Abstract
Partial delignification of alder wood specimens was performed with alkaline sulfite and peracetic acid (PAA) methods, and subsequently the delignified samples were saturated with furfuryl alcohol, as an eco-friendly chemical. The aim of delignification before furfurylation was to facilitate the penetration of furfuryl alcohol into the cell wall, in order to overcome the weak mechanical properties and hydrophilicity of alder wood as a fast growing species. The results showed that alkaline delignification removed a lower percentage of lignin despite more significant mass loss as compared to peracetic acid delignification method, which indicates more carbohydrates degradation in alkaline delignification. Infrared spectroscopy also confirmed more retention of carbohydrates in the sample delignified with peracetic acid. The polymerization of furfuryl alcohol in the cell cavities decreased the access of water molecules to the hydroxyl groups of the cell wall, increased the contact angle, and decreasing surface wettability. This effect was more noticeable in the PAA-delignified samples. Mechanical tests revealed that the delignified samples with peracetic acid had higher mechanical strength and modulus, compared to the alkaline sulfite delignified samples. This was attributable to the maintenance of carbohydrates during the delignification process.
Download PDF
Full Article
Effect of Delignification Pretreatment and Furfurylation on Practical Properties of Alder Wood (Alnus spp.)
Sayed Mohammad Hossein Yahyaee, Foroogh Dastoorian,* Maryam Ghorbani, and Seyed Majid Zabihzadeh
Partial delignification of alder wood specimens was performed with alkaline sulfite and peracetic acid (PAA) methods, and subsequently the delignified samples were saturated with furfuryl alcohol, as an eco-friendly chemical. The aim of delignification before furfurylation was to facilitate the penetration of furfuryl alcohol into the cell wall, in order to overcome the weak mechanical properties and hydrophilicity of alder wood as a fast growing species. The results showed that alkaline delignification removed a lower percentage of lignin despite more significant mass loss as compared to peracetic acid delignification method, which indicates more carbohydrates degradation in alkaline delignification. Infrared spectroscopy also confirmed more retention of carbohydrates in the sample delignified with peracetic acid. The polymerization of furfuryl alcohol in the cell cavities decreased the access of water molecules to the hydroxyl groups of the cell wall, increased the contact angle, and decreasing surface wettability. This effect was more noticeable in the PAA-delignified samples. Mechanical tests revealed that the delignified samples with peracetic acid had higher mechanical strength and modulus, compared to the alkaline sulfite delignified samples. This was attributable to the maintenance of carbohydrates during the delignification process.
DOI: 10.15376/biores.19.4.9169-9190
Keywords: Peracetic acid delignification; Alkaline delignification; Mechanical properties; Physical properties; Contact angle
Contact information: Sari Agricultural Sciences and Natural Resources University, Dep. of Wood and Cellulosic Products Engineering; *Corresponding author: f.dastoorian@sanru.ac.ir
INTRODUCTION
Wood, as one of the most sustainable “green” building materials on Earth, has been widely used from ancient times to the modern day (Akpan et al. 2021; Ding et al. 2022). In addition to advantages such as naturalness, acceptable specific strength, ease of use, and renewability (Dogu et al. 2016), wood has also several disadvantages, such as moisture absorption, dimensional instability, and the possibility of biodegradation (Navi and Heger 2004). With increasing population growth and development of wood demand, the supply of natural forests has decreased sharply (Karadeniz et al. 2009). Therefore, fast-growing species such as eucalyptus, poplar, and alder, having short growth periods, can be considered as alternatives to slow-growing species (Yahyaee et al. 2022). Caucasian alder (Alnus subcordata) is one of the native fast-growing trees in the forests of northern Iran and Caucasus, which reaches its maximum growth at the age of 50. Due to its high biomass production (with volume growth of 10 to 15 cubic meters per hectare), this species is suitable for wood cultivation to rebuild and rehabilitate deforested areas. Undesirable properties of Alnus spp. wood such as low density (385 to 535 kg/m3), low hardness, low mechanical strength, high moisture absorption, and mediocre natural durability (Bakhshi, 2022; Reh et al. 2024), limit the use of these fast-growing species (Tu et al. 2014). To overcome these limitations, various modifying methods, such as mechanical, thermal, and chemical treatments, can be used. The chemical modification leads to reaction of a chemical with cell wall components (cellulose, hemicellulose, and lignin), changes the nature of cell wall macromolecules, which in turn leads to higher dimensional stability and natural durability (Hill 2006). In recent years, the chemical modification of wood by the furfurylation method has been regarded considerably (Yao et al. 2017; Yang et al. 2019; Li et al. 2020). Furfuryl alcohol is one of the furfural derivatives (Fig. 1). Furfural is a low price, bio-derived product of pentose-rich agricultural residues such as bagasse and rice husk. Due to its high polarity and solubility in water, furfuryl alcohol can penetrate the wood cell wall and form a polymer (Kong et al. 2018). According to previous reports, the amount of volatile organic compounds and aromatic hydrocarbons resulted from the burning of the furfurylated wood is meager (Lande et al. 2004), and the toxicity of this substance has not been reported so far. Therefore, furfuryl alcohol can be used as an eco-friendly chemical for wood modification to increase its dimensional stability, biological durability, and significantly improves mechanical properties (Lande et al. 2008). Because of these valuable properties and environmental benefits, the commercial production of furfurylated wood has been initiated since 2004, by Kebony Products in Norway. The resulting wood has been utilized for laboratory countertops, parquet flooring, knife handles, decking, siding, and walkways in acid plants (Schneider 2007; Lande et al. 2008).
In addition to reacting with lignin and hemicellulose, furfuryl alcohol is also capable of self-condensing and can form polymers in the cell wall cavities and wood lumens (Lande et al. 2008; Gerardin 2016; Li et al. 2016). Partial lignin removal before furfurylation can facilitate the penetration of furfuryl alcohol into the cell wall by increasing the porosity and the possibility of reacting with carbohydrates. Likewise, covalent bonding between residual lignin and furfuryl alcohol/poly furfuryl alcohol (Fig. 2) probably occurs more easily as a result of partial delignification due to the development of more porosity. This study aimed to evaluate the effect of partial replacement of cell wall lignin with polyfurfuryl alcohol on the mechanical and hydrophilic properties of alder wood.
