Investigations on hygrothermal aging of thermoplastic polyurethane material
A. Boubakri, K. Elleuch, N. Guermazi
, H.F. Ayedi
Unité de Recherche de Chimie Industrielle et Matériaux, URCIM – ENIS, BPW 1173-3038, Sfax, Tunisia
Received 16 March 2009
Accepted 29 May 2009
Available online 23 June 2009
A. Thermoplastic polyurethane
E. Hygrothermal effect
I. Polymer degradation
In this investigation, the absorption and diffusion of water in thermoplastic polyurethane (TPU) was stud-
ied. Water-sorption experiments, physical and mechanical property tests were performed after immer-
sion in water at a set temperature of 70 °C for up to 6 months. Emphasis was given to the effect of
immersion aging on thermal, mechanical and tribological properties. Fickian diffusion behaviour was
found during the initial immersion time followed by saturation stage as time progressed. The use of dif-
ferential scanning calorimetry (DSC) analysis showed that the glass transition temperature (Tg) was sen-
sitive to the effect of hygrothermal aging and it decreased with increasing water uptake.
More signiﬁcantly, the mechanical properties in bulk material, obtained from tensile test, were affected
by aging. Elastic modulus and stress at 200% of strain of the studied TPU were decreased after sufﬁcient
exposure to moisture. On the other hand, the mechanical properties of the material surface were equally
investigated with the help of an abrasive wear test. A decrease in wear resistance of the aged TPU was
discerned. The reversibility of mechanical and physical properties after moisture exposure was also
assessed in this study. The polymer degradation was found as irreversible phenomenon.
Finally, the evolution of the mechanical properties seems to be well correlated to structural modiﬁca-
tions obtained from Fourier transform infrared spectroscopy (FTIR) characterization.
Ó 2009 Elsevier Ltd. All rights reserved.
Polymeric materials are being increasingly used in a wide num-
ber of applications where the ratio mechanical properties/weight is
required. In particular, the thermoplastic polyurethanes (TPU),
which are generally described as ‘‘bridging the gap between rubber
and plastics”, impart high elasticity combined with high abrasion
resistance . That is why TPU are widely used in industry and
consumer products, particularly in the ﬁelds of heavy pressure,
load, conveyor belts, protective coating, impact, wear, etc. [2–4].
However, their use in a variety of conditions such as weather
exposure and aggressive elements (sunlight, UV exposure, water
(humidity), polluting agents and temperature) can enormously af-
fect their durability. For these considerations, several studies, in
the last few decades, were focused on the impact of aging on the
materials behaviour [5–12].
On the other hand, natural aging tests are very representative
because they make it possible to reproduce identical conditions
of exposure; however, they are time consuming. That is why accel-
erated aging tests were generally used in order to reproduce the
service conditions of the tested materials [9,10,12,13].
As reported in the literature of polymer aging and from the var-
ious results, the common point that there was harmful effect on
the mechanical and the thermal (physical) properties of the aged
materials, particularly following hygrothermal environments
[10,12,14]. In this context, several studies showed that TPU mate-
rials, exposed to aggressive conditions, undergo signiﬁcant struc-
tural changes which have been found to depend on their
structure and morphology [5,7,15]. These changes lead to a deteri-
oration of the physical properties .
Following the glass transition temperature (Tg) evolution dur-
ing hygrothermal aging can be considered the most used indicator
revealing the degradation of the material structure [16–21]. A de-
crease of Tg was generally found and associated to the plasticiza-
tion phenomenon which takes place during the immersion
period. Similarly, in order to characterize the evolution of the
mechanical properties, several techniques were generally adopted
[12,22]. For example, Alawsi et al.  and Aldajah et al.  have
used the bending (ﬂexural) test in order to evaluate the durability
of certain composites after exposure environments. Aldajah et al.
have shown that the laminate-composites loose most of the ﬂexure
stiffness and the polymeric matrix deteriorated due to the water
absorption. Other authors have used the tensile test
[10,13,23,24]. As results, a decrease in bulk properties (in term of
material structure), such as the tensile strength and the elastic
modulus, was reported in theses studies. This degradation was
generally related to the moisture ingress of water into bulk mate-
rial. Wan et al.  have already shown that the degree of mechan-
ical deterioration of certain composites depended on the level of
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* Corresponding author. Tel.: +216 99 614 199; fax: +216 74 275 595.
E-mail address: firstname.lastname@example.org (N. Guermazi).
Materials and Design 30 (2009) 3958–3965
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