The Novel Use of Sodium Borohydride as a Protective Agent for the Chemical Treatment of Vegetable Fibers

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1 Fibers and Polymers 2012, Vol.13, No.5, DOI /s The Novel Use of Sodium Borohydride as a Protective Agent for the Chemical Treatment of Vegetable Fibers A. G. O. Moraes*, M. R. Sierakowski 1, and S. C. Amico Department of Materials Engineering, Federal University of Rio Grande do Sul, Porto Alegre 15010, RS, Brazil 1 BioPol, Department of Chemistry, Federal University of Paraná, Curitiba 19081, PR, Brazil (Received February 10, 2011; Revised November 11, 2011; Accepted December 4, 2011) Abstract: The vegetable fibers used as reinforcement for polymer matrix composites are usually treated to improve their adhesion with the matrix. The chemical treatment with sodium hydroxide (NaOH) is widely employed, but it may damage the fiber surface structure, reducing its strength. This novel study is related to the use of hydride ions (H ) as protective agent for vegetable fibers, under alkaline treatment, as a way to promote their use in polymeric composites. Sisal fibers were modified by immersion in a NaOH aqueous solution (2, 5, and 10 % wt/vol) with or without the addition of sodium borohydride (NaBH 4 ) (1 % wt/vol) under different treatment conditions. The treated fibers were characterized via density and moisture content analyses and also using scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The effectiveness of NaBH 4 to protect the sisal fiber was more pronounced in moderate NaOH concentrations (5 %) at room temperature or higher for shorter alkaline treatment times. Keywords: Vegetable fibers, Alkaline treatment, NaOH, Polysaccharide degradation, Borohydride ions Introduction Vegetable fibers represent an alternative for the partial replacement of glass fibers in polymer composites. Indeed, many industries started using composites reinforced with vegetable fibers, such as the automotive sector [1] which already uses them in a variety of components. Problems related to heterogeneity, fiber/matrix adhesion, surface characteristics [2], water absorption, moisture content, and limited thermal and mechanical properties, however, hinder their use in certain environments and more stringent loadbearing requirements. The main components of vegetable fibers are cellulose, hemicellulose, lignin, pectins, and waxes. Cellulose is a natural polymer consisting of D-anhydroglucose (C 6 H 11 O 5 ) repeating units joined by β-1,4-glycosidic linkages at C 1 and C 4 position. Each repeating unit contains three hydroxyl (OH) groups and the degree of polymerization (DP) is around 10,000 [3]. Hemicellulose has an open structure containing many OH and acetyl (COCH 3 ) groups, being partly soluble in water and hygroscopic. Lignins are amorphous, highly complex, mainly aromatic, polymers of phenylpropane units and show the least water sorption among the main vegetable fiber constituents [4]. Brazil is the world's largest producer of sisal [3], one of the most used vegetable fibers, followed by Kenya and Tanzania. Sisal fibers are extracted from the leaves of the sisal plant (Agave sisalana), which is mainly exploited in Northeast region of Brazil. Some benefits of this fiber include easy cultivation and surface modification [5]. Surface treatments are carried out in vegetable fibers to *Corresponding author: alvaro.moraes@ufrgs.br add specific chemical groups or to remove chains of weakly bonded material and low molecular weight groups [2,6], improving interfacial adhesion with a hydrophobic polymeric material and decreasing water absorption. Many papers report on the effect of surface treatments on distinct vegetable fibers, among them physical treatments [7] (e.g. corona and plasma) and a variety of chemical treatments, e.g. sodium hydroxide (NaOH), acetylation [3], silane coupling agents, N-isopropylacrylamide, N-substituted methacrylamide, benzoylation, acrylation, permanganate, peroxide and