JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 50

WPC

Fig. 1: Thermogravimetric analysis of WPC (60% PP, 40% wood); sample mass:
12.7 mg, heating rate: 10 K/min, Al2O3 crucible; the mass-loss curve (solid line)
and the first derivative with respect to time (DTGA) are shown

of 60% PP and 40% wood during heat treatment. Initially,
the sample was heated up to 600°C in a nitrogen atmosphere, followed by an oxidizing step up to 800°C in air.
The first mass loss starts at about 150°C (with a 165°C
DTGA peak temperature), due to the release of water
(1%). In the temperature interval between approx. 230°C
and 600°C, two further mass-loss steps occur. The first
one, at a temperature of 367°C (principal DTGA peak),
can be assigned to the decomposition of wood and the
second one, at 467°C (DTGA peak as well), is related to
the PP degradation. This assumption is confirmed by the
fact that the experimental mass loss of 61.5% is close to the
nominal composition of 60% PP and 40 % wood as well as
by Figure 2, which demonstrates that polypropylene undergoes a complete decomposition between 400°C and 500°C.
After switching to air, combustion of the residual carbon
takes place. The corresponding mass loss of 8.5% must be
added to the mass-loss step of 27.8%, resulting in a total
proportion of 36.3%. The remaining ash content is about
1.4%.
Both the residual amount of carbon and the water release
increase with increasing wood content (Figure 2).
As wood turns out to be less thermally stable than polypropylene (PP), its decomposition range should be taken into
account when defining the processing temperature of wood
plastic composites for manufacturing.

Determining the mechanical stability and the
glass transition temperature by DMA
Fig. 2: TGA comparison of three WPC samples with various polymer/wood
contents (sample mass: 12.7 mg to 13.6 mg, heating rate: 10 K/min, atmosphere:
N2/air, Al2O3 crucibles) and one PP sample (sample mass: 10.5 mg, heating rate:
20 K/min, nitrogen atmosphere)

Fig. 3: DMA experiment on a WPC sample containing 60% wood and 40% PP;
3-point bending with a 50-mm free bending length, heating rate: 2 K/min, amplitude: 40 mm, max. dynamic force: 6.6 N, proportional factor: 1.2

50 jec composites magazine / No93 December 2014

Dynamic mechanical analysis (DMA) is a technique that
provides information on the mechanical properties of a
sample material under a small, mostly sinusoidal dynamic
force as a function of temperature, time and/or frequency.
The graph in Figure 3 shows the storage modulus (E´, solid
line) and loss modulus (E´´, dashed line) curves of a WPC
compound with a wood proportion of 60% as determined
through DMA testing. The storage modulus is linked to the
material's stiffness while the loss modulus is a measure for
its damping behaviour. In the present case, the E´ value (at
-50°C) is approximately 9000 MPa at the beginning of the
measurement.
Polypropylene (PP) is a semi-crystalline polymer with a
melting range at around 160/165°C and a glass transition
which can typically be found in the interval between -20°C
to +20°C (3). The glass transition describes the transition
from a hard, brittle state to a viscous, rubber-like state.
By passing through a glass transition region, dynamic
mechanical analysis measures the increase in flexibility of
the sample material resulting in a decline of the storage
modulus curve as soon as the compliance of the sample
(in consequence of the stress) goes up. This makes DMA

JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

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