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M. Dolores La Rubiaª, Ana B. Lópezª, Rafael Pachecoª, Francisco Navasb
ªDepartment of Chemical, Environmental and Materials Engineering, Higher Polytechnic School of Jaen, University of Jaen, 23071 Jaen, (Spain) bAndaltec, Plastic Technological Centre, Martos, Jaén (Spain)
Key words:
The main objective of this work is to evaluate the
changes that occur in the mechanical and thermal properties of
polypropylene used as bumpers of cars after several mechanical
recycling cycles consisting of alternating cycles of grinding, extrusion
and injection. A comparative study is made with the results of the
virgin polypropylene.
References
[1] Sadrmohaghegh C., Scott G. Polymer Degradation and Stability 3, 333-340 (1981).
[2] Jansson A., Möller K., Gevert T. Polymer Degradation and Stability 82, 37-46 (2003).
[3] Jansson A., Möller K., Hjertberg T. Polymer Degradation and Stability 84, 227-232 (2004).
Acknowledgment
Financial support for this work was provided by the Andalucía Regional Government (Teluris
project, reference number: PI 26369)
OBJETIVE
Influence of the recycling in the mechanical and thermal
properties of post-consumer polypropylene materials used
as bumpers of cars
Elongation-at break
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Impact Strength
The large amount of waste currently produced by the society is a serious environmental problem.
Recycling is presented as a way to reduce waste. The fact that plastics are organic matter that
degraded by oxidation during use and when they are subjected to high temperatures and shear forces
during processing, may explain the limited use of recycled plastics. It is therefore necessary further
knowledge of the properties of recycled plastics in order to find appropriate applications and thus
increase the use of these materials.
The post-consumer polypropylene was from parts of cars. The recycling cycle consisted in
grinding, extrusion and injection (Fig.1), obtaining different test samples (PPpc1) according to the
UNE-EN standards (Fig.2). After analyze them, the test samples were recycled for two and three
times (PPpc2 and PPpc3). In order to investigate the influence of the recycling cycles on the
material, the mechanical properties were studied by tensile testing according to UNE-EN-ISO-527
standard and Impact test (UNE-EN-ISO-179). Vicat Softening temperature (VST) test (UNE-EN-ISO-
306) and Differential Scanning Calorimetry (DSC) were also performed. Figure 3 shows the
experimental procedure.
The polypropylene (PP) is one of the most widely used plastics and maintains its mechanical
properties after undergoing successive recycled. One of the most interesting properties of
polypropylene is its good recycling behavior as it maintains its mechanical properties such as impact
strength, modulus of elasticity and thermal after undergoing extrusions and repeated injections.
According to several authors [1], after these processes only properties fell by 4.4% and the index
carbonyl remains virtually unchanged .
Mechanical properties
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%)
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a)
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Experimental Procedure Introduction
Results and Discussion
Figure 3: Experimental procedure Figure 2: Test samples
Figure 1: Grinding-extrusion-injetion line
Plastic waste
Thermal properties
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HDT
VST
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Tg
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DSC heating scans were performed to determine the influence of the recycling cycles on the thermal behavior and
crystallinity of the PP post-consumer. The melting temperature values (Tm) increased and crystallinity index (Wc)
for the studied samples diminished after recycling. Vicat and HDT temperatures showed slightly variation.
Young’s Modulus Yield stress
Most of the mechanical properties of the post-consumer polypropylene decrease after recycling compared to virgin polypropylene During simulated
recycling the mechanical properties (elongation-at-break, yield stress and Young’s Modulus) dropped initially and increased again upon second and third
recycling cycle. However, impact strength showed opposite behavior.
This zig-zag behavior is probably explained by a combination of three mechanisms: (1) changes in degree of crystallinity, (2) surface degradation, and (3)
dilution of degraded polymer chains after re-extrusion.
Conclusions
Post-consumer recycled PP has
lower mechanical properties than
virgin PP. Hoewever, after several
recycling cycles maintains the
mechanical and thermal properties.
