23. - 25. 10. 2012, Brno, Czech Republic, EU
POLYPROPYLENE FIBERS MODIFIED BY ATMOSPHERIC PRESSURE PLASMA AS
REINFORCEMENT FOR CONCRETE
Monika FIALOVÁ a, Tomáš MORÁVEK a, Daniel KOPKÁNĚ b, Jozef RÁHEĽ a,c, Pavel SŤAHEL a,
Mirko ČERNÁK a,c
a
Department of Physical Electronics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno,
Czech Republic ([email protected])
b
Faculty of Civil Engineering, Brno University of Technology, Veveří 95, 66237 Brno, Czech Republic,
([email protected])
c
Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius
University, Mlynska dolina F2, 842 48, Bratislava, Slovak Republic ([email protected])
Abstract
This contribution deals with the atmospheric pressure plasma treatment of PP fibers that leads to the
increase of surface energy of polypropylene and the improvement of cohesion of PP fibers to cement matrix
as well. The plasma modification was performed by Diffuse Coplanar Surface Barrier Discharge operating at
atmospheric pressure in ambient air. The surface energy of PP fibers was studied by the method based on
the measurement of the weight of absorbed water as a function of time. The change in cohesion was tested
by restrain shrinkage test on mortar samples. It was confirmed that the plasma treatment improved the
surface wetting properties of PP fibers and the mechanical performance of the plasma treated PP fibers
reinforcing concrete.
Keywords:
polypropylene fibers, fiber-reinforced concrete, surface energy, Washburn method
1.
INTRODUCTION
Polypropylene fibers are popular for reinforcing concrete composites due to their unique properties such as
high melting point, chemical stability and relatively low cost compared to other fiber types. Particularly these
fibers provide secondary concrete reinforcement for a reducing of shrinkage cracking and its spreading.
However polypropylene is typically hydrophobic, resulting in poor bonds with concrete matrices [1].
Until now several methods leading to the bonding have been suggested. These methods have been based
mainly on mechanical, chemical or low-pressure plasma pretreatment of PP fibers [2,3,4]. Recently the
atmospheric pressure plasma treatment of PP fibers seems to be more economical, ecological and
commercially attractive. This contribution presents new method of plasma treatment of PP fibers based on so
called Diffuse Coplanar Surface Barrier Discharge (DCSBD) operating in atmospheric pressure and
generating a thin layer of macroscopically homogenous plasma. The possibility of using the DCSBD for
plasma treatment of polypropylene nonwovens textile to increase the surface free energy has been
successfully tested [5].
2.
EXPERIMENTAL ARRANGEMENT
For the plasma treatment of PP fibers the DCSBD electrode system was used. Its schematic diagram is
shown in Fig.1. The DCSBD panel is consist of two systems of parallel strip-like electrodes (1,5 mm wide, 1
mm strip to strip) embedded in alumina ceramic. The ceramic layer between the plasma and electrodes was
0,4 mm. A sinusoidal high-frequency high voltage was applied between electrode systems. Due to the
23. - 25. 10. 2012, Brno, Czech Republic, EU
specific arrangements of the electrode panel, the thin layer (about 0,3 mm) of macroscopically diffuse
ambient air plasma is generated on the ceramic surface. The effect of plasma treatment on PP fibers was
tested for discharge power 300 W and various treatment time.
The apparatus for in-line plasma treatment of PP fibers shown in Fig.1 was built. It enables continuous
setting of the treatment time and automatic plasma treatment. Commercially produced polypropylene fibers
by KrampeHarex CZ Company were tested in this study. Diameter of these fibers with round cross section
-3
2
was (32±10%) μm, density of fibres was 910 kg.m and the tensile strength was (300±15%) N.mm .
dielectric Al2O3
plasma
electrode
Fig.1 The schematic illustration of the DCSBD electrode system and apparatus for in-line plasma treatment
of PP fibers.
3.
DIAGNOSTIC METHODS
3.1
Washburn Wicking Test
Standard laboratory analytical scales DENVER IN SL123 were adapted for performing Washburn wicking
test into porous medium of closely packed fibers.
Known mass PP fibers of approximately the same
length were closely packed into the glass tube of 10
mm inner diameter, forming a numerous inter-fibrous
capillaries oriented in parallel with glass tube axis. The
position of fibers bundle was adjusted so it slightly
stuck out of the bottom glass tube end and cut to obtain
well-defined flat surface. Glass tube was mounted onto
custom made holder attached to scales (see Fig.2) and
consequently the fibers cut surface was brought into
the contact with the level of wicking medium (mainly
distilled water). The increase of glass tube mass
caused by liquid drawn by capillary elevation forces
was recorded by attached PC.
Fig.2 Balance adjusted to measure the increment of mass.
In order to determine the contact angle, the following mathematical model of capillary elevation was used.
According to Fries and Dryer [6] the momentum balance of a liquid inside a capillary tube gives
23. - 25. 10. 2012, Brno, Czech Republic, EU
2 cos 
8 h
d (hh)
  gh sin  2 h  
R
R
dt
(1)
where σ is surface tension, θ contact angle, R pore radius, ρ density of liquid, h height of the liquid level, ψ
angle formed between the inclined tube and the free liquid surface and μ dynamic viscosity. Measuring the
height of the water level h directly is impractical, therefore the mass increment is measured. Then Eq.(1) can
be modified into a form more suitable for linear regression
m
A
 R cos  2 2
 B, where A 
S 
m
4
(2)
and where S is total inter-fibrous area. The contact angle is only present in the term A. That is the reason,
why the slope of dependency dm/dt=f(1/m) in the given interval is of great interest to us. Eq.(2) shows, that A
depends linearly on cos θ and the pore radius R. Bigger values of A correspond to hydrophilic fibers and the
smaller values to hydrophobic ones. Using the above mentioned procedure and measured dependences
m(t), we were able to determine the contact angles of untreated, sized and plasma-treated fibers.
3.2
Restrain Shrinkage Test
Since the PP fibers are added mainly for improve the capacity of concrete to overcame early age shrinkage
strain the experimental part made on cement paste involving restrain shrinkage test. The method is based
on steel ring surrounded by tested mixture which undergoes volume changes. Because the steel ring is
obstacle for shrinkage, certain strain will be induced in to the specimen. When the strain overcame strength
of the sample a ruptures will occur. Delicate changes in geometry of the steel ring can be measured by strain
gauge.
Procedure steps involved:

Mixing of cement, water and PP fibers (see Tab.1)

Placing of the mixture between the steel ring and complementary external ring.

