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Green Laminate Composites Based on Polypropylene (PP) and Flax
Fiber
Ngo, T.-D.; Nofar, M.; Ton-That, M.-T.; Sepehr, M.; Hu, W.; Denault, J.
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https://nrc-publications.canada.ca/eng/view/object/?id=a718e13d-7835-4328-9552-626b7cd07d67 https://publications-cnrc.canada.ca/fra/voir/objet/?id=a718e13d-7835-4328-9552-626b7cd07d67FLAX FIBER
T.-D. Ngo, M. Nofar, M.-T. Ton-That, M. Sepehr, W. Hu, J. Denault
Industrial Materials Institute National, Research Council Canada
Abstract
As the demand of green materials and green products are growing, the use of renewable resources and recycle materials are of great attraction. Natural fiber composites have been extensively studied during the last ten years. However, the main focuses were laminate thermoset composites and extrusion/injection composite products.
New approach in fabricating thermoplastic composite parts and composite formulation with flax fiber at low cost has been developed to reduce energy consumption and improve the mechanical performance. The laminate composites were prepared by compression moulding. The results demonstrate that the formulation and the fiber treatment play important roles to the performances of the composites.
Introduction
Nowadays, natural fibers are becoming very interesting reinforcement for composite materials due to their reasonable mechanical properties and environmental benefits. Natural fibers mainly include hollow celluloses which join to each other by a matrix of hemicellulose and lignin [1]. Natural flax fiber consists of 60% cellulose, 15% hemi cellulose, 2-3% pectin, 2% lignin, and 1% waxes [2-3]. Natural flax fibers are one of the best choices to be replaced with glass fibers due to economical and environmental friendly points of view.
Polypropylene (PP) is a thermoplastic polymer which is used in various applications such as packaging, laboratory equipments, automotive components, and etc. It has the density in the range of 0.855 g/cm3 (amorphous) and 0.946 g/cm3 (crystalline). The melting point of PP is 165°C. Also, regarding to literatures other properties of PP can be mentioned as strength of 26-41 MPa, stiffness of 0.95-1.77 GPa, and Tg of -23 up to -10°C depending on the crystalline percentage [4]. Different approached have been achieved regarding to optimizing these factors and using this method to produce flax fiber fabrics and PP sheets [5, 6].
This paper focused on the fabricating thermoplastic composite parts and composite formulation from PP and
flax fiber at low cost. The effect of coupling agent maleated polypropylene, CaO additive, flax fiber treatment on the properties of polypropylene flax fiber composite were investigated.
Experimental
The polypropylene (PP) used was profax 1274. Maleated polypropylene Epolene G-3015 Polymer (E43) from Eastman Chemical Company. CaO was used as additive. Satin plain weave flax fabric was supplied by Moss Composites (Belgium).
The different PP films with the thickness of about 180µm were prepared by extruder. Composite descriptions are shown in Table 1.
Treatment of the fiber does not only affect the surface properties but it was also reported that micro fibrils rearrange themselves in the fiber [7]. In this work the fibers were treated with 2% solution of NaOH at 80 °C for 2 h. Fibers then were dried at room temperature for at least 6 hrs, then in the oven at temperature of 140 °C for about 2 hours to remove the humidity before use.
Table 1. Composite descriptions.
Designation Description PP/flax PP and flax fiber composite PPE/flax (PP + E43) and flax fiber
composite
PPEC/flax (PP + E43 + CaO) and flax fiber composite
PPEC/ tr flax (PP + E43 + CaO) and NaOH treated flax fiber composite Hydraulic press was used to fabricate the composites following specific temperature and pressure profiles (Figure 1).
1 min 7 min 3 min Time (minutes) T e m p er at u re ( oC) P res su re ( T o n s) 9.0 27.0 170 23
Figure 1. Pressure and temperature profiles
The tensile, and interlaminate shear strength properties of the PP/flax composites were determined at room temperature and 50% relative humidity according to ASTM D638-03, and ASTM D 2344-00 on an Instron 5500R machine, with crosshead speeds of 5.0 mm/min, and 1.3 mm/min respectively.
