Thermogravimetric Analysis For Wood Fiber Reinforced Polypropylene Base Composites

THERMOGRAVIMETRIC ANALYSIS FOR WOOD FIBER REINFORCED POLYPROPYLENE BASE COMPOSITES

 

Jae Gyoung Gwon, and Jung Hyeun Kim, University of Seoul, Seoul (Korea)

 

 

Introduction

      Needs for the WPC have been increased due to effective application of limited natural resources and environmentally friendly characteristics. Because of the outdoor durability and low maintenance cost, the WPC is being widely used in various applications such as deck, fence, marina, sound-proof wall, furniture, and automotive parts. Although the WPC has been commercialized in various applications, the base materials are so sensitive to flame that their usages have been limited [1, 2].

       Metal hydroxides can release water vapor above their decomposition temperature due to their endothermic reaction. In addition, their solid residues after the reaction can reduce the heat release rate from burning[3]. Aluminum hydroxide (AH, 1) and magnesium hydroxide (MH, 2) decompose according to the following reactions:

2Al(OH)₃(s) à Al₂O₃(s) + 3H₂O(g)                   (1)

Mg(OH)₂(s) à MgO(s) + H₂O(g)                     (2)

These two reactions were well summarized in a literature [3]. The endothermic loss of water resulting from the thermal decomposition of aluminum hydroxide is between 1170 J/g and 1300 J/g, and the decomposition starts at about 180-200°. In case of magnesium hydroxide, the endothermic loss is between 1244 J/g and 1450 J/g , and it starts to decompose at about 300-330°. Although the WPC has been commercialized, only a few studies about thermal decomposition of the metal hydroxides in the WPC have been performed. The WPC is more fire-sensitive than the wood itself because common polymers have higher heating values than the wood. Therefore, this study is aimed at exploring synergistic effect of hybridization of wood and metal hydroxides on thermal stability of polypropylene in WPC through thermo gravimetric analysis. We also examined the effect of the wood fiber and the metal hydroxides loading contents on thermal stability of the polypropylene in WPC.

Results and Discussion

Thermal degradation behavior of base components

          Fig. 1 shows thermal degradation curves of the base materials used in fabrication of WPC, under dehumidified N₂ atmosphere. As can be seen in Fig. 1a, PP is completely degraded before the temperature reaching around 465°, whereas AH and MH reveal substantial amounts of residue after the primary decomposition process due to the remaining metal oxide materials. Furthermore, the wood fiber case shows gradual weight loss after the first abrupt reduction as the temperature increases.

Fig. 1. Weight change(a) and mass loss rate(b) curves of base materials (wood, PP, AH, and MH) as a function of temperature.

          In addition, in Fig. 1b, the mass loss rate is plotted as a function of temperature, and it clearly demonstrates different decomposition temperatures of all constituent materials (wood fiber: 200°~385°, PP: 276°~465°, MH: 277°~ 394°, and AH: 209°~295°). In addition, their maximum mass loss rates (T_max) are observed at 356°, 422°, 378°, and 279°, respectively.

Thermal degradation behavior of WPC

Thermal stability of WPC materials combining those two base components is much dependant on the amount of wood fiber. Fig. 2 shows thermo-gravimetric analysis results of WPC containing various amount of wood fiber as a function of temperature. Fig. 2a demonstrates one step weight loss for the wood fiber 10wt% composite and two step weight losses for the wood fiber 30wt% and 50wt% composites. The resulting degradation temperatures increased from 422° for the PP base material to 434°, 448°, and 452° for the wood fiber 10wt%, 30wt%, and 50wt%, respectively. Therefore, thermal protective carbonaceous layer produced from the degradation of wood fibers can enhance the thermal stability of the PP base WPC materials. Fig. 2b shows the mass loss rate of WPC. WPC samples also show the two mass loss peaks from the wood fiber degradation and the PP decomposition. However, the peak for the wood fiber degradation from the wood fiber 10wt% composite is hardly seen, and it may be from the WPC degradation governed by the dominant PP decomposition. Instead, the wood fiber 30wt% and 50wt% composites show increased mass loss rate in the wood fiber decomposition temperature ranges but reduced mass loss rates in PP decomposition temperature ranges. This possibly indicates that the retardation of PP decomposition is very effective from the carbonaceous residues formed from the degradation of the increased wood fiber contents.

Fig. 2. Weight change (a) and mass loss rate (b) curves for various amount of wood fiber contents in WPC.

Thermal degradation behavior of metal hydroxides filled WPCs

          T_max of the PP degradation step of the MH filled WPC case is slightly higher than that of the AH case (463~466°). The thermal degradation of MH occurs at 300~400°, but the degradation occurs at 200-300° in the AH case. Therefore, the MH case shows high thermal stability in the WPC system because the endothermic reaction directly affects the retardation of the PP decomposition.

References 1.       H. Seefeldt and U. Braund (2011), "Burning Behavior of Wood-Plastic Composite Decking Boards in End-Use Conditions: The Effects of Geometry, Material Composition, and Moisture," Journal of fire science, 30, pp. 41-51.

2.       M. B. A. Bakar, Z. A. M. Ishak, R. M. Taib,H. D. Rozman, and S. M. Jani    (2010), "Flammablility and Mechanical Properties of Wood Flour-Filled Polypropylene Composites," Journal of polymer science, 116, pp. 2714-2722.

3.       L. A. Hollingbery and T. R. Hull (2010), "The Thermal Decomposition of Huntite and Hydromagnesite-A Review," Thermochimica acta, 509, pp. 1-11.