Experimental study of relaxation behavior of injected composites with polypropylene reinforced by short flax fibers

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.

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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

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

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

Speed of screw rotation 122 tours/min

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

Deformation (%)

10%

20%

30%

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

Time (s)

30

% 10

%

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

Time (s)

TE=21°C TE=40°C TE=60°C

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

Time (s)

TM30 TE21

TM30 TE40

TM30 TE60

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