Fiber composition is shown in Table 1. After mercerization, a reduction in lignin content and an increase in cellulose content were observed, combined with an accentuated loss in mass. Untreated fibers exhibit lignin contents of about 60% and 70%, for macambira and caroá respectively, which is significantly high as compared with some lignocellulosic fibers such as coconut, whose lignin content usually ranges from 37 to 44% (Fahma et al. 2011; Rosa et al. 2010; Rosa et al. 2009).
Lignin content of macambira fibers decreased about 70%, from 59.8% to 18.4%, after alkaline treatment that removed the amorphous phase of fibers, as lignin, polysaccharides and some other components such as wax (Hashim et al. 2012; Kim and Netravali 2010; Wang et al. 2003). Cellulose content, on the other hand, increased from about 28.6% to 74.5% with the same treatment. This did not indicate that cellulose increases in absolute values, but in the percentage of cellulose in relation to the total of sample.
Lignin content in caroá reduced from 69.1% to 31% after mercerization, followed by bleaching and a second mercerization. This fiber has a large percentage of amorphous material, which requires a harsher treatment. Three bleaching stages were carried out to fully remove lignin and other amorphous components. The reaction yield shows a loss of 35 wt.% for caroá fibers. The relatively cellulose content increased from 26.9% to 56.9% with alkaline treatments.
Table 1 Contents of lignocellulosic macambira and caroá fibers
Fiber
|
Fiber composition (%)
|
Cellulose
|
Hemicellulose
|
Lignin
|
Others*
|
Macambira
before mercerization
|
28.6±0.6
|
2.4±0.9
|
59.8±3.5
|
9.2
|
Macambira
after mercerization
|
74.5±1.2
|
3.4±0.5
|
18.4±0.5
|
3.7
|
Caroá
before mercerization
|
26.9±0.9
|
0.4±0.1
|
69.1±0.5
|
3.5
|
Caroá
after mercerization
|
56.9±1.1
|
8.1±0.6
|
31.0±0.9
|
4.0
|
*Others means the difference of absolute values in Table to complete 100%
Caroá and macambira fibers underwent alkaline treatment, followed by bleaching, as described in methods. Differences in morphology can be observed in SEM micrographs (Fig. 1). Non-cellulosic components are observed in untreated fibers, as shown for caroá and macambira. SEM images evidence that fibers were covered with a non-cellulosic and non-fibrous material layer. Through the treatment, mainly after bleaching, fibers exhibit a uniform appearance and a linear arrangement of fibrils as seen in the bottom images of Fig.1, which are of bleached fibers. Figure 1 shows that waxes covering fiber surface were removed after treatment.
Lignin is a natural biding agent of cellulose that makes vegetable fibers a complex structure. However, the chemical structure of lignin and cellulose are quite different (Tobimatsu and Schuetz 2019). In this sense, process as mercerization and bleaching act degrading lignin while keeping crystalline domains of cellulose practically intact (Ilyas et al. 2018) as can be seen in SEM images of Fig.1 and corroborated by the results presented in Table 1.
AFM images of Fig. 2show the morphology of nanocrystals extracted from macambira and caroá fibers. Nanocrystals from caroá fibers had diameters ranging from 31 to 59 nm and lengths from 203 to 296 nm. Crystals extracted from macambira fibers showed similar average sizes, with diameters ranging from 34 to 51 nm and lengths between 214 and 262 nm. The results showed that the aspect ratios were 5.3±1.6 and 5.2±0.7 for of caroá and macambira, respectively. The morphology of caroá and macambira whiskers is presented in Figure 2(a) and (b), respectively. This low aspect ratio, as compared to other natural fibers (Chazeau et al. 1999; Wang et al. 2006), is attributed to the alkaline/bleaching processes that not only removed lignin and other non-cellulosic components, but also may have removed the amorphous regions of cellulose, thus reducing the aspect ratio. In addition, the sulfuric acid extraction also tends to attack cellulose crystals (Li et al. 2009; Visakh et al. 2012; Wang et al. 2006).