Fig. 1. Conversion of furfural to furfuryl alcohol
Previous research has shown that furfuryl alcohol can form covalent bonds with lignin (Fig. 2) and occupy lignin-rich areas i.e., the middle lamella and corners of the wood cells (Nordstierna et al. 2008; Thygesen et al. 2010). Because of the similarity of polyfurfuryl alcohol with lignin structure, it may play a similar role in the cell wall.
Fig. 2. Self-condensation reaction of furfuryl alcohol and its reaction with lignin (Gerardin 2016)
Lignin removal with sodium hydroxide, known as the soda method, is a prevalent delignification technique that causes substantial degradation of hemicellulose and also, to some extent, cellulose (Song et al. 2018). In previous studies, delignification using sodium hydroxide-sodium sulfite was performed at variable temperatures and times, which reduced density, increased porosity, and simplified impregnation with different resins (Naseeruddin et al. 2013; Frey et al. 2019). Further research by Keplinger et al (2018) showed that it is possible to fabricate a composite by impregnating wood with epoxy resin following the delignification pretreatment, while maintaining the structure of fibers similar to natural wood. This process considerably increased the mechanical strength and decreased the moisture absorption. Maturana et al. (2022) evaluated alkaline sulfite partial delignification depends on three tropical hardwood samples. They found morphological structures affect the flow of the alkaline solution and subsequently the efficiency of the delignification process.
Peracetic acid delignification leads to less degradation of carbohydrates (Zhao et al. 2010) and consumes far less energy than the conventional processes such as soda and kraft. In addition, because this process is free of inorganic chemicals and the cooking organic acids are completely recyclable using distillation technology, it is considered as an environmentally friendly process (Zhao et al. 2010; Palamae et al. 2014). Investigations that have been conducted so far in the field of wood furfurylation have confirmed the positive effect of this treatment on the significant improvement of water absorption, thickness swelling, and wettability of surfaces, and that by increasing the amount of furfurylation, the physical properties will be greatly improved. But on the other hand, increasing the furfurylation level weakens the mechanical strength. The research conducted by Yang et al. (2019) showed that lignin removal facilitates the saturation of furfuryl alcohol. In the current research, delignification with an organic peracetic acid solvent, which leads to less degradation of carbohydrates, was compared with alkaline sulfite delignification relative to the physical and mechanical properties of furfurylated alder wood. According to the investigations, no research has been reported on the effect of delignification with organic solvent on the physical and mechanical properties of furfuryl wood.
EXPERIMENTAL
Materials
Alder (Alnus spp.) wood veneers with a thickness of 1.5 mm were prepared from Nekachoob Co. (Mazandaran province, Iran), peeled tangentially from the sapwood of a 65-cm diameter log. The chemicals were furfuryl alcohol (purity of ≥98%wt), hydrogen peroxide, sodium sulfite, citric acid (Merck Co., Germany), sodium hydroxide (Mojallali Co., Iran), and acetic acid (AsiaPahohesh Co. Iran).
Sodium hydroxide-sodium sulfite delignification
An aqueous solution containing 400 g of water, 20 g of sodium hydroxide, and 10.1 g of sodium sulfite was prepared. Then, wood specimens were immersed in the prepared solution (L/W = 0.10 by weight) and heated in an oven at temperature of 103 °C for 6 h. Afterward, the samples were removed from the solution and washed with boiled, distilled water until a colorless washing was obtained.
Peracetic acid delignification
Alkaline peroxide was prepared according to Misson et al. (2009). Peracetic acid (PAA) was prepared by mixing 600 mL of refrigerated acetic acid with 400 mL of alkaline hydrogen peroxide. Sulfuric acid (15 mL) was added as a catalyst, and the reagents were stirred at room temperature (35 °C) for 72 h. The wood samples were immersed in water and peracetic acid solution by 1:15 weight ratio. Delignification was performed for 2 h in an oven at 75 °C. Afterward, the samples were removed from the solution and washed with boiling water. The Klason lignin content was measured according to the TAPPI T222 om-02 (2002) on three replications from both PAA and alkaline sulfite delignified powdered samples. The percentage of delignification was calculated as Eq. 1,
where Lc and Lt denote Klason lignin values of the control and delignified samples, respectively.
The amount of mass loss (ML) after delignification was calculated according to Eq. 2, where, mo and mt denote oven-dry mass values before and after delignification respectively.
Fluorescence Microscopy
Transverse sections of 5 µm were prepared using a microtome (Did sabz Co, Iran) and placed on a microscope slide. In order to examine the residual lignin distribution in fiber’s cell wall regions of the partially delignified samples, a fluorescence microscope (Smart 300, Canada Smart Co. Newmarket, Canada) equipped with an ultra-pressure mercury lamp was used. Because of its autofluorescence property, lignin can be excited with blue light (wavelength of 488 nm) and emission occurs at wavelength of 495 to 600 nm. Therefore, lignin rich fractions would be appeared as luminescent regions (Ji et al. 2013; Donaldson 2020).
Furfurylation
First, a 50:50 mixture of furfuryl alcohol (FA) and deionized water was prepared at ambient temperature, and then 10% citric acid was added as a catalyst. The delignified samples were immersed in the mixture, and the saturation process was performed for 30 min at intervals of 5 min under 0.8 MPa vacuum. Then, the samples were covered with aluminum foil and placed in an oven at 120 °C for 8 h. In order to complete the polymerization process, the samples were removed from the foil and placed in an oven at 70 °C for 24 h. The percentage of weight gain (WPG) due to furfurylation was calculated according to Eq. 3. The specified codes for different treated samples and their density are presented in Table 1,
where M0 and M1 are the oven-dry mass values before and after impregnation, respectively.
