Abstract
The effects of glycerol pretreatment and thermal modification on poplar wood was examined using thermogravimetric analysis (TGA). The total mass losses of thermally-treated samples before and after glycerol impregnation were studied. The thermal degradation process was divided into three stages based on natural breaks in the slope of the TGA curves. The set-on and set-off temperatures, mass loss, and activation energy (Ea) of each stage were compared. Pretreatment with 60% glycerol followed by thermal modification at 160 °C produced pronounced differences in the three decomposition stages. Fewer wood components were decomposed in the first stage in glycerol-pretreated wood, which suggested that the pretreatment had modified the wood components into more thermally stable substances. However, the mass losses were higher in the next stage, suggesting that the effect on thermal stability was limited. The Ea values of wood decomposition during the first stage were decreased, while those during the second and third stages were increased. These results illustrate the potential for using a glycerol pretreatment to alter the thermal stability of wood.
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Thermogravimetric Analysis Studies of Thermally-treated Glycerol Impregnated Poplar Wood
Rui Liu,a Jeffrey J. Morrell,b and Li Yan a,*
The effects of glycerol pretreatment and thermal modification on poplar wood was examined using thermogravimetric analysis (TGA). The total mass losses of thermally-treated samples before and after glycerol impregnation were studied. The thermal degradation process was divided into three stages based on natural breaks in the slope of the TGA curves. The set-on and set-off temperatures, mass loss, and activation energy (Ea) of each stage were compared. Pretreatment with 60% glycerol followed by thermal modification at 160 °C produced pronounced differences in the three decomposition stages. Fewer wood components were decomposed in the first stage in glycerol-pretreated wood, which suggested that the pretreatment had modified the wood components into more thermally stable substances. However, the mass losses were higher in the next stage, suggesting that the effect on thermal stability was limited. The Ea values of wood decomposition during the first stage were decreased, while those during the second and third stages were increased. These results illustrate the potential for using a glycerol pretreatment to alter the thermal stability of wood.
Keywords: Thermogravimetric analysis (TGA); Glycerol impregnation; Thermal treatment; Kinetic analysis
Contact information: a: College of Forestry, Northwest A&F University, Yangling, Shaanxi, China 712100; b: Department of Wood Science & Engineering, Oregon State University, Corvallis, OR, USA 97331; *Corresponding author: liliyan@nwsuaf.edu.cn
INTRODUCTION
Thermal modification is an environmentally attractive wood treatment method that is performed in a protected atmosphere or in a liquid medium under high temperature conditions. The process has been reported to improve wood color (Ayadi et al. 2003; Shi and Jiang 2011), dimensional stability (Stamm and Hanson 1937; Burmester 1973), and decay resistance (Kamdem et al. 2002; Schwarze and Spycher 2005).
Thermogravimetric analysis (TGA) has been used widely to study the pyrolysis process and thermal stabilities of thermoplastic wood (Scott et al. 2004), liquefied wood polymer composites (Doh et al. 2005), thermally-modified wood (Chen et al. 2011), and ionic liquid-modified wood (Patachia et al. 2013).
Analysis of the thermogravimetric (TG) curve can be used to determine the pyrolysis parameters, such as residual mass, pyrolysis stage division, set-on temperature, and set-off temperature, of the pyrolysis stages (Kercher and Nagle 2001; Grønli et al. 2002; Doh et al. 2005). The differential thermogravimetric (DTG) curve is derived from the first derivative of the TG curve and reveals the pyrolysis rate of the sample. Safi et al. (2004) studied the thermal degradation of pine needles and reported that the entire degradation range could be divided into four stages based upon the natural breaks in the slope of the TG curves. The activation energy of each stage was calculated and compared. Shen et al. (2009) studied the thermal properties of four species of wood and separated the DTG curves into a first stage with a range of 200 °C to 370 °C characterized by hemicellulose and cellulose degradation, and a second stage with a temperature range of 370 °C to 490 °C where lignin was decomposed. Several studies report three main stages of thermal decompositions, including dehydration (26.85 °C to 106.85 °C), active pyrolysis (176.85 °C to 386.85 °C), and passive pyrolysis (> 386.85 °C). Two peaks observed in active pyrolysis are related to hemicellulose and cellulose decompositions, while lignin is decomposed during both the active and passive pyrolysis stages without any characteristic peaks in the thermogram (Strezov et al. 2003; Gašparovič et al. 2010; Slopiecka et al. 2012).
