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1. http://www.iaeme.com/IJMET/index.asp 10 editor@iaeme.com
International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 12, December 2019, pp. 10-16, Article ID: IJMET_10_12_002
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=12
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
EXPERIMENTAL STUDY OF RELAXATION
BEHAVIOR OF INJECTED COMPOSITES WITH
POLYPROPYLENE REINFORCED BY SHORT
FLAX FIBERS
Houssam Ourchid, Mariam Benhadou, Abdellah Haddout
Laboratory of Industrial Management and Energy and Technology of Plastics and Composites
ENSEM - University Hassan II, Casablanca Morocco
Basma Benhadou
International University of Casablanca, Casablanca Morocco
ABSTRACT
The use of thermoplastic composites reinforced with flax fibers in several fields,
particularly in the automotive and aeronautics sectors, provides a favorable response
to environmental requirements and new regulations on the recycling composite
materials.
In this work, we present the experimental study of mechanical behavior, relaxation
and morphological evolution of injected polypropylene / short flax fibers, depending
on the mold temperature, the fiber content and the nature of coupling agent. This
approach is motivated by the existence of relations between the microstructure, the
thermal and mechanical parameters of injection molding and the viscoelastic
properties of the material that we seek to highlight. The mold temperature and the
coupling agent rate have a decisive importance on the morphology and the
mechanical properties of the injected composites.
Finally, we improve the control of the injection molding process to optimize the
visoelastic properties of manufactured parts.
Keywords: Thermoplastic Bio-Composites, Relaxation, Injection Molding,
Morphology
Cite this Article: Houssam Ourchid, Mariam Benhadou, Abdellah Haddout and
Basma Benhadou, Experimental Study of Relaxation Behavior of Injected Composites
with Polypropylene Reinforced by Short Flax Fibers. International Journal of
Mechanical Engineering and Technology 10(12), 2019, pp. 10-16.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=12
1. INTRODUCTION
Composite materials injected and reinforced with plant fibers and thermoplastic matrices are
developing considerably in the various industrial fields, in particular in the automotive,
medical, building, electronics fields.

2. Experimental Study of Relaxation Behavior of Injected Composites with Polypropylene
Reinforced by Short Flax Fibers
http://www.iaeme.com/IJMET/index.asp 11 editor@iaeme.com
These materials constantly provide increasing performance allowing manufacturers to
consider innovative and competitive technical solutions [1-2]. Indeed, the advantages of
natural fibers compared to their synthetic counterparts (glass fiber, carbon, ...) are numerous,
citing their lower cost and their specific properties / density ratios comparable to glass fibers.
They are also renewable and recyclable [3 -4]. Using short flax fibers instead of glass fibers
has a clear advantage for the automotive industry. This economic and ecological gain is an
asset for large companies.
The thermomechanical history of injection process produces morphological variations in
the fabricated part leading to structural domains that depend both on the nature of the
composite material and the processing conditions.
In many cases of thermoplastic composites, the poor dispersion of the fibers or a lack of
cohesion between the fibers and the matrix prevents the obtaining of the desired properties.
The size and orientation of the fibers affect the mechanical properties of the injected parts:
tensile strength, elongation at break, impact resistance.
The determination of the influence of these parameters on the microstructure and the
knowledge of the relationships which connect it to the mechanical characteristics of the
injected parts, have the advantage of being able to predict particular transformation conditions
as a function of the geometry of the parts to be manufactured and the field of application [5].
The use properties of thermoplastic polymers reinforced with short fibers (mechanical
properties, shrinkage, etc.) can be very complex. The existence of anisotropy is mainly related
to the heterogeneity of the fibers orientation created during the non-isothermal flow and at
high pressures of the polymer-fiber system in the mold cavity, as well as the quality of the
interfacial matrix-fiber area.
Different parameters can influence the properties of plant fibers such as the nature, the
variety of the fiber, its structure, the micro-fibrillar angle and its cellulose content [6 - 7]. Flax
fiber has specific properties (ratio between the mechanical quantity and the volumetric mass)
equivalent to or even greater than fiberglass, and it is the fiber that makes it possible to obtain
the composite with the best mechanical properties. These properties will also strongly depend
on the fiber / matrix interface [8-9]. Polypropylene has a low surface energy, resulting in poor
fiber / matrix adhesion. The use of a coupling agent or a compatibilizer such as maleic
anhydride grafted polypropylene makes it possible to improve the fiber / matrix adhesion [10-
11-12].
Jandas et al [12] studied the influence of surface treatments of banana fibers/ PLA . The
properties of the composites were evaluated by mechanical tests, DSC and TGA, while the
viscoelastic properties were measured by DMA. The viscoelastic measurements using DMA
confirmed the increase of the storage modulus and the reduction of the damping coefficient
for the treated fiber biocomposites.
The effect of fiber orientation on the viscoelastic properties of a thermoplastic composite
has been the subject of numerous studies. Kurivilla et al. [5] have observed in the case of
cellulose fiber-reinforced polyethylene, that the modulus of elasticity increases with the
length of the reinforcing fibers.
The objective of this work is to predict the long-term behavior of polypropylene / short
flax fiber composites based on a characterization of their relaxation behavior. The effect of
fiber content and the nature of treatment on viscoelastic properties have been reported