isocyanate, cyanethylation, to name a few [4,8]. The most reported treatment for vegetable fibers is the alkaline treatment, which is usually performed using a NaOH solution. The important modification carried out by this treatment is the disruption of the hydrogen bonding (in the OH group) in the network structure due to the ionization of this group to alkoxide [4], leading to an increase in surface roughness and active surface area. This treatment removes a certain amount of lignin, wax, and oils covering the external surface of the fiber cell wall and also depolymerizes cellulose. Investigation of the degradation mechanism [9-12] of polysaccharides (e.g. cellulose and xyloglucan) under alkaline aqueous conditions and the use of protective agents containing borohydride ions (BH 4 - ) [10] have recently been applied to a vegetable fiber [13]. Chemical compounds containing hydride ions (H - ), e.g. sodium borohydride (NaBH 4 ), can act as a reducing agent for the end-group (i.e. aldehyde) present in the C-1 free polysaccharides chain (consisting of monomeric units, the monosaccharides, 3-Oand/or 4-O-substituted) during treatment under alkaline conditions. For the cellulose, the use of an appropriate reducing agent could prevent degradation of terminal 641

2 642 Fibers and Polymers 2012, Vol.13, No.5 A. G. O. Moraes et al. Table 1. Nomenclature used to identify the treatments applied to the sisal fibers H 2 O_t30, H 2 O_t60 Washing with water for 30 or 60 min, respectively OH2_t60, OH2_BH_t60 Treatment with 2 % (wt/vol) of NaOH for 60 min, without or with 1 % (wt/vol) of NaBH 4, respectively OH5_t15, OH5_t30, OH5_t60 Treatment with 5 % (wt/vol) of NaOH for 15, 30 or 60 min, respectively OH5_t60_T40, OH5_BH_t60_T40 Treatment with 5 % (wt/vol) of NaOH for 60 min at 40 C, without or with 1 % (wt/vol) of NaBH 4, respectively OH5_BH_t15, OH5_BH_t30, Treatment with 5 % (wt/vol) of NaOH and 1 % (wt/vol) of NaBH 4 for 15, 30 or 60 min, respectively OH5_BH_t60 OH10_t60, OH10_BH_t60 Treatment with 10 % (wt/vol) of NaOH for 60 min, without or with 1 % (wt/vol) of NaBH 4, respectively monosaccharide units by a mechanism called end-wise degradation or β-elimination (peeling or unzipping) [12]. On this context, this research aims to investigate the use of H as a protective agent for vegetable fibers under alkaline treatment in a way to promote their use in polymeric composites. Experimental Materials Sisal fibers were supplied as ropes by Casa Gaúcha de Barbantes (Porto Alegre/RS, Brazil) and the linear density for the ropes and the fiber itself was 2588 and 31 tex, respectively. Glacial acetic acid 99.7 wt.% (Quimex), sodium borohydride 97.0 wt.% (Nuclear), and sodium hydroxide 98 wt.% (Vetec) were used as received. Fiber Treatment and Characterization The sisal fibers were surface modified by immersion in a sodium hydroxide NaOH aqueous solution (2, 5, and 10 % wt/vol) under mechanical stirring (70 rpm) with or without the addition of sodium borohydride NaBH 4 (1 % wt/vol) in batches of 3.8 g of fiber per 500 ml of solution for 15, 30, or 60 min at room temperature (20 o C) or at 40 o C. The remaining solution on the fibers was neutralized with an acetic acid aqueous solution HAc (0.2 % wt/vol), followed by washing with distilled water, drying in an air-circulation oven for 1 h at 105 ± 1 o C, and conditioning. The various fiber samples studied were referred to as shown in Table 1. The in natura fiber and the fibers which had only been immersed in distilled water for 30 or 60 min (named H 2 O_ t30 and H 2 O_t60, respectively) were used as control groups. Apparent density of the fibers was determined according to ISO with the use of a pycnometer with distilled water at room temperature. The moisture content of the fibers was estimated according to ASTM D2654, using an air-circulation oven (2 h at 105 ± 1 C). Surface morphology of the fibers was studied using scanning electron microscopy (SEM) in a JEOL microscope model JSM The samples were pre-coated with gold and the