To evaluate the polymer morphology, X-ray diffraction (XRD) patterns were obtained from the surface of the samples with a Bruker Discover 8 powder X-ray diffractometer using CuKα radiation (λ = 1.54250 Å) and a 2θ scan range from 10° to 30°.
A Hitachi-S4700 field emission gun scanning electron microscope (FEGSEM) was used to observe the fracture surface of composites with different magnifications. Samples need to be made conductive by covering the sample with a thin layer of conductive Au/Pd. This was done by using an EMITECH K575X high resolution sputter coater in which an electric field and argon gas were applied.
Melting and crystallization characteristics were also determined using a PerkinElmer DSC7 instrument under the flow of N2. Samples were heated from room
temperature to 220°C with a heating rate of 20°C.min-1 then cooled to room temperature at 20°C.min-1
Results and Discussion
SEM images of the flax fibers before and after treatment with 2% solution of NaOH are shown in Figure 2. SEM indicates that the surface of fiber before treatment was not clean and many lumps of contaminations on the surface as can be seen on Figure 2a. The result shows that after treatment of the fiber with 2% NaOH solution for 2 h at 80°C, the outer surface of the fibers is quite smooth as impurities and contaminants on the fiber surface have been removed.
Figure 2. SEM photos for (a) non-treated and (b) treated flax fibers.
Figure 3 displays the X-ray patterns of different polypropylene-flax composites in the range 2θ = 10°-30°. All specimens show α peaks in this range. The results also indicate that no significant difference on the crystallization is observed for the PP/flax composite with the presence of coupling agent E43 (PPE/flax). Similar observation can be seen when comparing the PPEC/flax composite, and PPEC/treated flax composite. There is no significant difference on the crystallization between them. However, the presence of both E43 and CaO has an effect on the β form of PP/flax. The β form of the PP/flax disappeared with the presence of both coupling agent E43 and CaO. 0 2500 5000 7500 10000 10 15 20 25 30 PP/flax PPE/flax PPEC/flax PPEC/tr flax In te n s it y ( C o u n ts ) 2θ (o) (a) β
Figure 3. XRD diffraction curves for PP/flax composites A thermal analysis of PP/flax composites was performed on the DSC. Figure 4 shows the DSC curves for the first heating (Figure 4a) and cooling (Figure 4b). The DSC curve of the PP/flax composite shows one major melting endotherm at 167°C, corresponding to the melting of alpha phase PP. A slight shoulder on the lower temperature (around 155°C) side of the melting peak corresponds to the melting of a small amount of beta crystals. However, there is no beta form for the PPEC/flax and PPEC/treated flax. These observations correspond to the result that obtained from the XRD.
Figure 4b shows that the PP/flax and the PPE/flax show the crystallization onset at 115°C and the crystallization peak at 106°C while the PPEC/flax with and without treatment exhibit crystallization onset at
indicates that the presence of the CaO faster the crystallization of the PP/flax composites.
0 2 4 6 8 40 60 80 100 120 140 160 180 200 PP/flax PPE/flax PPEC/flax PPEC/treated flax Hea t F lo w ( W /g ) Temperature (oC) (a) β -5 -4 -3 -2 -1 0 40 60 80 100 120 140 160 180 200 PP/flax PPE/flax PPEC/flax PPEC/treated flax Hea t F lo w ( W /g ) Temperature (oC) (b)
Figure 4. DSC curves for different PP/flax composites (a) heating, and (b) cooling
Figure 5 shows the tensile properties for different PP/flax composites. As shown, considering three types of PP base matrix reinforced with untreated flax tissues, including PP, and the other two by adding E43 to PP as the coupling agent and subsequently E43 plus CaO to PP as the coupling agent and reactive additive, respectively. The results shows that the young’s modulus, the tensile strength, and the yield strength increase with the presence of coupling agent and further increase with the CaO reactive additive. However, the composite seems to be more brittle and less tough. This may due to the better interface between the fiber and matrix with the presence of coupling agent, and further with combination between coupling agent and reactive additive CaO. Thus result in higher tensile properties.