Table 1. The Specified Codes of Different Samples
Fourier Transform Infrared Spectroscopy
Infrared spectroscopy was carried out in the Attenuated Total Reflectance (ATR) mode, using a Cary 630 spectrometer (Agilent, USA), equipped with a ZnSe diamond crystal and a deuterated triglycine sulfate (dTGS) detector. A 10×10 mm2 sample was placed in contact with the ATR crystal to absorb the evanescent waves. Spectra of three different points of a sample were recorded, and an average spectrum was obtained. The spectra of all samples were determined in the range of 4000 to 600 cm−1 with a resolution of 4 cm−1. All spectra were averaged, and baseline corrected using SpectraGryph software.
The water absorption test was conducted according to ASTM D 570 (1998) on specimens with the size of 1.5×60×60 mm3 (R × T × L). Measurements of water uptake (WA), anti-swelling efficiency (ASE), and water-repellent efficiency (WRE) were carried out with ten replicates at intervals of 2, 4, 6, 12 and 24 h and then every 24 h until a constant water absorption and thickness swelling was obtained.
The water absorption (WA), anti-swelling efficiency (ASE), and water-resistant efficiency (WRE) were calculated with Eqs. 4 to 6,
where Mo and Mt denote the oven-dried mass before and after water soaking, respectively.
In Eq. 5, Su and St stand for swelling of the control and the treated samples respectively.
In Eq. 6, WAc is the water absorption of the control samples and WAt is the water absorption of the treated ones.
Tensile Strength Parallel to Grain
Tensile test was conducted according to ASTM D638 (2003), on dumbbell-shaped type V test specimens, by using a universal testing machine (Santam Co., Iran) at speed of 5 mm/min, in 5 replications.
Bending Properties
The bending test was performed according to ASTM-D790 (2017) by the SANTAM test machine on test specimens of 50.8 mm length by 12.7 mm width on a 25.4 mm support span, at displacement rate of 1.2 mm/min, in 5 replications.
Contact Angle Analysis
The contact angles of water droplets were determined using a Digidrop contact angle measuring device equipped with an image processing system, according to the ASTM- D7334-08 (2013). A deionized water droplet was released onto the sample’s surface (10×20 mm2), and the contact angle was measured from the droplet profile until 140 s of exposure using Dino-Lite software.
RESULTS AND DISCUSSION
Mass Loss and Delignification Efficiency
As shown in Fig. 3, for the alkaline sulfite delignified samples, the mass loss was 30%, while this parameter was 25% for the delignified sample with PAA. Furthermore, Klason lignin content showed 45% of lignin removal in the alkaline process, but this value for the PAA-delignified sample was calculated as 52%. It can be concluded that in the typical delignification process (i.e., alkaline delignification), lignin, cellulose, and especially hemicellulose are largely destroyed and removed, while in the PAA delignification, despite lower mass loss, higher lignin destruction and removal was observed, which indicates the concentration of PAA on removal of lignin and other cell wall polymers (cellulose and hemicellulose) have remained essentially unchanged (Zhao et al. 2011).
During alkaline delignification at temperatures below 170 °C, at the reducing end group of a cellulose molecule, glucose units are progressively detached one by one, which is known as peeling or unzipping (Loon and Glaus 1997; Knill and Kennedi 2003; Li et al. 2017). In the present research, delignification at 103 °C for 8 h probably led to peeling and shortening of cellulose molecules, especially at the amorphous parts. Hemicelluloses, as low molecular weight carbohydrates, are extensively prone to dissolution/degradation in alkaline solutions, even at ambient temperature (Wigell et al. 2007). In PAA delignification, hydroxonium ions (OH+) produced by peracetic acid, attack the electron-rich moieties of lignin, cleave ether bonds between lignin monomers, and produce phenolic compounds (Ma et al. 2016). Aromatic ring hydroxylation leads to its demethoxylation and subsequently aromatic ring opening, which makes the oxidized lignin soluble (Barros et al. 2010). Kumar et al. (2003) reported that cellulose reducing ends had no alteration during peracetic acid delignification.
Another advantage of the PAA delignification process, considering energy consumption, compared to the alkaline one, is lower process temperature and a shorter time (Palamae et al. 2014). Kundu et al. (2021) found that the delignification of biomass at temperature of 90 °C for 5 h resulted in the effective removal of lignin with a negligible loss of carbohydrates.
Fig. 3. Comparison of mass loss and lignin removal percentage of peracetic acid (PAA) and alkaline sulfite (Alk sulfite) delignification processes.
Fluorescence Microscopy
The fluorescence microscopic method was used to examine the distribution of lignin in fiber’s cell wall regions in the control and delignified samples, as a function of lignin autofluorescence. The observations indicated delignification, leading to bulking of the cell wall structure caused by hydrophilicity of the residual sulfonated lignin in the alkaline sulfite delignified sample (Fig. 4b), and also may be attributed to more carbohydrate ratio in the PAA delignified sample (Fig. 4c).
Fig. 4. Fluorescence microscopic images of transverse section of the control (a); Alk-sulfite delignified (b); and PAA-delignified samples (c)
In the alkaline sulfite sample, lignin in the cell corners and middle lamella, shown by white arrows in Figs. 4b and 4c, appeared with a thicker luminescent layer compared to the PAA sample. Concerning the higher lignin removal of PAA delignified samples (Fig. 3), it can be concluded that in alkaline delignified samples, the major lignin removal occurred between the cell wall microfibrils. At first stages of Alk-sulfite delignification, up to 50% of lignin removal, major delignification occurs in the cell wall, while in organosolv PAA-delignification, major lignin removal occurs in middle lamella and the cell wall lignin remains almost intact (Albrecht 1971; Behera 1985; Sjostrom 1987).
