Impact of Various pH Levels on 4-Methyl Catechol Treatment of Wood
Md. Tipu Sultan,a,* Md. Rezaur Rahman,a Sinin Hamdan,b Josephine Chang Hui Lai,a Zainal Abidin Talib,c and Fahmi Asyadi Bin Md Yusof d
Four types of treated wood (TW) were prepared by the impregnation of solid wood each with a solution of 4-methyl catechol at pH 8, a solution of 4-methyl catechol at pH 9, a solution of 4-methyl catechol at pH 10, and a solution of 4-methyl catechol at pH 11. These TW were characterized by Fourier transform infrared spectroscopic, X-ray diffraction (XRD), scanning electron microscopic, 3-point bending, free-free-vibration, and thermo-gravimetric analysis. FT-IR result showed that TW had more than one carbonyl absorbance band between 1745 and 1690 cm-1, while raw wood had only one carbonyl absorbance band in this regard. TW at pH levels of eight and nine showed higher crystallinity index (CIXRD) than that of raw wood. The SEM micrograph of TW at pH 8 had a smoother surface compared with other treated wood and raw wood. The modulus of elasticity (MOE), and modulus of rupture (MOR) of TW at pH 8 and pH 9 significantly increased. The raw wood exhibited a higher water uptake compared with the TW. The TGA results showed that TW were thermally less stable between 267 and 400 °C than raw wood.
Keywords: TW; Mechanical properties; TGA; XRD; SEM; pH
Contact information: a: Department of Chemical Engineering and Energy Sustainability, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia; b: Department of Mechanical Engineering, Faculty of Engineering, Universiti Malaysia Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia; c: Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia, and; d: Department ofPolymer Engineering Technology, University of Kuala Lumpur, Malaysia; *Correspondent author: email@example.com
Wood is a combination of cellulose, hemicellulose, and lignin. One can regard hemicellulose as a binder that holds crystalline hydrophilic cellulose together with the amorphous hydrophobic lignin. Since the beginning of human civilization, wood has been used to make shelters, boats, wheels, and construction tools. Recently, scientists and economists have shown a preference for wood over non-renewable synthetic materials because of its renewability. Wood lowers both the cost of the materials and energy consumption, and it recycles better than glass material (O’Donnell et al. 2004; Ku et al. 2011). Wooden materials have long been used to construct buildings because of their excellent specific strength, high specific modulus, and aesthetic properties. The plastics industry uses wood sawdust as reinforcement to improve the stiffness of composites (Stark and Rowlands 2003). One of the examples of wood usage is in musical instruments due to the better sound, color, and acoustic properties. Besides, the cracks can be well determined through the natural frequency of a wooden beam (Landis and Navi 2009).
The main drawbacks of wood are that it is dimensionally unstable, it is susceptible to microorganism attack, and that it absorbs water from the environment in its cell walls and cavities because of hydrophilic and capillarity properties (Singh and Mohanty 2007; Belgacem and Gandini 2008; Kojiro et al. 2008; Mohebby and Militz 2010; Mattos et al. 2014; Redman et al.2016). Wood also discolors when used indoors because of the oxidation of lignin (Sahin and Mantanis 2011).
Recently, wood has been treated with a variety of chemicals, such as styrene, epoxy resins, urethane, phenol formaldehyde resin, methyl methacrylate (MMA), acrylic, and vinyl monomers, and to improve its physical, hydrophobic, mechanical, and biological properties (Sultan et al. 2016; Rahman et al. 2012). Thermosetting and thermoplastic monomer wood composites have shown certain improvements in wood properties, with some limitations (Li et al. 2011). Thermosetting-related polymers, such as phenolic resins, urea-formaldehyde, and melamine-formaldehydes, show better compressive strength, moisture-shrinking, and swelling behaviors. However, wood treated with these types of polymers may be more brittle and displays only marginal improvement in morphological properties. Thermoplastic-type monomers, such as acrylate, styrene, and methacrylate, slightly improve the dimensional stability because there is no bond formation between the hydroxyl groups of cellulose and the monomer. These polymers simply fill the void spaces within the wood structure (Esteves et al. 2014). Poor chemical and physical interfacial interactions between the wood surface and chemical are the most important mechanisms of bond failure (Rowell 2012).
