Abstract
The thermal degradation behavior and pyrolysis kinetic models of wood flour (WF)/PVC composites before and after adding chitosan were studied using thermogravimetry (TGA) and nine common kinetic model functions (f(α)). The results indicated that the thermal degradation temperature of WF/PVC composites was delayed to a higher value after adding chitosan. The first-order reaction order (L1) model and second-order reaction order (L2) model were found to be the best reaction order functions for the description of mass loss kinetics of WF/PVC without chitosan during the first and second stages. Two L2 models were suitable for both degradation stages of WF/CS/PVC. Activation energy (E) and frequency factor (A) for WF/PVC and WF/CS/PVC corresponded to 26.05 kJ·mol-1, 4.08×103 s-1, and 40.89 kJ·mol-1, 2.11×1010 s-1 at the first degradation stage, respectively, and 97.83 kJ·mol-1, 1.11×107 s-1 and 92.88 kJ·mol-1, 1.56×1011 s-1 at the second degradation stage.
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Effects of Natural Chitosan as Biopolymer Coupling Agent on the Pyrolysis Kinetics of Wood Flour/ Polyvinyl Chloride Composites
Kaimeng Xu,a Taian Chen,a,* Zhifeng Zheng,a,* Suyong Huang,a Kaifu Li,b and Tuhua Zhong c
The thermal degradation behavior and pyrolysis kinetic models of wood flour (WF)/PVC composites before and after adding chitosan were studied using thermogravimetry (TGA) and nine common kinetic model functions (f(α)). The results indicated that the thermal degradation temperature of WF/PVC composites was delayed to a higher value after adding chitosan. The first-order reaction order (L1) model and second-order reaction order (L2) model were found to be the best reaction order functions for the description of mass loss kinetics of WF/PVC without chitosan during the first and second stages. Two L2 models were suitable for both degradation stages of WF/CS/PVC. Activation energy (E) and frequency factor (A) for WF/PVC and WF/CS/PVC corresponded to 26.05 kJ·mol-1, 4.08×103 s-1, and 40.89 kJ·mol-1, 2.11×1010 s-1 at the first degradation stage, respectively, and 97.83 kJ·mol-1, 1.11×107 s-1 and 92.88 kJ·mol-1, 1.56×1011 s-1 at the second degradation stage.
Keywords: Chitosan; Wood flour; PVC; Pyrolysis kinetic model
Contact information: a: University Key Laboratory of Biomass Chemical Refinery & Synthesis of Yunnan Province, College of Material Engineering, Southwest Forestry University, Kunming 650224, P.R. China; b: Department of Forestry, South China Agricultural University, Guangzhou 510642, P.R. China; c: Division of Forestry and Natural Resources, West Virginia University, Morgantown, WV 26505, USA;
* Corresponding authors: chentaian6868@163.com; zhengzhifeng666@163.com
INTRODUCTION
The decline in forestry resources and a rise in awareness of environmental protection have greatly kindled the fast development of emerging alternative materials. Wood plastic composites (WPC) are one of many potential novel materials to replace solid wood and conventional wood-based composites, such as particle board (PB), medium density fiberboard (MDF), and plywood (Lu et al. 2008; Fabiyi et al. 2009; Yue et al. 2011; Segerholm et al. 2012; Cavdar et al. 2014). Wood flour/polyvinyl chloride (WF/PVC) composites are a primary subset of WPC, which had great growth in different applications in the global market in the last few decades due to their unique wood-like appearance, the fact that they are easy to paint, their ability to readily form foams, their excellent resistance to biodegradation, and low cost of maintenance (Jiang and Kamdem 2004; Muller et al. 2012; Kositchaiyong et al. 2013; Xu et al. 2013; Kositchaiyong et al. 2014).
Interfacial bonding and thermal stability are currently two difficult problems that impede the use of WF/PVC in more fields. Compared to polyolefin resins, such as polyethylene (PE) and polypropylene (PP), PVC contains additional carbon-chlorine (C-Cl) bonds in its molecular structure, resulting in a special molecular chain of polarity, which has a significant effect on interfacial bonding of PVC and wood flour. With respect to thermal stability, it is well known that hydrogen chloride (HCl) is eliminated from PVC during the heating process. As for WF/PVC, the initial dehydrochlorination of the PVC macromolecular chains would be accelerated owing to the acidic products in wood flour (McNeill et al. 1995), which would lead to inferior organoleptic properties (odor and color) and deterioration of physical-mechanical properties (Fang et al. 2012).
