(217dq) 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.