Weight Percentage Gain
Comparing the weight percentage gain (WPG) after furfurylation in FA, Alk- sulfite/FA, and PAA/FA samples (Fig. 5), a significant difference was observed between the FA sample and the two delignified ones (P-value = 0.00). The WPG values of Alk- sulfite/FA and PAA/FA samples (83% and 85%, respectively) were almost threefold of the FA sample (27%). It can be concluded that the delignification pretreatment led to higher permeation of furfuryl alcohol. In PAA pretreatment, lignin removal was slightly higher than Alk sulfite pretreatment (Fig. 3). However, WPG values of PAA/FA samples and Alk sulfite/FA samples were statistically insignificant. In PAA delignified samples, the major lignin removal occurred in middle lamella and cell corners (Fig. 4c). Therefore, FA can penetrate more easily into them and because of adequate space, FA self-condensation reaction and grafting with residual lignin (Fig. 2) could take place. Xu et al. (2022) stated that methylene bridge linkage between lignin and PFA was created in the middle lamella and cell corners. In Alk sulfite delignified samples, as a result of more lignin removal from cell wall, FA can penetrate between the cell wall microfibrils and may just deposit on them without crating linkage (Ehmcke et al. 2017).
Fig. 5. Percentage of weight gain due to furfurylation in FA, Alk-sulfite/FA, and PAA/FA samples.
Attenuated Total Reflectance FTIR Analysis
The results of infrared spectroscopy for the control sample, Alk-sulfite, and PAA-delignified samples are shown in Fig. 6. The results showed that the peaks due to C=C or C=O stretching vibrations in the aromatic ring of lignin (1528 to 1626 cm-1) were observed strongly in the control sample and almost disappeared for the delignified ones. The peak associated with the C=O stretching vibration of acetyl or carboxylic acid groups of lignin and hemicellulose (1734 cm-1) was observed only in the control sample. The peak related to the alcoholic hydroxyl groups appeared in the control sample (3291 to 3490 cm-1) and with the highest intensity in the PAA delignified sample (3334 cm-1), while the peak disappeared in the alkaline delignified samples. Due to the preservation of carbohydrates, the delignified sample with PAA had a higher peak intensity and bandwidth compared to the control sample. Also, the peak related to C-O stretching vibration in carbohydrates (1027 to 1031 cm-1) was more intense in the PAA-delignified sample, which confirms the preservation of carbohydrates in this method. Appearance of three new peaks in Alk-sulfite delignified samples, at wave numbers of 1403, 1031 and 874 cm-1, assigned to S=O stretching vibration, implies the presence of remaining lignosulfonate. The peak in the range of 2860 to 3000 cm-1 corresponding to asymmetric C-H stretch vibration of alkane appeared with the lowest intensity in Alk sulfite sample, may be attributed to more degradation of carbohydrates in addition to lignin removal.
Fig. 6. FTIR spectra of control and delignified samples with alkaline sulfite (Alk sulfite) and peracetic acid (PAA)
Figure 7 presents the FT-IR spectra of untreated and furfurylated samples. The absorption band at 2850 to 2950 cm−1 related to the stretching C-H vibrations of alkanes (methoxyl group and methyl or methylene groups) while alkenes and aromatic compounds show a C-H band at 3020 to 3080 cm-1 (Liu 2021). The highest peak intensity in the range of 2850 to 2950 cm-1 was found in Alk sulfite/FA samples. This can be related to the free hydroxymethyl groups of FA monomers deposited between microfibrils (as described in WPG section). The lowest peak intensity belonged to PAA/FA samples, which is attributable to more polymerization of FA in middle lamella and cell corners. This creates methylene linkage between FA monomers or residual lignin and FA monomers and leads to band broadening slightly higher than 3000 cm-1 (indicated in Fig. 7 by a red circle).
Fig. 7. FTIR spectra of furfurylated (FA), alkali delignified and furfurylated (Alk sulfite/FA) and the PAA delignified and furfurylated (PAA/FA) samples
The intensity of the peak related to hydroxyl groups (3300 to 3600 cm-1) in furfurylated samples did not change appreciably compared to non-furfurylated one. This phenomenon implies no reaction between the hydroxyl groups of the cell wall and furfuryl alcohol. The lower peak intensity in PAA/FA samples compared to the Alk Sulfite/FA samples could be related to more FA polymerization. Beck et al. (2019) and Alvarez et al. (2021) reported similar results. The appearance of a new peak in treated samples at 787 to 790 cm-1 attributed to skeletal vibration of furan ring of FA structure (Alvarez et al. 2021) was observed, with the highest intensity in Alk-sulfite one demonstrating polymerization of FA in the cell cavities. The peak around 1707 to 1719 cm-1 in FA treated samples was attributable to C=O stretching of γ-diketons formed by some furan ring opening in the polymerized FA structures (Pranger et al. 2008; Lems et al. 2019; Li et al. 2020).
Water Absorption and Thickness Swelling
The water absorption values of the test specimens as a function of immersion time and the correlation curve between WPG and water absorption at 96 hours of immersion are shown in Figs. 8a and 8b. The highest amount of water absorption was related to the control sample, and the lowest was related to the PAA/FA, Alk sulfite/FA, and FA samples, respectively. Comparing Figs. 8a and 8b shows a direct relationship between the WPG and decrease of water absorption (Dong et al. 2015). Furfurylation causes polymer formation in the lumen and cell wall and prevents water from entering the pores. In the delignified samples, due to the more significant penetration of FA into the wood structure and subsequently its polymerization in the pores, a significant decrease in water absorption was observed, especially in the PAA-delignified samples. Also, as shown in Fig. 9, the highest water resistance efficiency was found in PAA-delignified samples, which is due to the reduction of available spaces for water absorption, which in turn, increases the water resistance efficiency (Thygesen et al. 2010). On the other hand, the presence of furan compounds in phenolic structures (confirmed by IR spectrum, Fig. 7) that could form a chemical bond with cell wall lignin in middle lamella (Thygesen et al. 2010), decreases the accessibility of hydroxyl groups of carbohydrates to water molecules.
Fig. 8. Water absorption of the control and treated samples (a), Correlation curve of water absorption values after 96 hours and WPG (b)
Fig. 9. Water resistance efficiency of FA, Alk sulfite /FA, and PAA/FA
As can be seen in Fig. 10, thickness swelling of the control sample was significantly higher than the furfurylated samples. The furfurylated samples showed shrinkage of about 2% due to the filling of the pores with the hydrophobic poly furfuryl alcohol, as discussed in ATR-FTIR section and shown in Fig. 7 (Li et al. 2015).