NaOH-treated wood performs better than wood treated with thermosetting-related polymers. In general, wood’s strength is affected when it is treated with an acid or a base at high temperatures. Alkaline solutions are more destructive to wood fibers than acidic solutions because wood absorbs alkaline solutions more readily than acidic solutions (Junior et al. 2010). Acids with pH values above 2 and bases with pH values below 10 do not greatly degrade wood fibers over short periods at low temperatures (Kollmann and Cote 1968). Undesirable water uptake also can be controlled by removing lignin and hemicellulose (Glasser et al. 1999).
Mechanical properties and the spiral angle of alkali-treated wood decrease after the removal of lignin-hemicellulose resin and pigment from the wood cell. The resulting surface of the wood is rougher, with high aspect ratio fibers to enhance the mechanical interlocking with external polymer (Mohanty et al. 2001; Barreto et al. 2011; Rahman et al. 2011). Natural fibers treated by NaOH increase their molecular orientation, thus increasing their stiffness. During alkali treatment, the fibers swell, kink, and curl, and the shape of the cell cavity changes from flat to oval (Wang et al. 2016). In addition, alkali-treated wood has an increased crystallinity index, which results in stiff wood and lower water absorption (Ghali et al. 2009).
Polymer-treated wood combines the performance and cost attributes of both wood and thermoplastics. Therefore, after chemical treatment of fast-growing and low-quality wood, it can be used effectively for sustainable development (Rahman et al. 2010a; Islam et al. 2012a; Devi and Maji 2013). To improve the adhesion and compatibility of polymer to cellulose, wood samples can be chemically pretreated with a 5% alkaline NaOH solution, then treated with an MMA/ST monomer mixture to manufacture pretreated wood polymer composite (Islam et al.2012a).
Alkaline-treated fibers show better thermal stability than untreated fibers because of a decrease in hemicellulose and an increase in residual char formation (Das and Chakrabarty 2008). However, there has been no work on tropical wood impregnation with 4-methyl catechol monomer at various pH levels.
In this study, raw wood was treated with 4-methyl catechol at various pH levels to increase its mechanical and hydrophobic properties. 4-methyl catechol was chosen to be used to treat wood, as 4-methyl catechol can be polymerized in presence of pH above 7 (Jha and Halada 2011). The thermal stability is also reported.
Defect-free and straight-grained raw wood (RW) of Kumpang (Gymnacranthera eugenifolia) was obtained from Sematan (Sarawak, Malaysia). The chemicals used to produce TW were 4-methyl catechol, methanol, and sodium hydroxide. 4-methyl catechol (95%) as a monomer and methanol (99%) as a solvent were supplied by Sigma Aldrich (St. Louis, MO). Sodium hydroxide was supplied by Merck KgaA (Darmstadt, Germany). In this study, all chemicals were of analytical grade.
The Kumpang wood was cut into three sections 1.2 m long. Each bolt was quarter-sawn to produce planks 4 cm thick and subsequently conditioned to air drying in a room with 60% relative humidity and an ambient temperature of 25 °C for one month prior to testing. The planks were ripped and machined to 340 mm (L) x 20 mm (T) x 10 mm (R) for three-point bending and free-free vibration testing.
Preparation of Treated wood (TW)
All the samples were oven dried at 105 °C to constant weight before impregnation. The oven-dried specimens and 5% 4-methyl catechol in 1000 mL methanol at pH levels of 8, 9, 10, and 11 were separately placed in an impregnation vacuum chamber at 75 mm (Hg) for 30 min. These specimens were then removed from the chamber, and excess chemicals were wiped off with a dry cloth. Specimens wrapped with aluminum foil were placed in an oven for 24 h at 105 °C for the polymerization process. When the solid raw wood specimens were treated with a solution of 4-methyl catechol at pH 8, it was named TW at pH 8; at pH 9, it was named TW at pH 9; at pH 10, it was named TW at pH 10; and at pH 11, it was named TW at pH 11.
Determination of weight percentage gain (WPG)
Weight percentage gain (WPG) was calculated using the following equation,
WPG (%) = [(Wf – Wi) / Wi] × 100 (1)
where Wf and Wi are the oven-dried weight of the TW and the raw wood, respectively.
Determination of water uptake (WU)
The oven-dried specimens were immersed in distilled water for fifteen days according to ASTM D570-98 (D570 1985) to measure the water uptake. The increase in the weight of the specimens was calculated using the following equation,
WU (%) = [(W2 – W1)/W1] × 100 (2)
where W2 is the weight of treated (or untreated control) wood samples after immersion and W1 is the weight the oven-dried sample, respectively.