Also shown in Figure 5, the composite with alkaline treated flax becomes more brittle and the less toughness compared to flax fiber without treatment. Considering the tensile and yield strength, it can be seen that these values decrease compared to that of untreated flax fiber. However, the young’s modulus of the composite increases with the treated fabric compared to the untreated flax fabric. As we can see in Figure 2 that the treatment results smooth fiber surface as impurities and
This is expected to improve performance of composite. However, the result only shows the improvement on the modulus, and not the strength. It is still not clear about the effect of fiber treatment on the impregnation and the performance of the composite. Further experiment should be carried out to get a better understanding.
60 80 100 120 PP/ fl a x PPE /f la x PPE C/ fl a x PPE C /t r f la x Tensi le S tr e ng th ( M Pa) 6 8 10 12 PP /f la x P P E /fla x PPE C /f la x PPE C /t r f la x T e ns il e M o du lu s ( G Pa )
Figure 5. Tensile properties for different PP/flax composites
Interlaminate shear from short beam shear test (SBS) were evaluated on the composite samples and the results are shown in Figure 6. The result indicates that shear strength of the composites are increasing by adding E43 and subsequently both E43 and CaO to PP as the coupling agent and also it decreases when using treated flax fibers due to its brittle behaviour.
7 9 11 13 PP/ fl a x PPE /f la x P P E C /f la x PPE C/ tr f la x SBS ( M P a )
Figure 6. Interlaminate shear strength for different PP/flax composites
Figure 7 shows the SEM photos of fractured composites. It is shown very clearly in Figure 7a that the interfacial adhesion between hydrophilic flax fiber and hydrophobic PP matrix of composites without coupling agent is poor, which can be seen by the clean fiber surface and there is not much adhering polymer on it. While in Figure 7b, the interfacial property of composites with the presence of coupling agent is much better. Some fibers can be found to be broken off near the surface. And there is no obvious gap between fiber and PP matrix, which suggested the flax fiber is coated and adhered by PP tightly. This is due to the better bonding between hydrophilic flax fiber and hydrophobic PP matrix which promoted by the maleic groups.
This good interfacial adhesion can improve the stress transfer ability from the matrix to the fiber, which results in a better mechanical property of PP/flax composites. Similar or even better phenomenon of good interface between fiber and matrix in composites can be observed for the composite with the presence of both coupling agent and CaO (Figures 7c an 7d). The Figures show more PP adhered on the fiber surface and more broken fibers. One even could not differentiate flax fiber from PP matrix. This better interface proved that the presence of CaO further improves the interfacial property of composites.
Figure 7. SEM photos of fractured for (a) PP/flax; (b) PPE/flax; (c) PPEC/flax; and (d) PPEC/ treated flax composites.
The study on the respond of the properties of the polypropylene flax fiber composites under humid condition is on the way.
Conclusions
Laminate polypropylene flax fiber composites can be fabricated easily by compression molding with acceptable performance. The study has been proven that the addition of maleic anhydride grafted PP to the composites improve the interfacial adhesion between hydrophilic flax fibers and these hydrophobic polymeric matrices. This adhesion is further improved with a new formulation which adding of CaO additive. The micrographs obtained by means of scanning electron microscopy have shown that better adhesion at the fiber-matrix interface exists when grafted matrices are added to the composite. Similar or even better phenomenon of good interface between fiber and matrix in composites can be observed for the composite with the presence of both coupling agent and CaO. In addition, mechanical measurements are in accordance with these observations, showing a considerable increase in the composite properties when the grafted matrices and CaO were added to the composite.
Acknowledgements
The authors would like to thank the financial support from different research programs of the Canadian Government via the National Bioproduct Program 2 of the National Research Council of Canada and the ABIP-NAFGEN of the Agriculture Agri-Food Canada. Nofar would like to thank the National Science Research Council Canada for the financial support for the research fellowships (Grant# N00784).
(a) (b)
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