Fig. 10. Thickness swelling of control, FA, Alk sulfite /, and PAA/FA
Despite higher WPG values of delignified samples, there was no significant difference between the thickness swelling values of delignified and non-delignified furfurylated samples, although the water absorption values had a direct correlation with WPG (Fig. 8b). Furfuryl alcohol polymerization or deposition in the lumen and cell wall acts as a physical barrier and decreases the accessibility of hydroxyl groups of carbohydrates to water molecules, so the thickness swelling of all furfurylated samples was almost the same, while the amount of water absorption was dependent to WPG.
The results of FTIR spectroscopy (Fig. 7) did not show any change in the intensity of hydroxyl groups, which are the hydrophilic positions of the cell wall. Gaitán-Alvarez et al. (2021) acknowledged that the formation of polyfurfuryl alcohol in the cell wall and cavity acts as a barrier to accessing the hydroxyl groups of the cell wall and enhances the dimension stability. Also, as can be seen in Fig. 11, the anti-swelling efficiency of the furfurylated samples was almost the same and above 70% due to polymerization of furfuryl alcohol and filling of the pores in the structure (Dong et al. 2015). Lande et al (2004) reported that even small amounts of furfurylation can significantly improve the anti-swelling efficacy.
Fig. 11. Anti swelling efficiency of FA, Alk sulfite /, and PAA/FA
Tensile Properties Parallel to Grain
Figure 12 illustrates the tensile strength and tensile modulus values of the control, FA, Alk sulfite/FA, and PAA/FA samples. The results of the analysis of variance indicated a significant difference between the average tensile strength values of the samples (P value = 0.00). The lowest strength was related to the Alk sulfite/FA sample (27 MPa), which can be attributed to the destruction of a significant part of the hemicellulose and the amorphous part of the cellulose (peeling) during the alkaline delignification process (Lu et al. 2020). The 30% decrease in the tensile strength of the FA sample compared to the control sample. Li et al. (2020) found that the enhancement of mechanical properties of furfurylated wood was dependent on the wood species because of different morphological structures, affecting the resin distribution and also FA concentration. They reported that higher FA concentration (>15%), had a destructive influence on the mechanical properties, and attributed this phenomenon to the cell wall bulking caused by excess FA infiltration, which unfavorably affect the integrity of cell walls.
No significant difference was observed between the strength properties of the PAA/FA and the control samples, which indicates the superiority of the peracetic acid delignification process over the alkaline delignification process due to the preservation of cellulose during delignification. In addition, the removal of lignin from the middle lamella and the substitution of furfuryl alcohol, in PAA delignified samples, do not cause a decrease in the tensile strength of fibers. In contrast, the lignin removal from the cell wall, in alkaline delignified samples, weakens the connection between cellulose microfibrils. Therefore, FA can easily penetrate into the cell wall. It just creates deposits between microfibrils without linkage and reduces the tensile strength.
The highest amount of tensile elasticity modulus was related to the PAA/FA sample, and the lowest tensile modulus was related to the Alk/FA sample. Wimmer and Lucas (1997) found that the Young’s modulus of lignin in the middle lamella is 50% lower than that of the secondary wall lignin. Degradation of middle lamella lignin as a result of PAA delignification, as depicted in Fig. 4a, and substitution of poly-furfuryl alcohol improved the tensile modulus, but in the alkaline-sulfite delignified samples, the tensile modulus decreased drastically due to the destruction of cell wall lignin (Fig. 4b). Also, lower mass loss and higher lignin removal in the PAA delignified samples (Fig. 3), implies maintaining the cellulose, which could be another reason for increasing the tensile modulus of the PAA/FA sample.
Fig. 12. Tensile Strength and modulus of the control, FA, Alk sulfite/FA, PAA/FA samples
Bending Properties
Figure 13 illustrates the bending strength and bending modulus of the samples. The highest amount of bending strength was related to the PAA/FA sample (132.3 MPa), control (104.2 MPa), FA (100 MPa), and Alk sulfite/FA (72.5 MPa), respectively. However, no significant difference was observed between the bending strength values of the control sample and the FA sample. Similar results were reported by Esteves et al. (2011). The comparison between Alk sulfite/FA and PAA/FA samples showed a significant decrease of bending strength in alkaline delignified sample, while no statistical difference was observed between PAA/FA and the control sample. The reason can be attributed to more lignin removal from the middle lamella as a result of PAA delignification and substitution of furfuryl alcohol. Li et al (2020) mentioned the positive effect of furfurylation on the bending strength of Chinese fir and poplar wood at concentration of 15%. But by increasing the concentration of furfuryl alcohol up to 70%, the bending strength decreased. The probable cause of this phenomenon was attributed to the entry of furfuryl alcohol into the cell wall at a high concentration, the bulking of the cell wall, and subsequently the disruption of the integrity of the cell wall. There was no significant difference between the flexural modulus values of the control samples (8.33 GPa), FA (7.41 GPa), and Alk sulfite/FA (6.73 GPa). However, the flexural modulus of the PAA/FA sample (12.31 GPa) was significantly higher than other ones. The cause of this phenomenon can be related to the lower role of lignin in the middle lamella on improving the modulus (Wimmer and Lucas 1997), which is destroyed by peracetic acid during the delignification process and replaced by poly-furfuryl alcohol. While in the alkali delignification process, most of the lignin is removed from the secondary wall. Also, maintaining the crystalline areas of cellulose in delignification with organic solvent is another reason for increasing the modulus of the PAA/FA sample.