Kinetic analysis can also be applied to thermal analysis. According to the Arrhenius formula, the kinetic parameters, such as activation energy (Ea) and pre-exponential factor (A), are obtained after a series of calculations (Safi et al. 2004; Fang et al. 2006; Gašparovič et al. 2010). Gašparovič et al. (2010) calculated that the activation energy of wood chips (residual processing wood) during the main decomposition stage ranged from 190 kJ/mol to 217 kJ/mol. Studies from the pyrolysis of several hardwoods and softwoods have revealed that wood devolatilizes at temperatures above 280 °C in three parallel reactions (Grønli et al. (2002). The activation energies for hemicellulose, cellulose, and lignin were determined as 100 kJ/mol, 236 kJ/mol, and 46 kJ/mol, respectively. Bartocci et al. (2017) studied pyrolysis of biomass pellets (10% glycerol and 90% sawdust) and described glycerol pyrolysis as a first order reaction. They also calculated activation energies and pre-exponential factors for hemicellulose, cellulose, lignin, and glycerol.
Glycerin is a major by-product of biodiesel production. Increased biodiesel production should provide abundant supplies of low cost glycerol. Manara and Zabaniotou (2016) suggested that co-valorization of crude glycerol with waste biomass could help decrease biodiesel production costs, while providing an application for waste glycerol. Fantozzi et al. (2016) studied the pyrolysis kinetics of glycerol and developed a skeletal kinetic model of glycerol pyrolysis. They found that non-condensable gas yields of 70% (wt basis) could be achieved at 750 to 800 C with hydrogen concentrations up to 44 ~ 48% (volume basis). Yields increased the increasing temperature.
Previous studies have suggested that glycerol impregnation prior to thermal modification improved the physical properties of wood. For example, the dimensional stability of compressed wood was improved when wood was pretreated with 40% glycerol solution prior to compression (Inoue et al. 2000). Yan et al. (2010) reported that the compression deformation was more permanent when the wood was impregnated with 50% glycerol followed by thermal modification at 160 C for 60 min. Douglas-fir (Pseudotsuga menziesii), when impregnated with 20% glycerol solution prior to thermal modification at 200 C for 6 h, showed a higher anti-swelling efficiency (48%) than normal thermally-modified wood (26%) (Yan and Morrell 2014). However, the effects of glycerol on thermal degradation reactions are unknown.
The objective of this study was to assess the effects of a glycerol treatment followed by thermal modification on the thermal properties of wood using thermogravimetric analysis (TGA). Activation energy (Ea) was also used to study the effects of the glycerol pretreatment on the thermal degradation reactions.
EXPERIMENTAL
Materials
Poplar wood (Populus tomentosa), from Xian Yang City, Shaanxi Province, China, that was free of defects or biological attack was ground to pass through 40- to 60-mesh screens.
Methods
Glycerol treatment
Wood powder ground to pass a 20 mesh screen was first mixed with an excess of glycerol solution (99 % concentrate) diluted with water to 20%, 60%, or 100% (wt/wt) and a screen was placed over the wood material to limit the particles from floating. The material was then subjected to vacuum (0.09 MPa) for 30 min. The vacuum was released and the glycerol was drained. The treated ground wood was then oven-dried at 60 C for 3 d.
Thermal modification
The glycerol-impregnated and non-impregnated wood powders were thermally-treated by placing them in an oven at 120 C, 140 C, 160 C, 180 C, or 200 C for 4 h. The resulting powders were washed with anhydrous ethanol in an ultrasonic cleaner for 30 min, then the ethanol was removed, and the wood was oven-dried to a constant weight at 60 C. This step removed residual, non-reacted glycerol that might interfere with subsequent thermogravimetric analyses.
TGA measurement
The TGA (thermogravimetric analysis) measurements were performed using a Mettler TGA/DSC 3+ synchronous thermal analyzer (Mettler-Toledo, Zurich, Switzerland). High purity nitrogen was used at a flow rate of 50 mL/min as the carrier gas to maintain pyrolysis conditions. Five milligrams (± 1.0 mg) of wood powder was added to the holder and the temperature was raised from room temperature to 600 C at a rate of 10 C/min. The decomposition stages were divided using the TG curves as described earlier, the set-on and set-off temperatures were found, and reaction enthalpy was calculated using Mettler STARe Software (V15.00, Mettler-Toledo, Zurich, Switzerland).