In previous studies by the authors it was concluded that natural chitosan is an excellent biopolymer coupling agent and anti-bacterial agent to promote interfacial adhesion and added value of WF/PVC when being added in the optimum amount (Xu et al. 2014a). Meanwhile, due to the effects of alkalescence and the coupling function from the -NH2 groups in CS molecular chains, it was also deduced that the autocatalytic effect of HCl on continuous dehydrochlorination of PVC macromolecular chains was partly crippled due to the adsorption and acid-base neutralization by CS (Karayildirim et al. 2006; Xu et al. 2014b).
The pyrolysis kinetic model of polymeric materials is acknowledged as an accurate prediction of the flammability and fire hazard properties, especially for polymers and wood fibers that are very sensitive to flame (Sain et al. 2004; Snegireva et al. 2012; Czégény et al. 2015). Therefore, it is also of great importance in understanding the mechanism of thermal degradation reactions of composites. Despite some studies that focused on thermal degradation properties of WF/PVC, a universal and acceptable pyrolysis kinetic model for WF/PVC composites with and without chitosan are not yet available.
The aim of our present work is to firstly investigate the pyrolysis kinetic models of WF/PVC before and after adding chitosan at optimum addition amounts and particle size for its best performance of naturally coupling function according to our previous conclusions using thermogravimetry (TGA). Kinetics parameters of pyrolysis based on two dominant reaction regions, consisting of activation energy, reaction order, and frequency factor, were also studied. This data will be very crucial and useful support for both the design and optimization of reactors and the further utilization of WF/CS/PVC.
EXPERIMENTAL
Raw Materials
PVC (DG-800) with an average degree of polymerization and density of 800 and 1.35 g/cm3, respectively, was bought from Tianjin Dagu Chemical Ltd. Co., China. Wood flour of Cunninghamia lanceolata with particle size of 80 to 100 mesh was provided by Guangzhou Minshan New Material Ltd. Co., China. The flakes of chitosan had a degree of deacetylation, average molecular mass, and particle size of 95%, 820,000, and 180 to 220 mesh, respectively, and were obtained from Golden-shell Biochemical Ltd. Co., China. Other additives, including heat stabilizers, processing aids, and lubricants, were supplied by Yunnan Zhongqian Ltd. Co., China.
Preparation of Composites
WF and CS were pre-dried at 105 °C for 72 h in an oven prior to blending. Both WF (40 phr) and PVC (100 phr) were mixed in a high speed blender (SHR-10A, Zhangjiagang, China) at 1800 rpm for 8 min. Then CS was added into 2 wt% acetic acid solution while stirring at 40 °C for 30 min. After cooling down to room temperature, the solution was sprayed onto the compounds of wood flour and PVC and mixed for 10 min at 85 °C. Other additives, including heat stabilizer (4 phr), processing aids (6 phr), and lubricants (0.8 phr), also were added and mixed at 105 °C for 10 min. Subsequently, the mixture was extruded as a sheet by conical twin-screw extruders (LSE-35, Guangzhou, China) with a screw diameter of 40 mm and length to diameter ratio of 13:1, corresponding to the temperatures of 130, 145, 165, 180, and 178 °C from hopper to die zone; its rotation speed was set at 25 rpm.
Thermal Degradation Properties Measurement
The measurement of thermal degradation properties for WF/PVC and WF/CS/PVC were carried out on a thermogravimetric instrument (TGA 209-F1, Netzsch Co., Germany) to obtain the data about weight loss during the continuous heating. Samples of 10 mg were heated from 30 to 800 °C under the protection of a high purity nitrogen atmosphere at a heating rate of 10 °C·min-1.
Pyrolysis Kinetic Model Calculation
The pyrolysis process of samples can be expressed as follows: samples 
volatile + solid residue. The decomposition rate of samples is generally described as follows according to the Arrhenius equation (Eq. 1),
(1)
where, f(a) is a characteristic function depending on the reaction mechanism; k is the rate constant of decomposition; A is the frequency factor (s-1); E is the apparent activation energy of decomposition (J·mol-1); R is the universal gas constant (8.314 J·mol-1·K-1); and T is the absolute temperature, in K. Also, α is the fractional conversion at certain time (s), which can be defined (Eq. 2) as,
(2)
where, W0 is the initial weight of sample (g); 
is the weight of sample at t time (g); 
is the final weight of the sample (g); 
is the mass loss at t time (g); and 
is the mass loss at the end of the reaction (g).