Fig. 13. Bending Strength and bending modulus of the control, FA, Alk sulfite/FA, and PAA/FA samples
Contact Angle
As can be seen in Fig. 14, in the control sample, the contact angle at the moment of drop formation was 65º, and after about 30 s, the drop was entirely absorbed and disappeared. In the FA sample, the contact angle at the moment of drop impact was 125.5º; in the Alk-sulfite/FA sample, it was 125º; and in the PAA/FA sample, it was 144º. The comparison of drop behavior after 140 s also showed that the highest contact angle values belonged to the PAA/FA, Alk sulfite/FA, and FA samples, respectively. With the increase in WPG caused by furfurylation, the polymerization of furfuryl alcohol in cell cavities and cell walls increased, leading to blocking of cell lumens and cell wall pores, and decreasing access to hydroxyl groups, which increased contact angle and decreased surface wettability.
Fig. 14. Contact angle images in the first second and after 140 s of control samples (a and b), FA (d and c), Alk sulfite/FA (e and f), and PAA/FA (g and h)
CONCLUSIONS
In this work, partial delignification of alder wood specimens was done by alkaline sulfite and peracetic acid methods, and the delignified specimens were then impregnated by furfuryl alcohol. In general, peracetic acid (PAA) delignification pretreatment led to higher stiffness, higher flexural strength, and higher water repellency of furfurylated alder wood. Investigating the effect of partial replacement of lignin with poly-furfuryl alcohol on mechanical properties and hydrophilicity showed that:
- The amount of lignin removal in the alkaline-sulfite delignified and PAA-delignified samples (45% and 52%, respectively) compared to their corresponding mass loss (30% and 25%), indicated more degradation of carbohydrates in the alkaline-sulfite delignified samples.
- The fluorescence microscopic observations indicated that in alkaline sulfite delignified sample, lignin in the cell corners and middle lamella appeared with a thicker luminescent layer while in the PAA delignified sample; the major lignin removal occurred between the cell wall microfibrils.
- The results of Fourier-transform infrared (FTIR) spectroscopy confirmed the retention of carbohydrates in the PAA-delignified samples. Skeletal vibration of furan ring of furfuryl alcohol (FA) structure, observed with the highest intensity in alkaline sulfite sample demonstrating polymerization of FA in the cell cavities.
- Poly-furfuryl alcohol polymerization in the lumen and cell wall decreased the access of water molecules to the hydroxyl groups of wood, increased the contact angle, and decreased surface wettability. This effect was more noticeable in the PAA-delignified samples.
- The tensile strength values of PAA/FA and control samples were almost the same and the bending strength values of PAA/FA was significantly higher than the control sample, related to cellulose preservation during PAA delignification. Alk sulfite/FA sample had the lowest tensile and bending strengths as a result of cellulose peeling during delignification. The 30% decrease in the tensile strength of FA sample compared to the control one was attributed to the cell wall bulking caused by excess FA infiltration, which unfavorably affect the integrity of cell walls.
- The highest and lowest tensile modulus values were related to PAA/FA and Alk/FA samples, respectively.
- The flexural modulus of the PAA/FA sample was significantly higher than other samples while, was no significant difference between the flexural modulus values of control, FA, and Alk sulfite/FA samples.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Dr. Friedrich Menges for providing the SPECTRAGRYPH open-source software. The author would like to thank Sari Agricultural Sciences and Natural Resources University for financial support of this research under contract number “04-1399-11”. The authors would like to acknowledge the Central laboratory staff at Sari Agricultural Sciences and Natural Resources University (SANRU), for their help and support in FTIR analysis and fluorescence microscopy.
REFERENCES CITED
Akpan, E. I., Wetzel, B. and Friedrich, K., 2021. “Eco-friendly and sustainable processing of wood-based materials,” Green Chemistry 23(6), 2198-2232. DOI: 10.1039/D0GC04430J
Albrecht, J. S. (1971). An Investigation of the Physical-Chemical Mechanism of Selective Delignification of Wood with Peracetic Acid, Doctoral dissertation, Georgia Institute of Technology. DOI: 10.5555/19750623526
Alvarez, J., Moya, R., Mantanis, G. I. and Berrocal, A. (2021). “Furfurylation of tropical wood species with and without silver nanoparticles: Part I: Analysis with confocal laser scanning microscopy and FTIR spectroscopy” Wood Material Science and Engineering pp.1-10. DOI: 10.1080/17480272.2021.1886166
ASTM-D570 (1998). “Standard test method for water absorption of plastics,” ASTM International, West Conshohocken, PA, USA
ASTM D-638 (2003). “Standard test method for tensile properties of plastics,” ASTM International, West Conshohocken, PA, USA
ASTM-D790 (2017). “Standard test method for flexural properties of unrainforce and rainforce plastic and electrical insulated materials,” ASTM International, West Conshohocken, PA, USA
ASTM-D7334 (2013). “Standard practice for surface wettability of coatings, substrates and pigments by advancing contact angle measurement,” ASTM International, West Conshohocken, PA, USA
Bakhshi, R. (2022). “Effect of altitude on biometric, physical and mechanical properties of Caucasian alder species (Case study in Savadkuh region)” Iranian Journal of Wood and Paper Industries 13(2), 239-248.
Barros, D. P., Silva, V. L., and Colodette, J. L. (2010). “Effect of last stage bleaching with peracetic acid on brightness development and properties of eucalyptus pulp,” BioResources 5(2), 881-898.
Beck, G., Hill, C., Cocher, P. M. and Alfredsen, G. (2019). “Accessibility of hydroxyl groups in furfurylated wood at different weight percent gains and during Rhodonia placenta decay,” European Journal of Wood and Wood Products 77(5), 953-955. DOI: 10.1007/s00107-019-01445-4
Behera, N.C. (1985). Topochemistry of Delignification and its Effect on Fiber Properties of Spruce Organosolv Pulp, Doctoral dissertation, University of British Columbia. DOI: 10.14288/1.0096536
Ding, T., Cheng, Y. and Jiang, T., (2022). “Preparation and characterization of heat-treated Douglas fir wood with core–shell structure,” Forests 13(6), article 825. DOI: 10.3390/f13060825
Dogu, D., Bakir, D., Tuncer, F. D., Hizal., K. T., Unsal, O., and Kandan, Z. (2016). “Microscopic investigation of defects in thermally compressed poplar wood panels,” Maderas – Ciencia y Technology 18(2), 337-348. DOI: 10.4067/S0718-221X2016005000031
Donaldson, L. (2020). “Autofluorescence in plants,” Molecules 25(10), article 2393.