All specimens were kept in the oven at 103 oC for 72 h before density determination. The oven-dry density of each sample was then determined by using the water immersion method (Bowyer et al. 2003). The calculation was as follows:
Density = Mass of wood / Volume of wood (3)
Fourier transform infrared spectroscopy (FTIR)
The infrared spectra of all specimens were recorded on a Shimadzu FT-IR 81001 Spectrophotometer (Kyoto, Japan). The wavenumber range of the scan used ranged from 4000 to 600 cm-1. The technique used was attenuated total reflection.
X-ray diffraction (XRD) analysis
X-ray diffraction (XRD) analyses for raw wood and treated wood were performed with a Rigaku diffractometer (Tokyo, Japan), operating with Cu K radiation, wavelength of 0.154 nm, 40 kV, and 30 mA. The XRD crystallinity index (CIXRD) was calculated using the following height ratio of diffraction peaks,
where I002 is the maximum intensity of the peak at 2θ at approximately 22° and Iam is the minimum intensity above baseline at 2θ at approximately 30°, which corresponds to the crystalline and amorphous parts, respectively. The d-spacing between the  lattice planes (d002) of the sample was calculated using the Bragg equation,
where λ is the X-ray wavelength (0.154 nm).
Scanning electron microscopy (SEM)
Fractured surfaces were examined to investigate the interfacial bonding between the wood cell and polymer with filler. These surfaces were imaged using a Hitachi TM3030 scanning electron microscope (SEM) supplied by JEOL (Tokyo, Japan). The SEM specimens were sputter-coated with gold by using JFC- 1600, Tokyo, Japan, prior to observation. The micrographs were acquired at a magnification of 1500.
Modulus of rupture (MOR), modulus of elasticity (MOE), and dynamic Young’s modulus (Ed) measurements
Determination of MOE, and MOR were carried out according to ASTM D-143 (2006). Three-point bending tests were conducted using a Shimadzu MSC-5/500 universal testing machine (Kyoto, Japan) operating at a crosshead speed of 5 mm/min. The modulus of rupture (MOR) and modulus of elasticity (MOE) were calculated as follows:
MOR = 1.5LW / bd2 (6)
MOE = L3m / 4bd3 (7)
where W is the ultimate failure load, L is the span between the centers of support, b is the mean width (tangential direction) of the sample, d is the mean thickness (radial direction) of the sample, and m is the slope of the tangent to the initial line of the force displacement curve.
Determinations of the dynamic Young’s modulus (Ed) for all specimens were carried out using a free-free flexural vibration testing system. The Ed was calculated from the resonant frequency using the following equation,
where I is the second moment of area of the beam’s cross-section (which is equal to bd3 / 12), d is the beam’s depth, b is the beam’s width, l is the beam’s length, fn is the beam’s natural frequency at the nth flexural mode, ρ is the beam’s density, A is the beam’s cross-sectional area, and mn is a constant related to the free-free beam vibration at the nth flexural mode. The value of mn at the first mode (n = 1) is 4.730 (i.e., m1 = 4.730).
Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) measurements were performed on 5- to 10-mg samples at a heating rate of 10 °C/min in a nitrogen atmosphere using a thermogravimetric analyzer (Model SDT Q600, TA Instruments, Selangor, Malaysia). The thermal decomposition of each sample occurred within a programmed temperature range from 30 to 600 °C under a constant nitrogen flow rate of 5 mL/min. The continuous weight loss and temperature were recorded and analyzed.
RESULTS AND DISCUSSION
Weight Percentage Gain (WPG) and Water Uptake (WU)
Table 1 shows the weight percentage gain (WPG) and the water uptake (WU). As can be seen in Table 1, the TW at pH 8 and pH 9 showed higher WPG compared with raw wood. This higher WPG was due to the –OH group of polymerized 4-methyl catechol that created the covalent bonding with the –OH groups of the wood cell wall (reflected in FTIR analysis) (Rahman et al 2010b). The weight of the degraded hemicellulose and lignin from wood were lower compared with the weight that was added by the polymerized 4-methyl catechol in wood. At pH 8 and pH 9, strong polymerization took place between 4-methyl catechol and the wood cell wall (reflected in the SEM morphology). At pH 10 and pH 11, the weights of the TW were lower than raw wood due to the removal of hemicellulose and lignin, and less polymerization of 4-methyl catechol occurred in the wood.