Thermogravimetric analysis
Decomposition of the untreated fibers occurred in several stages, indicating the presence of components which decompose at different temperatures. Thermogravimetric analyses were carried out in untreated and treated fibers. The main difference between thermogravimetric curves of untreated and each treated fiber was the loss over the range of 30 to 100°C, which has been attributed to moisture uptake. In addition, other differences have been observed for onset temperature. Due to the low decomposition temperatures of hemicellulose and lignin, the mass loss of the original fiber sample started at about 210ºC, with a DTG peak observed at around 350°C. This thermal event can be associated to pyrolysis of the cellulose. For both cases, it was observed an increase in thermal stability after treatment with sodium hydroxide due to the conversion of cellulose I to cellulose II, which is more thermally stable (Barud et al. 2011; Borsoi et al. 2016).
Table 2 Crystallinity index and TONSET for caroá and macambira fibers before and after undergoing mercerization, bleaching and acid hydrolysis
Fiber
|
Treatment
|
IC (%)
|
TONSET (°C)
|
Caroá
|
Untreated
|
47.0
|
257
|
Mercerized
|
61.1
|
268
|
One bleaching step
|
61.4
|
280
|
Two bleaching steps
|
68.1
|
290
|
Three bleaching steps
|
71.6
|
288
|
Cellulose nanocrystals
|
63.8
|
184
|
Macambira
|
Untreated
|
47.1
|
238
|
Mercerized
|
67.5
|
282
|
One bleaching step
|
72.6
|
294
|
Two bleaching steps
|
70.1
|
298
|
Cellulose nanocrystals
|
69.2
|
185
|
Thermal degradation curves for caroá and macambira fibers are presented in Fig. 3. DTG curves are also shown for both samples, in which peaks indicates volatiles loss, chemical transformations and decomposition of analyzed fibers. All caroá samples, except for whiskers, are thermally stable over the region of 100°C and 250°C. This temperature range is typical for processing thermoplastic-based composite materials. Macambira samples, except whiskers, have similar thermal stability, 100-240°C for fibers without treatment and 100-280°C for treated fibers.
The whiskers exhibited lower thermal stability when compared with untreated fibers and treated with sodium hydroxide and sodium chlorite, as already reported in the literature. (Azeredo et al. 2015; Carrier et al. 2011; Dorez et al. 2014; Moran et al. 2008; RazaIi et al. 2015; Thakur and Thakur 2014; Yue et al. 2015). These studies have shown that the process used (acid extraction) results in the incorporation of sulfate groups on crystal surface. Such sulfate groups have a catalytic effect on the thermal degradation reactions of cellulose.
In temperatures below 250 °C for vegetable fibers, the degradation reactions are consistently related to dehydration of water and to the formation of components such as peroxides, which works as catalyzer for cellulose degradation. The zone over the range between 250 and 409°C is known as degradation of hemicellulose component and also attributed to the break of crystalline regions of cellulose (De Rosa et al. 2010; Macedo et al. 2020; Yang et al. 2008). This degradation mechanism is quite different in comparison to that in low temperature, being treated as a reaction of fast degradation (Borsoi et al. 2016). From this range of temperature until the end of degradation, hydrogen bonds are destroyed giving rise to an accentuated change in crystallinity, yielding carbonyl and carboxyl groups that promote a considerable acceleration in the degradation of cellulose (Van De Velde and Kiekens 2002).
In addition, the whiskers have small size and larger contact surface, which facilitates thermal degradation. Whiskers from cellulosic pulp of macambira show similar thermal behavior of caroá samples, with Tonset at about 180°C and mass loss between this temperature and 600 °C.
In general, the partial removing of lignin might be controlled by the treatments of mercerization and bleaching, which means the hydrophobicity may also be managed by these treatments as our group has previously reported (Rosa et al. 2010).