microscope operated at 10 kv. Thermogravimetric analysis (TGA) was performed in a Table 2. Apparent density and moisture content of the various studied fibers Treatment Density (g/cm 3 ) Moisture content (%) In natura 1.28 (± 0.02) 9.1 (± 0.8) H 2 O_t (± 0.02) 7.8 (± 0.3) H 2 O_t (± 0.01) 7.7 (± 0.2) OH2_t (± 0.01) 6.1 (± 0.1) OH2_BH_t (± 0.01) 6.0 (± 0.4) OH5_t (± 0.01) 7.9 (± 0.2) OH5_BH_t (± 0.01) 6.9 (± 0.4) OH5_t (± 0.01) 7.7 (± 0.2) OH5_BH_t (± 0.01) 6.0 (± 0.2) OH5_t (± 0.01) 7.8 (± 0.2) OH5_BH_t (± 0.01) 6.5 (± 0.1) OH5_t60_T (± 0.01) 5.7 (± 0.1) OH5_BH_t60_T (± 0.03) 6.0 (± 0.4) OH10_t (± 0.02) 6.1 (± 0.4) OH10_BH_t (± 0.01) 5.9 (± 0.4) TA Instruments equipment (model 2050) under N 2 atmosphere and using a heating rate of 20 C/min from c.a. 20 to 1000 C. Fourier transform infrared spectroscopy (FTIR) was carried out in a Perkin Elmer spectrometer model Spectrum 1000, using the KBr disc technique containing about 6 wt.% of the sample in the 4000 to 400 cm -1 range. Results and Discussion Density of the in natura sisal fiber and the treated fibers are shown in Table 2. The results show that fiber density increased after chemical treatment (from 1.28 to g/ cm 3 ) but decreased after the simple washing of the fibers (1.13 g/cm 3 ). The water bath acts on the primary cell wall only, whereas the alkaline treatment can operate at the S 1, S 2 and S 3 layers of the secondary cell wall of the fiber, as reported by Mwaikambo and Ansell [5]. Interaction of the NaOH solution at the S 3 layer, leads to densification at the S 2 layer and the stricter the treatment, the higher the density [5]. The NaBH 4 predominantly acts on the S 1 and S 2 layers,

3 Use of Sodium Borohydride for Vegetable Fibers Fibers and Polymers 2012, Vol.13, No Figure 1. SEM micrographics of sisal fibers: (a) in natura, (b) H 2 O_t60, (c) OH2_t60, and (d) OH2_BH_t60. Magnification ( 400). preventing the decrease in the degree of polymerization of cellulose on the S 3 layer during alkaline attack. The NaBH 4 showed the most significant effect for shorter treatment times, leading to higher density, as can be seen for the 5 % NaOH treatment with NaBH 4 for 15 min. On the other hand, for longer times, density was found to be similar or to slightly decrease, suggesting that there was less aggregation of fibrils during chemical treatment, with greater interaction with the solution which may promote H action. This may imply that there is an ideal condition regarding NaOH concentration, solution ph, treatment time, and temperature to solubilize the various polysaccharides [10]. Table 2 also shows the moisture results of the various sisal fibers analyzed. This is an important characteristic considering that one of the drawbacks of using vegetable fibers as reinforcement in composites is their high water absorption, which contributes to material degradation, having a negative effect on their mechanical performance. The moisture content of in natura sisal (9.1 %) decreased after washing ( %) and with the various treatments ( %). However, the various treatments in the NaBH 4 presence resulted in a consistent reduction of moisture content compared to the respective treatment without this agent, which represents another positive feature of this type of treatment. The treatment with NaBH 4 at stricter conditions (at 40 C or using 10 % NaOH), however, did not decrease moisture content of the fibers. At high temperature, the NaBH 4 would somehow affect the transformation of the OH groups of the cellulose into alkoxide groups. The treatments also affected fiber texture. The in natura sisal is rough to touch and the treatments turned the fibers softer (H 2 O_t60) or less-colored (treatments with NaOH with and without NaBH 4 ). Figure 1 shows SEM micrographs of Figure 2. SEM micrographics of sisal fibers: (a) OH5_t60, (b) OH5_BH_t60, (c) OH5_t60_T40, (d) OH5_BH_t60_T40, (e) OH10_t60, and (f) OH10_BH_t60. Magnification ( 400). some of the studied fibers. Figure 1(a) depicts the longitudinal view of