The TW treated with 4-methyl catechol absorbed less water compared with raw wood because of the removal of hemicellulose and lignin (Hossain et al. 2014). The raw wood exhibited a higher percentage of water absorption followed by the TW at pH 11, pH 10, pH 9, and pH 8. This higher percentage was attributed to the fact that 4-methyl catechol solution at a lower pH filled the maximum number of lumens and tracheids of wood and then became polymerized.
Table 1. WU (%) and WPG (%) of the Raw Wood *
* Values represent means of 10 replicates
The void area on the wood surface facilitates the water penetration into wood cell walls with hydrophilic hydroxyl groups. At higher pH levels, the hemicellulose and lignin were reduced, and the water absorption was decreased compared with raw wood, as reflected in the SEM and XRD measurements.
The Fourier transform infrared spectroscopy (FTIR) spectra of the untreated and treated woods are shown in Fig. 1. The functional groups obtained by FTIR measurements are shown in Table 2.
Fig. 1. The FTIR of a) raw wood, b) TW at pH 11, c) TW at pH 10, d) TW at pH 9, and e) TW at pH 8
The broad peak from 3,600 to 3,200 cm-1 is associated with –OH stretching in hydroxyl groups from cellulose (Rahman et al. 2010a). Moreover, the relative intensities of TW at pH 8 and pH 9 increased compared with raw wood. This was due to the 4-methyl catechol itself, where –OH groups appeared in the treated wood. In the presence of NaOH, 4-methyl catechol polymerized with itself. At pH 10 and 11, the relative intensities of the –OH bands of the TW decreased compared with raw wood. The TW at pH 10 and pH 11 had more hemicellulose reduction compared with the TW at pH 8 and pH 9. The wavelength from 3750 to 3600 cm-1 originated from the –OH stretching of 4-methyl catechol in the TW whereas the raw wood did not represent this peak (Karpagasundari and Kulothungan 2014). The TW showed more than one peak between 1745 and 1690 cm-1, but raw wood displayed one peak at 1,735 cm-1 which indicated that the TW contained different types of carbonyl groups. This peak was due to the conversion of the aromatic ring of 4-methylcatechol to aliphatic alkene (Jha and Halada 2011). The absorbance at 1510 cm-1 corresponded to the benzene ring stretching vibration in lignin (Chen et al. 2010).
Compared with the raw wood, the absorption band at 1060 cm-1 decreased in TW above pH 8. This indicated that the 4-methyl catechol solution in pH levels over 8 reduced the C–O–C bond formation by removing some of the hemicellulose. The reaction between the –OH in 4-methyl catechol and the –OH in the wood cell wall produced a C–O–C bridge between them, which shown in Fig. 2.
According to the finding, networking between the polymerized 4-methyl catechol and the wood cell wall was better for the TW at pH 8, followed by the TW at pH 10 and pH 11. In addition, the absorbance at 1,600 cm-1 by the aromatic skeletal stretching vibration in lignin was reduced in the TW at pH 10 and pH 11, which may be attributed to the degradation of lignin (Xiao et al. 2012).
Table 2. Functional Groups Obtained from FTIR Measurements of NaOH-Influenced 4-Methyl Catechol-Treated Wood
The reaction scheme
- Polymerization of 4-methyl catechol:
- Wood and polymerized 4-methyl catechol reaction:
Fig. 2. Schematic reaction mechanism of a) polymerization of 4-methyl catechol and b) wood with polymerized 4-methyl catechol
The X-ray diffractogram of raw wood and the TW at various pH levels is shown in Fig. 3. The diffraction pattern showed two main peaks for cellulose in the TW and the raw wood.
