JEC COMPOSITES MAGAZINE - Issue #93 - December 2014 - 51

practice, the CTE is also often used as an average value between two indicated temperature points - even if these two
temperature points are not lying on a straight line.
Figure 4 illustrates the thermal expansion of the same WPC
material (60% wood - 40% PP) as used in the DMA experiment, this time in the temperature range between -30°C
and 100°C. Due to the glass transition, the presented relative length change curve of the WPC is slighly bent.

Fig. 4: Dilatometer measurement on a WPC sample containing 60% wood and
40% PP; sample length: 24.79 mm, heating rate: 3 K/min, He atmosphere

much more sensitive than differential scanning calorimetry
(DSC), for the determination of glass transition temperatures for samples with a low polymer content.
In Figure 3, the storage modulus drops down at -14°C (extrapolated onset temperature of E´) during heating due to
the glass transition of the PP matrix. The step displayed in
the E´ curve is accompanied by a peak in the loss-modulus
curve (E´´) at -6°C. Both points can be used to define the
glass transition temperature (Tg): standard ASTM E1640
favours the extrapolated onset of the E´ curve whereas
ASTM D4065 recommends the peak temperature of E´´.
In the E´ curve, the jump downwards between -20°C and
+20°C amounts to more than 30% (from 8168 MPa to
5594 MPa). Between 20°C and 50°C, there is a further
decrease to 4239 MPa (again more than 20% compared
to 5594 MPa), resulting in a total drop of almost 52% between -20°C and 50°C, which is a characteristic temperature difference between winter and summer under direct
solar radiation.
The position of PP's glass transition temperature together
with the dramatic change in stiffness is most probably the
reason why polypropylene-based WPCs have been mainly
used as non-structural components so far.

Depicting the length change as
a function of temperature by dilatometry
Dilatometry measures expansion and shrinkage phenomena during heating or cooling. It is closely related to
thermomechanical analysis (TMA). A typical application
of this method is the determination of the thermal expansion coefficient (CTE, often called technical alpha) within
a certain temperature interval.
Strictly speaking, the thermal expansion coefficient
describes the slope of a linear expansion curve, but in

Two CTE values are calculated. The one between -20°C
and 50°C, which simulates the temperature difference
between winter and summer, results in 39.0 · 10-6 1/K. The
other one, between 20°C and 50°C, is 42.1 · 10-6 1/K. For
a 1-m long WPC board, this correlates to an elongation
of 2.7 mm between -20°C and 50°C or of roughly 1.3 mm
between 30°C and 50°C, respectively. Contrary to that, the
thermal expansion coefficient of pure PP is much higher
and exhibits values of 130 · 10-6 1/K to 180 · 10-6 1/K (3).
Based on a mean CTE of 150 · 10-6 1/K, a 1-m long PP
board would expand by 10.5 mm when heated from
-20°C to 50°C.

Conclusion
Thermal analysis methods are powerful tools for characterising the properties of PP-based wood plastic composites in
terms of processing and application. While TGA provides
information about the WPC's wood/polymer ratio and
its thermal stability, DMA generates data regarding its
mechanical properties and is very sensitive for determining
the glass transition range of the polymer matrix component. Finally, dilatometry offers the opportunity to learn
more about the expansion behaviour of the material by
measuring its length change during heating. All measurement techniques prove to be easy to handle, reliable and
highly accurate. n

More information:
www.netzsch-thermal-analysis.com
References:
[1] M. Carus, A. Eder, Jec Composites Magazine, No. 89, May
2014, p. 23-26
[2] www.nova-institut.de/pdf/06-01_WPC-Studie.pdf, WPC
survey about markets (North America, Japan and Europe
with focus Germany), technical properties and application
areas edited by nova-Institute, Germany, January 2006
[3] Netzsch poster "Thermal
Properties of Polymers"

No93 December 2014 /

jec composites magazine 51

http://www.netzsch-thermal-analysis.com http://www.nova-institut.de/pdf/06-01_WPC-Studie.pdf

JEC COMPOSITES MAGAZINE - Issue #93 - December 2014

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #93 - December 2014