3. Houssam Ourchid, Mariam Benhadou, Abdellah Haddout and Basma Benhadou
http://www.iaeme.com/IJMET/index.asp 12 editor@iaeme.com
2. EXPERIMENTAL PROCEDURE
For this study, we chose isotactic polypropylene reinforced with different rate of short flax
fibers sized or not sized. Polypropylene is a semi-crystalline polymer. It is presented in the
form of colorless and translucent granules with a volumetric mass  = 908 kg / m3
, melt index
I = 15 g / min and melting temperature T = 167 °C.
Flax fibers used of 2.5mm average length before processing, and an average diameter of
240m. In order to improve the fiber-matrix interface, 5% of the maleic anhydride-grafted
polypropylene was added as a coupling agent.
The specimens were made using an industrial injection molding machine. This machine
has a closing force of 130 tons, equipped with a standard 35 mm diameter screw and an
instrumented mold. It is controlled by a microprocessor. This system allows in particular the
automatic adjustment of the press (closing force of the mold, temperature of the heating
collars of the barrel and the nozzle, etc.) and the adjustment of the parameters of the injection
cycle (injection speed, holding pressure, holding time, cooling time, injection pressure,
injection temperature ...).
Equipped with a double cavity mold, these are tensile test specimens defined according to
the ISO R527 standard.
The main injection parameters used are described in Table 1:
Table 1 The main injection parameters
Injection parameter Values
Injection temperature 195 °C
Mold temperature 30 et 60°C
Injection pressure 45MPa
Holding pressure 40MPa
Holding time 5 s
Speed of screw rotation 122 tours/min
Injection speed = 45 mm/s
3. RESULTATS ET DISCUSSION
3.1. Tensile behavior
Figure 1 shows the tensile behavior for different flax fiber rate of polypropylene thermoplastic
composites. This same figure shows that the stress at break increases very significantly with the
increase of the fiber volume fraction, with a significant decrease of the deformation at break.
Figure 1 Tensile behavior of polypropylene / short flax fiber composites at different fiber rate
0
5
10
15
20
25
30
35
40
0 0.01 0.02 0.03 0.04 0.05
constraint(MPa)
Deformation (%)
10%
20%
30%

4. Experimental Study of Relaxation Behavior of Injected Composites with Polypropylene
Reinforced by Short Flax Fibers
http://www.iaeme.com/IJMET/index.asp 13 editor@iaeme.com
3.2. Relaxation Tests
The relaxation tests were carried out using the LLOYD LR50K type apparatus on ISO 527
type specimens. This machine is equipped with a thermostatically controlled chamber cooled
by a circulation of nitrogen and possibly heated by an electrical resistance and air ventilation.
During each relaxation test, the constancy of the deformation is ensured by extensometers of
high precision and the temporal evolution of the stress is recorded with an acquisition rate of
4 points / second. The relaxation tests were carried out throughout the experiment at
controlled temperatures.
Study of the Incidence of Fiber Rate
We have studied the evolution of the relaxation over time of polypropylene reinforced at
different rate of short flax fibers. Figure 2 shows the evolution of relaxation stress over time.
Figure 2 Evolution of the relaxation stress of composite at different rate of short flax fibers in the
presence of the coupling agent, mold temperature Tm = 60°C.
The increase in the fiber content promotes a clear improvement in the relaxation stress. It
is noted that the initial stress is greater in the case where the rate is higher.
Incidence of Mold Temperature
During this part, we will treat the case of two types of molded parts at Tm = 30 ° C and Tm =
60 ° C, we studied the evolution of the relaxation stress as a function of time.
Figure 3 Evolution of the Relaxation Stress of a Composite reinforced with 10% Short sized flax
Fibers at Different Mold Temperatures
0
5
10
15
20
25
30
35
40
0 2000 4000 6000 8000
ConstraintN/mm²
Time (s)
30
%
10
%

