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wood flour composites: Effect of pulverization of wood flour with and without water
Accepted Manuscript Melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour composites: effect of pulverization of wood flour with and without water Md Minhaz-Ul Haque, Koichi Goda, Hirokazu Ito, Shinji Ogoe, Masaki Okamot, Tomoyuki Ema, Keiko Kagawa, Hidetaka Nogami PII:
S2542-5048(18)30037-X
DOI:
https://doi.org/10.1016/j.aiepr.2018.11.001
Reference:
AIEPR 14
To appear in:
Advanced Industrial and Engineering Polymer Research
Received Date: 26 September 2018 Revised Date:
12 November 2018
Accepted Date: 29 November 2018
Please cite this article as: M. Minhaz-Ul Haque, K. Goda, H. Ito, S. Ogoe, M. Okamot, T. Ema, K. Kagawa, H. Nogami, Melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour composites: effect of pulverization of wood flour with and without water, Advanced Industrial and Engineering Polymer Research, https://doi.org/10.1016/j.aiepr.2018.11.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour composites: effect of pulverization of wood flour with and without water Md Minhaz-Ul Haque1a*, Koichi Goda1, Hirokazu Ito2, Shinji Ogoe3, Masaki Okamot3, Tomoyuki Ema3, Keiko Kagawa3, Hidetaka Nogami4 1
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Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi, 755-8611 Japan 2 National Institute of Advanced Industrial Science and Technology, Hiroshima 739-0046, Japan. 3 Technology Development Center, TOCLAS Co., Shizuoka 432-8001, Japan 4 Okayama Prefectural Research Institute for Forest and Forest Products, Okayama 717-0013 Japan
Abstract: This study analyses the effect of pulverization of wood flour with and without water
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to the melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour (WF) composites. The composites were processed in an extruder and subsequent injection moulding. The effects of initial wood flour particle size, type of trees (namely cypress and European scots pine trees) as well as plate gap (200 µm and 350 µm) in pulverization were also evaluated in terms of melt-viscosity, tensile properties, Izod impact strength and fatigue behaviour of the prepared composites. PP/WF composites with pulverised wood flour with water exhibited higher
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melt-viscosity than that of nonpulverized and pulverized wood flour without water reinforced PP composites. Tensile strength values of composites were slightly affected by the pulverization of wood flour and the enhancement of tensile strength values were depended on the type of initial WF. From Izod impact test results of composites, it was found that the composites reinforced
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with pulverized wood flour with water displayed higher values of impact energy compared with the composites of pulverized WF without water. Depending on the type of initial WF particle, the positive effect of pulverization of wood flour with water to the fatigue life of the composites
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was also observed.
*Corresponding author at: Department of Mechanical Engineering, Yamaguchi University 2-16-1 Tokiwadai, Ube, Yamaguchi, 755-8611 Japan Cel: +8150 68650354; E-mail: [email protected] a
Permanent address: Department of Applied Chemistry and Chemical Engineering, Islamic University, Kushtia, Bangladesh Keywords: composites; mechanical properties; melt-viscosity; pulverization; wood flour
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1. Introduction Generally, hard materials are used as filler in soft polymeric materials to obtain materials of desired properties. These fillers can be natural such as clay, silica, wood flour (WF), cellulose fibres, or synthetic such as carbon black, aramid fibres, carbon fibres, glass fibres etc. Presence
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of these fillers into polymer matrices can cause a diverse change in the physical and mechanical behaviour of polymeric materials. Each filler may also has several advantages and disadvantages. Among the above fillers probably the most inexpensive one is wood flour. The advantageous characters of this green filler such as biodegradability, renewability, sustainability, light weight,
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less abrasive nature to processing equipment and cheapness have drawn much attention of polymer composite researchers. Since WF is inexpensive filler, hence, from economic point of view, it can be said, addition of WF into polypropylene (PP) matrix is a simple strategy of
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making inexpensive polypropylene composites. Further, incorporation of wood flour into PP matrix not only reduce the cost of the composite materials, but it can also impart its physical and mechanical properties to composite materials. However, WF, due to its hydrophilic nature, are not compatible with hydrophobic PP matrix. Consequently, the properties of WF is not contributed into PP matrix as much as expected. Thus the prepared PP/WF composites show lower performance than that of expected from it. Solutions to incompatibility problem as well as
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to have most probable contribution of WF properties in PP/WF composites exclusive research on PP/WF composites has been running in both academic institutions and industries for last three decades.