in natura sisal fiber bundle where some cell material may be seen holding together adjacent filaments at the surface. Washing with water removes impurities of various kinds commonly found at the surface of commercial sisal yarns without affecting its integrity (Figure 1(b)). In Figure 1(c), it may be seen that the chemical attack of the fiber with 2 % NaOH (wt/vol) left some loosely bonded cell material at the surface and may also have removed some of the fiber components. Nevertheless, Figure 1(d) for the fiber treated with 2 % NaOH (wt/vol) and 1 % (wt/vol) of NaBH 4 reveals again a smoother surface, indicating that this agent is helping protecting the fiber against chemical attack. Considering that all micrographs in Figures 1(a)-(d) were taken at the same magnification, a considerable variation in fiber size can be noticed. This heterogeneity is expected for a vegetable fiber such as sisal, but some of this variation may also be attributed to the chemical treatment itself as reported by John and Anandjiwala [3]. Figures 2(a)-(f) show SEM micrographs of sisal fibers after distinct treatments carried out for 60 min. Washing with water or treating under mild conditions (OH2_t60 and OH2_BH_t60, Figures 1(c) and 1(d), respectively) is not expected to remove amorphous fractions of the fiber surface

4 644 Fibers and Polymers 2012, Vol.13, No.5 A. G. O. Moraes et al. Table 3. Weight loss (%) under TGA of the various studied fibers Treatment Humidity Hemi Remaining cellulose Lignin Cellulose mass at 220 C In natura H 2 O_t H 2 O_t OH2_t OH2_BH_t OH5_t OH5_BH_t OH5_t OH5_BH_t OH5_t OH5_BH_t OH5_t60_T OH5_BH_t60_T OH10_t OH10_BH_t Figure 3. TGA (a) and DTG (b) curves for in natura and some treated sisal fibers. (primary cell wall), and is also not able to completely remove the S 1 layer of the secondary cell wall. However, the treatment with 5 % NaOH (wt/vol) was responsible for initiating the fiber defibrillation process i.e. fiber integrity was compromised (Figure 2(a)). The micrograph of the fiber after treatment with 5 % NaOH (wt/vol) shows less loose materials at the cell wall in comparison with the respective treatment containing NaBH 4, suggesting that, in the former, they have been partly dissolved during alkaline attack. Perhaps the loose material, mainly lignin and hemicelluloses, highly soluble in NaOH, was originally the binding material of the primary cell wall. It is important to stress out that the use of NaBH 4 during the treatment with 5 % NaOH (wt/vol) (Figure 2(b)) was responsible for preserving fiber integrity and, in fact, its visual aspect was not much different from the washed fiber (Figure 1(a)), but without impurities (clean fibers are visible) and with improved roughness which in turn promotes fiber/ matrix interaction in composites via mechanical locking. The OH5_BH_t60_T40 treatment (Figure 2(d)) promoted the exposure of fibrils (it is suggested that most of them are in the S 2 layer, which is responsible for the mechanical properties of the fiber [5]), which can indicate better fiber/ matrix interaction by interdiffusion. The treatment with OH10_t60 (Figure 2(e)) was too drastic, causing disintegration and considerable swelling of the fiber (probably due to action at the S 3 layer), even though the NaBH 4 (Figure 2(f)) may have helped preventing fiber swelling. Figure 3(a) shows the TGA curves for in natura and treated sisal samples. The fibers can be considered thermally stable up to 200 o C, with only minor mass loss attributed to moisture. High service temperatures are not suitable for polymer composites that use these fibers because severe fiber thermal degradation is expected. It is difficult to evaluate if thermal resistance of the fibers varied with the treatment, but it appears that the thermal stability of the OH5_BH_t60 sisal fiber is slightly higher than that for the OH5_t60. The mass loss between 200 o C and the peak temperature shown by the in natura fiber (Table 3) can be attributed to the thermal decomposition of hemicellulose, lignin (the rupture of α- and β-aryl-alkyl-ether bonds) and the rupture of the glycoside link of the cellulose molecule. The mass loss at higher temperatures involves the decomposition of cellulose oligomers yielding levoglucosans and low molecular mass volatile compounds, like ketone, aldehydes, furans, and pyrans [14]. On the other hand, the washed sisal fibers did not show evidence of the hemicellulose peak (Table 4). The various sisal fibers treated with NaOH showed lower mass loss and displayed only a single peak