Fig. 3. X-ray diffraction (XRD) diffractograms of a) raw wood, b) TW at pH 8, c) TW at pH 9, d) TW at pH 10, and e) TW at pH 11
The X-ray intensity was measured in the range of 20 theta to 80 theta due to the clear peak intensity would be shown within this temperature range (Zhang and Wang 2007). The peak observed at 2θ = 22.4° assigned to the  crystalline plane was used to calculate the CIXRD(Wada et al. 2004). The peak at 2θ = 36° showed  crystalline planes. According to the finding, the 2θ value of the raw wood was higher than that of the TW. The d-spacing of the TW at the  plane was higher than in the raw wood. This difference established that the wood sample had been effectively treated by polymer. The changes in the diffractograms of the TW were analyzed in order to determine the crystallinity index (CI); these values are reported in Table 3. The CIXRD value of the TW at pH 8 was higher, followed by the TW at pH 9, pH 10, pH 11, and raw wood, respectively (Chen et al. 2010). The resulting values are due to the removal of wax, hemicellulose, and pectin from wood, and the decrease in amorphous movement of the wood cell wall by polymerized 4-methyl catechol at lower pH levels (Howell et al. 2009; Agubra et al. 2013). There was no extra peak with respect to the raw wood. The lack of peaks resulted because the polymerized 4-methyl catechol only filled the wood cavities in the wood’s surface and did not form a strong polar bond with them.
Table 3. Crystallinity Index (CIXRD) and d-Spacing as Obtained by XRD Measurements on NaOH-Influenced 4-Methyl Catechol-Treated Wood
Scanning Electron Microscopic Analysis (SEM)
The scanning electron micrographs of raw wood and various TW at different pH levels are shown in Figs. 4a, 4b, 4c, 4d, and 4e.
Fig. 4. SEM images of a) raw wood, b) TW at pH 8, c) TW at pH 9, d) TW at pH 10, and e) TW at pH 11
The lumen and tracheids of the wood were filled by the solid polymers of 4-methyl catechol. In Figs. 4d and 4e, the TW at pH 10 and pH 11 showed rough surfaces compared with the RW at pH 8 and pH 9. This contrast was due to more degradation of the cementing hemicellulose and lignin (Hajiha et al. 2014; Stevulova et al. 2014). As can be seen in Figs. 4b and 4c, the TW at pH 8 and pH 9 showed smoother surfaces due to the polymerized 4-methyl catechol, which filled the voids and the cavities. The poor interaction between the polymer and the wood implied a more brittle composite (Li et al. 2011). From the SEM results it can be concluded that 4-methyl catechol was more polymerized at pH 8 and pH 9 compared with polymerization at pH 10 and pH 11.
MOR, MOE, and Ed
The MOE, MOR, and Ed analyses of the raw wood and the TW are shown in Table 4. The TW at pH 8, pH 9, and pH 10 showed higher MOE and MOR compared with raw wood due to the polymerized 4-methyl catechol that formed the covalent bonds with the wood cell wall. Under alkaline conditions, the hydrogen bond structure of wood ruptured when the concentrated alkali permeated into the fiber from outside to inside (Zheng et al. 2012). The proportion of intra-molecular hydrogen bonds increased, while the proportion of total intermolecular hydrogen bonds decreased (Xu et al. 2016). Consequently, it was expected that more hydroxyl groups would be exposed after the rupture of hydrogen bonds and the cohesion of the fiber would decrease, thus increasing the disintegration of the fiber performance of raw wood (Villasante et al. 2013; Boonstra et al. 2007). 4-Methyl catechol was also encouraged to react with the –OH group of the wood cell wall to form covalent bonds. Thus, the MOE and MOR of the alkali-influenced TW increased compared with the raw wood. Polar carbonyl groups and the –OH group of polymerized 4-methyl catechol created dipole bonds with the polar group of the wood cell wall (Jha and Halada 2011). The non-polar cyclic parts of the polymerized 4-methyl catechol produced van der Waals forces with the non-polar lignin material of wood. Consequently, the MOE and MOR of the TW at pH 8, pH 9, and pH 10 increased compared with raw wood. The MOR and MOE of the TW at pH 11 was the lowest among both the TW and the raw wood. This result occurred because the NaOH solution at a higher pH degraded more cementing hemicellulose and lignin than it did at lower pH solutions.
Table 4. MOE, MOR, and Ed of Raw Wood and Treated Wood
* Values represent means of 10 replicates.
Among the TW and raw wood, the Ed of TW at pH 8 was highest, followed by TW at pH 9, pH 10, pH 11, and raw wood, respectively. This was due to the densities of TW which were higher compared to raw wood. High density increases the shearing force in wood composite (Islam et al. 2012b). In the presence of shearing force, the free energy increases and thus the molecules continuously oscillate (Toh 1979). The shearing force varies sinusoidally with time. The Ed of raw wood was slightly lower than TW at pH 11 as more hemicellulose and lignin were removed from wood by NaOH, which reduced the density. This result demonstrated that cellulose and 4-methyl catechol bonding had improved the Ed in the absence of hemicellulose and lignin.