5. Houssam Ourchid, Mariam Benhadou, Abdellah Haddout and Basma Benhadou
http://www.iaeme.com/IJMET/index.asp 14 editor@iaeme.com
Figure 3 illustrates the evolution of the relaxation stress over time for a polypropylene
composite filled with 30% short flax fiber and injected at mold temperatures Tm = 30 ° C and
Tm = 60 ° C. We notice a very great increase of the relaxation stress with the temperature of
the mold. This evolution is explained by the impact of the temperature of the mold on the
kinetics of crystallization of the material.
Study of the Impact of Sizing
In order to study the impact of fiber surface treatment on the relaxation stress, we studied the
effect of the coupling agent and in particular the fiber-matrix interfacial state on the relaxation
behavior. Figure 4 shows the evolution of the relaxation stress of polypropylene reinforced
with 20% flax fiber, at mold temperature 30 ° C, with and without coupling agent.
Figure 4 Evolution of the relaxation stress of polypropylene reinforced with 20% short flax fibers,
with and without coupling agent.
This figure shows that the presence of the coupling agent produces an increase in the
relaxation stress. This result allows us to conclude that maleic anhydride plays an important
role in improving adhesion between the matrix and the fibers.
3.3. Temperature Relaxation Test
During this part, we will treat polypropylene composites reinforced with short flax fibers at
Tm = 30 ° C, we studied the evolution of the relaxation stress at different test temperatures.
Four temperature values were studied 21, 40, 60°C.
Figure 5: Relaxation stress of a composite loaded with 20% short flax fibers in the presence of the
coupling agent, and at different test temperatures.
0
10
20
30
40
0 500 1000 1500 2000 2500 3000
Constraint(N/mm²)
Time (s)
TE=21°C
TE=40°C
TE=60°C

6. Experimental Study of Relaxation Behavior of Injected Composites with Polypropylene
Reinforced by Short Flax Fibers
http://www.iaeme.com/IJMET/index.asp 15 editor@iaeme.com
Figures 5 and 6 show the evolution of relaxation stress of polypropylene composites at
different rate of short flax fibers as a function of time for different test temperatures. The
normal forces applied to obtain an initial strain equal to 1.9% are also illustrated.
It is observed that the relaxation stress decreased with time and the force to create the
initial strain decreases as the test temperature increases.
Figure 5 Relaxation stress of a composite reinforced with 30% short flax fibers, in the presence of the
coupling agent, and at different test temperatures.
It can be seen that the behavior of composites strongly depends on the test temperature.
However, because of the glass transition of polypropylene close to ambient temperature, the
test temperature is a factor influencing the response of the composites. Composites are stiffer
at low temperatures and their stiffness decreases by raising the temperature. Under the effect
of thermal agitation, the higher the temperature increases, the greater the molecular vibrations
and the more the molecules can move freely, which reduces the force required to apply the
initial strain 1.9% and the stress relaxation accelerated.
4. CONCLUSION
Our research focused on the determination of the optimal transformation parameters by the
industrial injection molding process of different composites formulation with polypropylene /
short flax fiber, in order to obtain materials with better mechanical properties, and also to
understand the effect of these parameters on the visual appearance, smell, shape and
distribution of the fibers and also on the stability of the injection process.
We studied the influence of flax fiber and a coupling agent on a polypropylene matrix
composite as well as that of the injection molding process on these composites. The
reinforcement of polypropylene by the flax fiber makes it possible to improve the mechanical
properties. We have studied the relaxation of the different polypropylene/ short flax fiber bio-
composites, depending on the importance of the injection processing parameters and the fiber
structure. An increase in the temperature of the mold, on the one hand, and the presence of a
coupling agent, on the other hand, led to an improvement in the relaxation stress. The
morphological analysis of the injected parts allowed us to establish a relationship between the
viscoelastic behavior and the characteristics of the fiber-matrix interface.
0
5
10
15
20
25
30
35
40
0 400 800 1200 1600 2000
Constraint(N/mm²)
Time (s)
TM30 TE21
TM30 TE40
TM30 TE60

7. Houssam Ourchid, Mariam Benhadou, Abdellah Haddout and Basma Benhadou
http://www.iaeme.com/IJMET/index.asp 16 editor@iaeme.com
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