Kinetic theory
The pyrolysis process in biomass is a series of complex reactions. The effect of reaction temperature on active energy was usually neglected, assuming that the reaction met the simple kinetic equation. The kinetic reaction equation (Eq. 1) of a sample pyrolysis reaction was expressed as follows:
Pyrolysis reaction rate.
where
and where W0 denotes the initial mass of the sample (mg), a represents the mass loss rate (%), Wis the mass of sample at temperature T (mg), and W∞ denotes the final quantity of the sample (mg).
According to the Arrhenius formula, the reaction rate constant is given by:
The heating rate 
in the experiment was constant.
A combination of Eqs. 1 and 2 gives the fundamental expression Eq. 3,
which was integrated to obtain the following Eq. 4:
With Doyle’s approximation (Flynn and Wall 1966; Tripathi and Srivastava 2011), which allows for E/RT ≥ 20, Eq. 5 now can be simplified as:
Thus, a linearized pyrolysis kinetic equation can be obtained.
When n = 1 , then
When 
, then
where 
the rate of heating (K/min), A is the pre-exponential factor (1/min), R is the gas constant [8.314 J/(K·mol)], T is the heating temperature (K), and E represents the activation energy (kJ/mol).
To facilitate the analysis and comparison of the results, the wood pyrolysis process was assumed as a primary reaction (n = 1).
Allow
,then a linear function was obtained as follows:
(9)
The linear function (Eq. 9) was obtained by linear regression of the temperature (T) and mass loss rate (a) of the TG curve in each wood pyrolysis process. The slope a1 and the intercept b1 of the linear function were obtained. These values were used to determine the activation energy (Ea) and the pre-exponential factor A.
RESULTS AND DISCUSSION
TGA Analysis
The TG (thermogravimetric) curves for the thermally modified (at 120 C to 200 C), non-impregnated, and glycerol-impregnated poplar wood are shown in Fig. 1. The TG curves for the control, thermally modified, and glycerol pretreated/thermally-modified wood samples showed similar trends. The mass loss processes of all samples could be divided into three stages, such as the water evaporation phase, thermal degradation phase, and carbonization phase, according to the rate of mass loss. Free and bound water evaporated during the water evaporation phase (35 C to 100 C) (Gašparovi et al. 2010; Slopiecka et al. 2012). Wood components degraded rapidly during the thermal degradation phase (220 C to 390 C), and the corresponding mass losses rapidly increased reflecting the decompositions of hemicelluloses, cellulose, and partial lignin. Thermal degradation slowed during the carbonization phase (> 390 C) and the mass losses reflected decomposition of the remaining lignin and combustion of char residues (Orfao et al. 1999; Safi 2004; Shen et al. 2009).
The residual mass of thermally modified wood ranged from 14.38% to 18.76%, which were higher than those for the control sample (14.31%) (Table 1). The residual mass of glycerol-pretreated wood ranged from 8.55% to 14.68%. However, the residual masses of glycerol-pretreated wood exposed to the same thermal modification temperature were lower than those for the material that was only thermally modified. These results suggested that the thermal degradation of wood decreased after thermal treatment, while glycerol impregnation before thermal treatment accelerated the degradation.
Table 1. Effect of Glycerol Pretreatment and Thermal Modification on Residual Masses of Polar Wood Following Thermogravimetric Analysis
The variations seen in the decomposition rates of wood components were well recognized in the second peak in Fig. 2. Glycerol impregnation followed by thermal modification appeared to affect the decompositions of wood components. These effects were further explored by dividing the second peak into three stages according to the three steps found from the DTG (differential thermogravimetric) curves. An example of this approach is shown for samples that were impregnated with 20% glycerol prior to thermal modification at 120 ℃ (Fig. 3).
Fig. 1. Effects of glycerol pretreatment and thermal modification on TG curves of poplar (Populus tomentosa)
The set-on and set-off temperatures were found at each treatment level in the DTG curve, and then the TG (thermogravimetric) curves were divided into three stages using the corresponding temperatures. The R2 for the regression lines for these curves were above 0.88, although the slopes of the lines indicated that reaction rates differed for each stage (Fig. 4). The set-on and set-off temperatures and calculated mass losses for the three stages are shown in Table 2. The temperature ranges of the three stages for the controls were 226.7 C to 287.4 C, 287.4 C to 338.7 C, and 338.7 C to 389.2 C, respectively.
