellulose and other forms of waxy compounds up to a certain threshold will expand fiber ductility.
The current DPF is compared in terms of strength with similar date-palm fibers and other types of fibers (extracted from different trees) which were treated with various forms of mercerization (Table 3). It can be concluded from the data presented in Table 3 with confidence that the new treatment procedure has accomplished a much better improvement in tensile properties.
Table 3
Effect of various treatment methods on tensile strength of natural fibers.
Fiber Type | Treatment Procedure | SUT (MPa) | Reference |
Untreated | Treated |
DPF-CH | Figure 9. | 80 | 297 | Current Work |
DPF-AH | Figure 9. | 80 | 444 | Current Work |
Date Palm Petiole | 2% NaOH | 174 | 243 | (Lassoued et al. 2018) |
Date Palm Stem | 1% NaOH | 176 | 310 | (Aly-Hassan et al. 2015) |
Abaca | 5% NaOH | 755 | 847 | (Cai et al. 2015) |
Banana | 5% NaOH | 780 | 536 | (Mejia Osorio et al. 2012) |
Coconut | 20% NaOH | 186 | 281 | (Arsyad 2017) |
Flax | 5% NaOH | 630 | 627 | (Aly et al. 2012) |
Husk | 5% NaOH | 109 | 135 | (Wang et al. 2014) |
Sisal | 2 M NaOH | 284 | 386 | (Kim and Netravali 2010) |
Sugarcane | 5% NaOH | 170 | 205 | (Hossain et al. 2014) |
Thermo-Gravimetric Analysis (TGA)
Figure 16 shows the thermal decomposition of the untreated and treated DPF when heated from ambient temperature to 500°C. TGA displays the ratio of weight loss over the temperature range, and derivative thermogravimetric (DTG) displays the rate of mass loss, of which peaks can be interpreted as highest degree of temperature decomposition. TGA graphs show that both untreated and treated DPF experienced an initial weight loss under 110°C due to moisture and volatile substances removal. Furthermore, degradation of bio-mass components starts at 210°C and 240°C for untreated and treated DPF, respectively. Such results imply that the treatment process improves the thermal stability of fiber. In reference to DTG curves, the untreated DPF exhibited two major degradation peaks which are associated to hemicellulose and cellulose degradation, respectively (Filho et al. 2020). However, the treated fiber exhibited only one degradation peak which is related to cellulose. The minor shoulder appeared in case of CH at 285°C is attributed to degradation of hemicellulose. A similar shoulder was not detected in the case of AH as cellulose content was dominant over other types of biomasses and therefore hemicellulose degradation is overlapped within the cellulose range. Findings confirm the non-crystalline biomass elimination as the fiber degradation tend to follow similar degradation characteristics of pure cellulose (Lanjewar et al. 2019). Finally, lower char residue content of untreated DPF (20.5%) in comparison with treated DPF (26.6%) can be explained by higher volatilization of waxes, impurities, and other volatile biomasses from the untreated fiber.