Under non-isothermal TGA, a constant heating rate 
is often used and Eq. 1 becomes Eqs. 3 and 4.
(3)
(4)
In this study, the Coats-Redfern integration method was applied to determine the kinetic parameters (Coats and Redfern 1964; Liu et al. 2002). The parameter 
was defined as shown by Eq. 5:
(5)
Equation 6 was formed by combining Eqs. 1 and 5.
(6)
Re-arranging and integrating Eq. 4 gives (Eq. 7):
(7)
The kinetic model is an algebraic expression which is usually associated with a physical model for the description of the kinetics in a solid state reaction. The most common forms of are listed in Table 1. A suitable model was estimated by the accuracy of linear regression of 
to 1/T.
It was known from equation (7) that the term 2RT/E is much less than 1 for the thermal decomposition of composites. Therefore, to 1/T should be a line with the slope of E/R when the correct is matched. Meanwhile, the activation energy (E) and frequency factor (A) can be calculated through the slope and intercept of the line.
Table 1. Kinetic Model Functions f(a) and Corresponding g(a) Usually Employed for the Solid State Reactions
RESULTS AND DISCUSSION
Thermal Degradation Properties Analysis
Figure 1 shows the TG curves of WF/PVC composites without chitosan and WF/PVC/CS with 30 phr chitosan. Two main mass loss processes (220 to 400 °C and 400 to 540 °C) were clearly evident. It was suggested that major pyrolysis of semi-cellulose, cellulose, and chitosan, as well as partial decomposition of lignin occurred, besides the volatilization of hydrogen chloride (HCl) at the first stage (Cardenas and Miranda 2004; Fang et al. 2012). Specifically, the amorphous structures of branches were removed from the main chains of hemicelluloses. Long chains of glucose units were broken for cellulose and chitosan. Aromatic rings in lignin appeared to be cross-linked (Levchik and Wei 2005; Yang et al. 2007). The second stage mainly involved thermal cracking of the carbonaceous conjugated polyene sequences in PVC (Diab et al. 2011).
Fig. 1. TG curves of WF/PVC and WF/PVC/CS composites
The observed thermal degradation curve of WF/PVC appeared to be different from that of the WF/PVC/CS in a nitrogen atmosphere. Specifically in Table 2, the temperature at 10% weight loss (T10%) of WF/PVC, which represents the volatilization of water, was almost the same (nearly 273 °C) as WF/PVC/CS due to the constant control of the moisture content of WF and CS in pre-drying process. However, T30% and T70% of WF/PVC/CS were prolonged to 312.7 °C and 459.2 °C from 304.8 °C and 455.5 °C of WF/PVC, which demonstrated that the thermal stability of WF/PVC/CS was promoted effectively because the ability of the interfacial interlock was strengthened with the autocatalytic effect of HCl on dehydrochlorination of PVC chains being weaken. Additionally there was a possible acid-base neutralization reaction taking place between HCl and alkaline chitosan. Moreover, the residue content of WF/PVC/CS was also higher than WF/PVC, since the carbon element of CS was brought into the whole compound system.
Table 2. The Data of TG Curve
T10%, T30% and T70% represented temperature at 10 %, 30 % and 70 % weight loss, respectively.
Pyrolysis Kinetic Model Analysis
Based on the analyses above, a two-stage model including two independent thermal degradation processes that occurred in the 240 to 400 °C and 400 to 540 °C temperature ranges is proposed to describe the mass loss kinetics of WF/PVC and WF/CS/PVC. The experimental curves with nine types of models were fitted as shown in Figs. 2 and 3. The specific data of fitted equations with correlation coefficients (R2) were listed in Table 3 and Table 4.
Fig. 2. Models of two thermal degradation processes of WF/PVC composites
As for WF/PVC composites, it was found from Fig. 2a that the function of the first-order reaction order (L1) showed the best extent of linearity. Moreover, it was also noticed in Table 3 that L1 with the fitted equation y =-3133.852x-2.037 and maximum R2=0.976 implied that the model function of L1 is the best reaction order function for the description of mass loss kinetic for WF/PVC composites without chitosan during the first stage.