Dong, Y., Yan, Y., Zhang, S., Li., J., and Wang, J. (2015). “Flammability and physical–mechanical properties assessment of wood treated with furfuryl alcohol and nano-SiO2,” European Journal of Wood and Wood Products 73, 457-464. DOI: 10.1007/s00107-015-0896-y
Ehmcke, G., Pilgård, A., Koch, G., and Richter, K. (2017). “Topochemical analyses of furfuryl alcohol-modified radiata pine (Pinus radiata) by UMSP, light microscopy and SEM,” Holzforschung 71(10), 821-831.
Frey, M., Schneider, L., Zirkelbach, M., Dransfeld, C., Masania, K., Keplinger, T., and Burgert, I. (2019). “Densified cellulose materials and delignified wood reinforced composites,” in: ICCM22.
Gerardin, P. (2016). “New alternatives for wood preservation based on thermal and chemical modification of wood— A review,” Annals of Forest Science 73, 559-570. DOI: 10.1007/s13595-015-0531-4
Hill, C. A. S. (2006). Wood Modification, Chemical, Thermal and Other Processes, John Wiley and Sons Ltd., The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England.
Ji, Z., Ma, J. F., Zhang, Z. H., Xu, F., and Sun, R. C. (2013). “Distribution of lignin and cellulose in compression wood tracheids of Pinus yunnanensis determined by fluorescence microscopy and confocal Raman microscopy,” Industrial Crops and Products 47, 212-217.
Karadeniz, N., Tiril, A., and Baylan, E. (2009). “Wetland management in Turkey: Problems, achievements and perspectives,” African Journal of Agricultural Research 4(11), 1106-1119.
Keplinger, T., Frey, M., and Burgert, I. (2018). “Versatile strategies for the development of wood-based functional materials,” in: Bio inspiration. Biomimetic and Bio replication VIII, Vol. 10593, article 1059313. DOI:10.1117/12.2296537
Knill, C. J., and Kennedy, J. F. (2003). “Degradation of cellulose under alkaline conditions,” Carbohydrate Polymers 51(3), 281-300.
Kong, L., Guan, H., and Wang, X. (2018). “In situ polymerization of furfuryl alcohol with ammonium dihydrogen phosphate in poplar wood for improved dimensional stability and flame retardancy,” ACS Sustainable Chemistry and Engineering 6, 3349-3357. DOI: 10.1021/acssuschemeng.7b03518
Kumar, R., Hu, F., Hubbell, C. A., Ragauskas, A. J., and Wyman, C. E. (2013). “Comparison of laboratory delignification methods, their selectivity, and impacts on physiochemical characteristics of cellulosic biomass,” Bioresource Technology 130, 372-381.
Kundu, C., Samudrala, S. P., Kibria, M. A., and Bhattacharya, S. (2021). “One-step peracetic acid pretreatment of hardwood and softwood biomass for platform chemicals production,” Scientific Reports 11(1), article 11183.
Lande, S., Westin, M., and Schneider, M. (2004). “Properties of furfurylated wood.” Scandinavian Journal of Forest Research 19(5), 22-30. DOI: 10.1080/0282758041001915
Lande, S., M, Eikenes., Westin, M., and Schneider, M. H. (2008). Furfurylation of Wood: Chemistry, Properties, and Commercialization. DOI: 10.1021/bk-2008-0982.ch020
Lems, E. M., Winklehner, S., Hansmann, C., Gindl-Altmutter, W., and Veigel, S. (2019). “Reinforcing effect of (polyfurfuryl alcohol) in cellulose-based porous materials,” Cellulose 26, 4431-4444. DOI: 10.1007/s10570-019-02348-6
Li, W., H, Wang., Ren, D., Yu, Y., and Yu, Y. (2015). “Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst,” Wood Science and Technology 49, 845-856. DOI: 10.1007/s00226-015-0721-0.
Li, W., Ren, D., Zhang, X., Wang, H., and Yu, Y. (2016). “The furfurylation of wood: A nanomechanical study of modified wood cells,” BioResources 11(2), 3614-3625. DOI: 10.15376/biores.11.2.3614-3625
Li, Q., Wang, A., Ding, W., and Zhang, Y. (2017). “Influencing factors for alkaline degradation of cellulose,” BioResources 12(1), 1263-1272.
Li, W., Chen, L., and Li, X. (2019). “Comparison of physical-mechanical and mould-proof properties of furfurylated and DMDHEU-modified wood,” BioResources 14(4), 9628-9644. DOI: 10.15376/biores.14.4.9628-9644
Li, W., Liu, M., Wang, H. and Yu, Y. (2020). “Fabrication of highly stable and durable furfurylated wood materials. Part I: process optimization,” Holzforschung 74(12), 1135-1146. DOI: 10.1515/hf-2019-0286
Liu, X. (2021). “Structural identification of organic compounds- IR and NMR Spectroscopy,” in: Organic Chemistry 1, Kwantlen Polytechnic University, British Columbia, Canada, Surrey, pp. 204-230.
Lu, M., He, W., Li, Z., Qiang, H., Cao, J., Guo, F., Wang, R. and, Guo, Z. (2020). “Effect of lignin content on properties of flexible transparent poplar veneer fabricated by impregnation with epoxy resin,” Polymers 12, 1-13. DOI: 10.3390/polym12112602
Ma, R., Guo, M., Lin, K.T., Hebert, V.R., Zhang, J., Wolcott, M.P., Quintero, M., Ramasamy, K.K., Chen, X. and Zhang, X. (2016). “Peracetic acid depolymerization of biorefinery lignin for production of selective monomeric phenolic compounds,” Chemistry–A European Journal 22(31), 10884-10891.
Maturana, J. C., Arroyave, C., Hurtado, A., Echeverría, F., and Correa, E. (2022). “Effect of alkaline delignification process on the anatomical characteristics and some mechanical properties of tropical woods of different densities,” Research Square, 1-14. DOI:10.21203/rs.3.rs-2188263/v1.