Fig. 2. Effects of glycerol pretreatment and thermal modification on DTG curves of poplar (Populus tomentosa)
Fig. 3. Effect of glycerol pretreatment and thermal modification of poplar wood at 120 °C on mass loss (%) and DTG curve
Table 2. Effect of Glycerol Pretreatment and Thermal Modification on Set-on and Set-off Temperatures and Mass Losses During the Three Stages of Thermogravimetric Analysis
Hemicelluloses, cellulose, and lignin in wood decompose rapidly at 180 C to 300 C, 240 C to 400 C, and 280 C to 550 C, respectively (Kuriyama 1967), suggesting that the hemicelluloses and a small amount of cellulose decompose in the first stage of heating. The remaining hemicelluloses’ decomposition and continuous cellulose decomposition occur in the second stage, while the reaction rate slowly increases. In the third stage, the remaining cellulose decomposes and the reaction rate further accelerates. Lignin decomposition also occurs more slowly over these three decomposition stages.
The set-on temperatures at stage I and stages III of the control, thermally modified, and glycerol pretreated/thermally modified materials were very similar. Set-on temperatures at stage Ⅱ of the control and thermally modified samples were also similar, but the set-on temperatures at stage Ⅱ were much lower of wood that was treated with 60% glycerol prior to thermal modification. The reasons for this temperature shift were unclear. The samples were washed to remove excess glycerol so that any effect on temperature, due to reaction between the glycerol and wood that was mediated by the original thermal modification, was eliminated.
Mass losses at stage I tended to be lower in the poplar samples that were pretreated with 60% glycerol/thermally modified at 160 C than those of samples only thermally modified or those that received no treatment. The mass losses of glycerol-pretreated wood were higher than those for the control and thermally modified wood during stage II of thermal degradation. These results suggested that the initial glycerol pretreatment reduced the susceptibility of wood components to thermal decomposition. It is unclear how glycerol reacted with the wood prior to thermal modification, but one possibility is that it accelerated the thermal decomposition of the carbohydrate components as these polymers were less abundant at the initial stage of TGA analysis. The mass losses of samples from all three treatments were similar at stage III, which suggested that neither glycerol pretreatment nor thermal modification affected the thermal sensitivity of the residual lignin.
Kinetic Analysis
The thermal degradation phases of the TG curve were divided into three decomposition stages, and the activation energy of each decomposition stage was calculated for each treatment. The activation energies (Ea), pre-exponential factor (A), and the correlation coefficient (R2) are presented in Table 3.
The Ea values of thermally modified wood at the three decomposition stages were higher than the Ea values for the controls, indicating that the thermal modification increased the thermal stability of wood components.
The Ea values of wood pretreated with 60% glycerol prior to thermal modification at 160 °C were much lower than those for the control and just thermally modified wood.
The Ea values of glycerol-pretreated wood at stage II were higher than those were for the control and thermally modified wood samples. The Ea values for all glycerol-pretreated wood samples at stage III were also higher. These results indicated that the glycerol pretreatment coupled with thermal modification altered the stability of wood components. The treatment initially accelerated decomposition at stage I, but improved stability at the second and third stages of thermogravimetric analysis. These results suggest that glycerol pretreatment altered the thermal modification process. These effects may be exploited to alter the process to improve specific wood properties, such as heat resistance, water repellency, or other desirable properties.
Table 3. Effect of Glycerol Pretreatment and Thermal Modification on Thermodynamic Parameters During the Three Stages of Thermogravimetric Analysis
CONCLUSIONS
- Thermal modification decreased the subsequent rate of degradation of wood components during thermogravimetric analysis while glycerol pretreatment appeared to accelerate the process.
- The combination of pretreatment with 60% glycerol followed by thermal modification at 160 °C appeared to be the optimum condition for initiating this effect. This effect was also demonstrated with the calculated Ea (activation energy) values.
- The results suggest that pretreatments may be a potential tool for altering the thermal modification process to produce products with differing resistances to thermal degradation or to create other attractive properties such as improved water resistance or stability.
ACKNOWLEDGMENTS
The authors are grateful for the support of the National Natural Science Foundation of China (NSFC) No. 31400500.
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Article submitted: October 11, 2017; Peer review completed: December 16, 2017; Revised version received and accepted: December 30, 2017; Published: January 16, 2018.
DOI: 10.15376/biores.13.1.1563-1575