Releasing of external ring after 12 hours (for higher drying rate).

Continual reading of tension of the steel ring by strain gauge.
The specimens were placed in environment with relative humidity of 45±3%, without forced movement of air.
After initial period of hardening of the mix, complementary external ring was removed. This step increased
the drying rate and therefore drying shrinkage.
Tab.1 Composition of tested mix.
Ingredient
Portland type cement - Mokrá 42,5 R
Tap water
Polypropylene fibers
Mass [g]
2000
800
10
23. - 25. 10. 2012, Brno, Czech Republic, EU
Fig.3 Experimental setup of restrain shrinkage test.
4.
RESULTS
The wetting properties of PP fibers were examined by the increment of water mass depending on time (see
the left graph in Fig.4). From this dependence using the equations mentioned above, the contact angle of
untreated, sized and plasma treated fibers were calculated. The right graph in Fig.4 shows the results. The
wettability of PP fibers after plasma treatment was increased. The 5s plasma treatment achieved the best
value, the contact angle was about 55°. For comparison the contact angle of untreated fibers was about 84°
and sized fibers 13°.
0
50
100
150
200
0
5
10
15
90
sized
plasma treated 5s
plasma treated 15s
plasma treated 30s
untreated
1,2
0,8
1,0
0,5
0,4
0,0
0
20
40
60
80
100
120
time [s]
140
160
180
200
220
90
80
80
70
70
60
60
50
50
40
40
30
30
20
0,0
30
1,5
contact angle [°]
weight of absorbed water [g]
1,6
25
Plasma treatment 300 W
2,0
2,0
20
20
sized fibers
10
10
0
0
0
5
10
15
20
25
30
exposure time [s]
Fig.3 Mass increment of water on different fibers (left) and calculated contact angle of treated and untreated
polypropylene fibers (right).
It is worth mentioning that the use of above mentioned procedure for determining the contact angle of nontreated polypropylene fibers (with low surface energy) proves itself to be quite difficult. In some cases the
non-treated fibers are reluctant to draw any water at all, indicating that the contact angle is greater than 90°.
23. - 25. 10. 2012, Brno, Czech Republic, EU
This is however of small interest in our case, since the untreated fibers are of almost no use to making
concrete mixtures.
relative lenght change [m/m]
It can be seen that sample without fibers had sudden failure after 50 hours. Similar behavior had also sample
with untreated fibers – the presence of fibers in the mix postponed the failure of approximately 10 hours but
the rupture happened almost at once. It is probably due to poor cohesion of untreated PP with cement
matrix. In the other hand commercial lubricated fibers (sized) and plasma treated fibers had significantly
different response. The
40
slow releasing of strain
plasma treated 5s
maximum
of
strain
plasma terated 30s
increased
and
rupture
30
untreated
happened in later time.
without fibers
sized
Moreover
the
cracks
20
opened slowly in many
steps. Also the sizes of
10
cracks were much smaller
but they appear in higher
0
number. This could be
sudden apperance of crack
beneficial
since
young
-10
concrete shows tendency
for healing – tiny cracks can
-20
0
10
20
30
40
50
60
70
80
be patched in few months.
time [hours]
Fig.4 f Results of restrain shrinkage test.
5.
CONCLUSION
The influence of atmospheric pressure plasma treatment on wetting properties of polypropylene and the
cohesion of PP fibers to cement matrix was investigated. By the means of the contact angle measurement it
was shown that plasma treatment enhances the wettability of polypropylene fibers. Even the 5s plasma
treatment improves the surface free energy of polypropylene. The results of restrain shrinkage test indicate,
that cohesion of plasma treated PP fibers has been significant improved. The manner in which the cracks
propagates through the samples has been modified by meaning of later occurrence and slow releasing of
induce strain.
ACKNOWLEDGEMENTS
Research has been supported by the project CZ.1.05/2.1.00/03.0086 funded by European Regional
Development Fund and project TA01010948 funded by Technology Agency of Czech Republic.
LITERATURE
[1]
ZHENG, Z., FELDMAN, D. Progress in Polymer Science 20, 185–210 (1995)
[2]
PEI, M., WANG, D., ZHAO, Y., HU, X., XU, Y., WU, J. Journal of Applied Polymer Science 92, 2637–2641 (2004)
[3]
FELEKOGLU, B., TOSUN, K., BARADAN, B. Journal of Materials Processing Technology 209, 5133–5144
(2009)
[4]
WEI, Q.F., Materials Characterization 52, 231–235 (2004)
[5]
CERNÁK, M., ČERNÁKOVÁ, L., HUDEC, I., KOVÁČIK, D., ZÁHORANOVÁ, A. The European Physical Journal
Applied Physics 47, 1–6 (2009)
[6]
FRIES, N., DREYER, M. J. of Colloid and Interface Sc ience 320, 259–263 (2008)
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POLYPROPYLENE FIBERS MODIFIED BY ATMOSPHERIC