Thermogravimetric Analysis (TGA)
The thermal stability of raw wood and wood treated with 4-methyl catechol at various pH levels is shown in Fig. 5. Raw wood, TW at pH 10, and TW at pH 11 showed two steps of thermal degradation and decomposition. However, TW at pH 8 and TW at pH 9 showed three steps of thermal degradation and decomposition (shown in Table 5). The first step of weight loss for both raw wood and TW occurred between 32 and 126 °C for the evaporation of absorbed moisture (Liu et al. 2013). The initial weight loss below 126 °C was the lowest for TW at pH 8, compared with TW at pH 9, TW at pH 10, raw wood, and TW at pH 11, respectively. At pH 8, polymerization of 4-methyl catechol was optimum compared with pH 9, pH 10, and pH 11. This was due to the removal of lignin and hemicellulose at higher pH levels. Therefore, the tendency to liberate moisture upon heating would be minimum for TW at pH 8 (Das and Chakraborty 2006).
Fig. 5. TGA of a) raw wood, b) TW at pH 11, c) TW at pH 10, d) TW at pH 9, and e) TW at pH 8
The second step of decomposition occurred between 110 and 282 °C, in which the decomposition of the hemicellulose of raw wood and TW occurred (Marques et al. 2015). In this temperature range, TW at pH 8 were less stable thermally compared with other treated wood. The TW at pH 8 contained higher hemicellulose followed by pH 9, pH 10, and pH 11, respectively. Thus, TW at higher pH levels were more stable thermally compared with lower pH levels. This was due to varying amounts of hemicellulose in TW at different pH levels.
The third step of decomposition occurred between 282 and 400 °C. This step occurred in order to break the bonds between wood (cellulose, lignin) and 4-methyl catechol. This step released combustible gases such as carbon monoxide, hydrogen, and methane, as well as carbon dioxide and tars that predominate as the temperature rises (Tomczak et al. 2007; TranVan et al. 2014). In this stage, TW at pH 8 were thermally more stable than TW at pH 9, because of cellulose–O–4–methyl catechol bonding in the glycoside chain.
The residual weight was higher for TW at pH 11 compared with the residual weight of TW at pH 10, TW at pH 9, raw wood, and TW at pH 8, respectively. This difference was due to the increasing pH for the 4-methyl catechol solution, which led to the formation of the lignin–cellulose complex (Ray et al. 2002). The residual weight of TW at pH 8 was lower than raw wood due to the presence of unstable cyclic diene in polymerized 4-methyl catechol.
Table 5. Results of TG Analysis for Raw Wood and TW
Ti: Onset temperature
Tm: Temperature corresponding to the maximum rate of mass loss
Tf: End temperature
- By studying FTIR, TGA, XRD, and SEM, it was concluded that wood was treated by 4-methyl catechol in the presence of NaOH.
- The CIXRD of the 002 plane of TW at pH 8 was more crystalline compared to TW at other pH level.
- The SEM micrographs showed that TW at pH 8 and TW at pH 9 had smoother surfaces compared with TW at pH 10, TW at pH 11, and raw wood.
- The TW at pH 8 had higher MOE, MOR, and Ed, followed by TW at pH 9, TW at pH 10, TW at pH 11 and raw wood, respectively.
- Raw wood gained weight after being treated by 4-methyl catechol at both pH 8 and pH 9, while it (raw wood) lost its weight at both pH 10 and pH 11.
- Water uptake by TW at pH 8 was the lowest among all the treated wood and raw wood because solution at a lower pH entered into maximum number of lumen and tracheids of wood and then it (4-methyl catechol) was polymerized.
- The TGA results showed that between 267 and 400 °C, TW were less stable thermally than raw wood due to the presence of unstable cyclic diene in polymerized 4-methyl catechol.
This work was financially supported by University of Malaysia Sarawak under the grant number FRGS/SG02 (01)/1085/2013(31).
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Article submitted: September 5, 2016; Peer review completed: February 6, 2016; Revised version received and accepted: March 21, 2017; Published: March 29, 2017.