5 Use of Sodium Borohydride for Vegetable Fibers Fibers and Polymers 2012, Vol.13, No Table 4. DTG peak temperatures ( o C) of the fibers as a function of the time, NaOH concentration and temperature Treatment Moisture Hemi cellulose Lignin Cellulose In natura H 2 O_t H 2 O_t OH2_t OH2_BH_t OH5_t OH5_BH_t OH5_t OH5_BH_t OH5_t OH5_BH_t OH5_t60_T OH5_BH_t60_T OH10_t OH10_BH_t found in branched hemicellulose chains and esterified and carboxylic groups of pectin [15], indicating that deesterification of the pectin is occurring during alkaline treatment. Furthermore, it is important to stress out that the use of NaBH 4 did not interfere in the chemical treatment with NaOH, i.e. lignin and impurities that contained the C=O group were still removed. Conclusion Sisal fibers can undergo morphological and compositional changes as a result of washing with water and, especially, after chemical treatment with NaOH, with or without NaBH 4 and/or temperature. Degradation of the fiber became more severe as the temperature and the concentration of the NaOH solution increased, and the treatment with 10 % NaOH (wt/vol) for 60 min at room temperature was considered inadequate. The investigation carried out in this work indicates the potential of using NaBH 4 as a protective agent for sisal fibers when an alkaline fiber treatment is employed, with promising results in terms of moisture content and morphological aspect of the fiber, without affecting, however, the effectiveness of the treatment, as evidenced by FTIR. The NaBH 4 was effective in protecting the sisal fiber during alkaline treatment in moderate concentration (5 % NaOH), even at higher temperature (40 o C), but this did not seem to occur at high NaOH concentrations (10 %). The combined analyses carried out for the various studied fiber treatments indicate the use of 5 % NaOH (wt/vol) and 1 % (wt/vol) of NaBH 4 for 60 min at room temperature as the most promising for sisal if this fiber is to be later used as reinforcement for composites, especially due to low moisture content and preserved fiber morphology. Acknowledgements Figure 4. FTIR spectrum for in natura and some treated sisal fibers. (Figure 3(b)), with temperatures that can be associated to that of the thermal decomposition of α-cellulose. The use of NaBH 4 caused an increase in the cellulose content for all 5 % NaOH (wt/vol) treatments. Figure 4 shows the FTIR spectrum of sisal fibers in natura, washed with water, and treated, at 20 or 40 o C, with 5 % NaOH with and without the 1 % NaBH 4. The most noticing feature of this figure is that the band at 1738 cm -1, which is present in the in natura and the washed fiber, disappears for all treatments with NaOH, even at room temperature. This band is assigned to the stretching of the carbonyl (C=O) bond in carboxylic groups that is mostly The authors would like to thank CNPq and CAPES for the financial support. References 1. R. Zah, R. Hischier, A. L. Leao, and I. Braun, J. Clean Prod., 15, 1032 (2007). 2. N. Cordeiro, C. Gouveia, A. G. O. Moraes, and S. C. Amico, Carbohydr. Polym., 84, 110 (2011). 3. M. J. John and R. D. Anandjiwala, Polym. Compos., 29, 187 (2008). 4. X. Li, L. G. Tabil, and S. Panigrahi, J. Polym. Environ., 15, 25 (2007). 5. L. Y. Mwaikambo and M. P. Ansell, J. Mater. Sci., 41, 2497 (2006). 6. B. Pukánszky, Eur. Polym. J., 41, 645 (2005). 7. A. Baltazar-y-Jimenez, M. Bistritz, E. Schulz, and A.

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