Misson, M., Haron, R., Kamaroddin, M. F. A., and Amin, N. A. S. (2009). “Pretreatment of empty palm fruit bunch for production of chemicals via catalytic pyrolysis,” Bioresource Technology 100(11), 2867-2873. DOI: 10.1016/j.biortech.2008.12.060
Naseeruddin, S., Yadav, K. S., Sateesh, L., Manikyam, A., Desai, S., and Rao, L. V. (2013). “Selection of the best chemical pretreatment for Lignocellulosic substrate Prosopis juliflora,” Bioresource Technology 136, 542-549. DOI: 10.1016/j.biortech.2013.03.053
Navi, P., and Heger, F. (2004). “Combined densification and thermo-hydro-mechanical processing of wood,” Materials Research Society Bulletin 29(5), 332-336. DOI: 10.1557/mrs2004.100
Nordstierna, L., Lande, S., Westin, M., Karlsson, O., and Furo, I. (2008). “Towards novel wood-based materials: Chemical bonds between lignin like model molecules and poly (furfuryl alcohol) studied by NMR,” Holzforschung 62, 709-713. DOI: 10.1515/HF.2008.110
Palamae, S., Palachum, W., Chisti, Y., and Chorit, W. (2014). “Retention of hemicellulose during delignification of oil palm empty fruit bunch (EFB) fiber with peracetic acid and alkaline peroxide,” Biomass and Bioenergy 1-9. DOI: 10.1016/j.biombioe.2014.03.045
Pranger, L., and Tannenbaum, R. (2008). “Biobased nanocomposites prepared by in situ polymerization of furfuryl alcohol with cellulose whiskers or montmorillonite clay,” Macromolecules 41(22), 8682-8687. DOI:10.1021/ma8020213
Reh, R., Kristak, L., Kral, P., Pipiska, T., and Jopek, M. (2024). “Perspectives on using alder, larch, and birch wood species to maintain the increasing particleboard production flow,” Polymers 16, article 1532.
Schneider, M. (2007). “Furfurylation of wood overview,” in: Proceedings 28th Annual General Meeting of Canadian Wood Preservation Association, pp. 118-123.
Sjostrom, E., Sorvari, J., Klernola, A., and Laine, J. (1987). “Delignification studies on spruce wood tissue fractions isolated from outer and inner cell wall regions,” Nordic Pulp and Paper Research Journal 2(3), 92-96. DOI: 10.3183/npprj–02-03-p092-096.
Song, J., Chen, C., Zhu, S., Zhu, M., Dai, J., Ray, U., and Yao, Y. (2018). “Processing bulk natural wood into a high-performance structural material,” Nature 554, 224-228. DOI: 10.1038/nature25476
TAPPI T222 om-02 (2002). “Acid-insoluble lignin in wood and pulp,” TAPPI Press, Atlanta, GA, USA.
Thygesen, L, G., Engelund E, T., and Hoffmeyer, P. (2010). “Water sorption in wood and modified wood at high values of relative humidity. Part I: Results for untreated, acetylated, and furfurylated Norway spruce,” Holzforschung 64, 315-323. DOI: 10.1515/hf.2010.044
Tu, D., Su, X., Zhang, T., Fan, W., and Zhou, Q. (2014). “Thermo mechanical densification of populus tomentosa var. tomentosa with low moisture content,” BioResources 9(3), 3846-3856. DOI:10.15376/biores.9.3.3846-3856
Van Loon, L. R., and Glaus, M. A. (1997). “Review of the kinetics of alkaline degradation of cellulose in view of its relevance for safety assessment of radioactive waste repositories,” Journal of environmental polymer degradation 5, 97-109.
Wigell, A., Brelid, H., and Theliander, H. (2007). “Degradation/dissolution of softwood hemicellulose during alkaline cooking at different temperatures and alkali concentrations,” Nordic Pulp & Paper Research Journal 22(4), 488-494.
Wimmer, R., and Lucas, B. (1997). “Comparing mechanical properties of secondary wall and cell corner middle lamella in spruce wood,” IAWA Journal, International Association of Wood Anatomists.
Xu, J., Hu, D., Zheng, Q., Meng, Q., and Li, N. (2022). “The distribution and polymerization mechanism of polyfurfuryl alcohol (pfa) with lignin in furfurylated wood,” Polymers 14(6), 1071.
Yahyaee, S. M. H., Dastoorian, F., Ghorbani, M., and Zabihzadeh, S. M. (2022). “Combined effect of organosolv delignification/polymerization on the set recovery of densified poplar wood,” European Journal of Wood and Wood Products pp.1-9. DOI: 10.1007/s00107-021-01756-5
Yang, T., Ma, E., and Cao, J. (2019). “Synergistic effects of partial hemicellulose removal and furfurylation on improving the dimensional stability of poplar wood tested under dynamic condition,” Industrial Crop and Products 139, 1-8. DOI: 10.1016/j.indcrop.2019.111550
Yao, M., Yang, Y., Song, J., Yu, Y., and Jin, Y. (2017). “Lignin-based catalysts for Chinese fir furfurylation to improve dimensional stability and mechanical properties,” Industrial Crop and Products 107, 38-44. DOI: 10.1016/j.indcrop.2017.05.038
Zhao, X., Heide, E. V. D., Zhang, T., and D, Liu. (2010). “Delignification of sugarcane bagasse with alkali and peracetic acid and characterization of the pulp,” BioResources 5(3), 1565-1580. DOI:10.15376/biores.5.3.1565-1580.
Zhao, X., Heide, E. V. D., Zhang, T., and Liu, D. (2011). “Single-stage pulping of sugarcane bagasse with peracetic acid,” Journal of Wood Chemistry and Technology 31, 1-25. DOI: 10.1080/02773811003637494
Article submitted: June 14, 2024; Peer review completed: July 4, 2024; Revisions received and accepted: September 22, 2024; Published: October 15, 2024.
DOI: 10.15376/biores.19.4.9169-9190