Table 3. The Fitted Equations with Correlation Coefficients (R2) of Two Stages for WF/PVC Composites without Chitosan
As for the second stage, the pyrolysis model curve in Fig. 2b corresponds to the optimum fitted equation of y=-11766.382x+12.968 (L2 model) with R2=0.987 in Table 3. Nevertheless the other fitted equation: y= -7796.445x+3.762 also achieved a high coefficient of determination value R2=0.985 (L1 model), which indicated that there were less obvious differences between the L1 and L2 model. This implied that the description of mass loss behaviors for WF/PVC composites without chitosan can be acceptable for both L1 and L2 at the second thermal degradation stage, but L2 was preferred. The reason may be that the two kinetic models for the two temperature ranges take effect together in the intermediate range from the first stage to the second stage (i.e., at the temperatures from the end of the first range to the beginning of the second range).
With respect to the WF/CS/PVC samples examined by nine model functions at two stages, it can be seen in Fig. 3 and Table 4 that the model functions of the second-order reaction order (L2) resulted in maximum coefficients of determination (R2) of 0.990 and 0.988, corresponding to the following two fitted equations: y = -4917.745x+4.542 and y = -11171.812x +11.844 for both the first and second thermal degradation processes. This demonstrated that the addition of chitosan did alter the pyrolysis behaviors at the two stages.
Fig. 3. Models of two thermal degradation processes of WF/CS/PVC composites without chitosan
Table 4. The Fitted Equations with Correlation Coefficients (R2) of Two Stages for WF/CS/PVC Composites
The kinetic parameters of WF/PVC and WF/CS/PVC, activation energy (E) and frequency factor (A), calculated from these equations’ best linearity, are summarized in Table 5. It was found that the activation energy value obtained from WF/CS/PVC sample (40.89 kJ·mol-1) was greater than the value estimated by WF/PVC (26.05 kJ·mol-1) without chitosan during the first thermal degradation stage. One major explanation was that adding chitosan allowed the bio-polymer coupling agent to improve the interfacial bonding of the PVC resin matrix and wood flour, WF, PVC, and CS, allowing these components to become combined into an integral system, which led to relatively high activation energy. However, the ability to resist thermal degradation for the whole system gradually got weaker when the temperature was over the maximum degradation temperature of pure CS (about 300 °C) (Diab et al. 2011), so the activation energy (E) was lower for WF/CS/PVC (92.88 kJ·mol-1) than for WF/PVC (97.83 kJ·mol-1). In addition, Table 4 also showed that the frequency factor (A) of WF/CS/PVC was much higher than the one of WF/PVC without chitosan. It can be deduced that adding chitosan successfully incorporated into the interfacial bonding system effect. The result of the calculated kinetic parameters for pyrolysis of WF/PVC and WF/CS/PVC correlated well with our previous research for WF/CS/PVC composites (Xu et al. 2014b).
According to the kinetic parameters and optimum models, the pyrolysis kinetic equations for WF/PVC and WF/PVC/CS samples at the first thermal degradation stage corresponded to 
4.08×103×(1-α)×exp(-26050/RT) and 2.11×1010×(1-α)2×exp(-40890/RT). At the second thermal degradation stage, they corresponded to 1.11×107×(1-α)2×exp(-97830/RT) and 1.56×1011×(1-α)2×exp(-92880/RT), respectively.
Table 5. Pyrolysis Kinetic Parameters of Different Group Samples
CONCLUSIONS
- The thermal degradation temperature of WF/PVC composites increased after adding chitosan.
- The L1 model with the fitted equation y=-3133.852x-2.037 and L2 model with fitted equation y=-11766.382x+12.968 were the best reaction order functions for the description of mass loss kinetics of WF/PVC at the first and second stages. Two L2 models with fitted equations: y=-4917.745x+4.542 and y=-11171.812x+11.844, were suitable for the first and second thermal degradation stages of WF/CS/PVC.
- The pyrolysis kinetic equations for the WF/PVC and WF/CS/PVC samples at the first thermal degradation stage were 4.08×103×(1-α)×exp(-26050/RT) and 2.11×1010×(1-α)2×exp(-40890/RT), respectively; at the second thermal degradation stage they were 1.11×107×(1-α)2×exp(-97830/RT) and 1.56×1011×(1-α)2×exp(-92880/RT).
ACKNOWLEDGMENTS
This work was sponsored by the grants of the National Natural Science Funds of China (No. 31160147), the Basic Research on the Application of the Key Project of Yunnan Province (No. 2011FA021), as well as the Scientific Funds of Southwest Forestry University (No. 111419).
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Article submitted: February 2, 2015; Peer review completed: June 9, 2015; Revised version received and accepted: June 12, 2015; Published: June 23, 2015.
DOI: 10.15376/biores.10.3.4903-4912