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Many studies on the effect of interfacial adhesion between PP matrix and wood flour to the mechanical properties of PP/WF composites by using either different coupling agents as for example maleic anhydride grafted polypropylene (PP-g-MA) or treated wood flours with various
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chemicals have been reported in different periodicals. Most of the researchers used maleic anhydride grafted polypropylene [1-10] as compatibilizer in PP/WF composites. Their studies reported that PP-g-MA can improve the mechanical performance of PP/WF composites as interfacial adhesion between WF and PP matrix is improved by PP-g-MA. It has also been reported that presence of less than 1 wt% PP-g-MA can produce sufficient increases in tensile strength [2]. The improved adhesion between PP and WF is obtained due to the chemical bonding between the hydroxyl groups available in wood particles and anhydride group in maleated polyolefins [6].
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Chemical treatments such as sodium hydroxide and vinil-tris-(2-metoxietoxi)-silane [11], surfactant and benzyl chloride [12], acetylation [13], butyl-acrylate and methyl methacrylate [14] etc. as well as thermal treatment [15, 16] of WF have also been carried out to improve interfacial adhesion between PP and WF. It has been reported that presence of treated wood flour improved
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the mechanical properties of PP/WF composites. The presence of a third component such as glass fibres [17], nanoclay [18] etc. in PP/WF composites can also improve the properties of PP/WF composites. The properties of PP/WF composites have also been affected by wood flour content and particle size [19-21].
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PP/WF composites or wood plastic composites already has found its application in several sectors such as automotive industries, office appliances, housewares, furniture, etc. [22-24]. The
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recent published article about the development of superparamagnetic Fe3O4@ PP/WF nanocomposites by Mahdi et al [25] also indicates a future prospect of new application of this composite material. Considering the outdoor applications of PP/WF composites water absorption [26], weathering behavior [27-30], degradability [31] and flammability [32-34] of PP/WF composites are also being studied.
In the recent years, researchers of PP/WF composites have also focused their attention to
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further pulverization of WF and its effect to the mechanical properties of PP/WF composites. The properties of PP/WF composites may largely depend on how effectively the WF particles are distributed and dispersed in PP matrix. Zhihai et al [35] developed solid-state shear milling method to prepare PP/WF composites. Their study reported that after pulverization, the aspect
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ratio of the obtained WF particles did not decrease but increased instead. The prepared PP/WF composite exhibited improved mechanical performance. However, Makise et al [36] investigated
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that excessive pulverization of WF in dry condition had a negative effect to the mechanical properties of PP/WF composites. In our previous study, we found that pulverized WF with water reinforced PP composites displayed higher tensile strength than that of nonpulverized WF reinforced PP composites [37]. Hence, further pulverization of wood flour with water has a positive effect to the mechanical properties of PP/WF composites. Moreover, pulverization of WF with water, a simple mechanical treatment, is a nonhazardous and a very suitable approach compared with any other existing treatment of wood flour. Thus, the aim of this present study was to investigate the effect of pulverization of wood flour with and without water to the melt-viscosity and some mechanical behaviour particularly
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tensile strength, impact strength and fatigue behaviour of PP/WF composites. Three mechanisms of wood particles splitting shown in Fig. 1 were assumed by further pulverization of WF. The first one was the splitting of longer particles along the width, the second mechanism was the splitting of short particles along the length and the third mechanism was the making fibrillation
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on the WF particles. The pulverization of WF without water was assumed to follow the first and second mechanisms of wood particles splitting mostly. Whereas pulverization of WF with water was assumed to follow the third mechanism of wood particles splitting mostly. If splitting of wood particles is brought about by the first mechanism only, then the surface area of WF will be
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increased but aspect ratio will be decreased. On the other hand, if splitting of wood particles is brought about by the second mechanism then both the surface area of WF and aspect ratio will
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be increased. We know the physical and mechanical behaviour of composite materials largely depends on the surface area and aspect ratio of particles. However, if splitting of wood particles is brought about by the third mechanism, then the roughness of WF particles surfaces will be increased due to fibrillation. The fibrillation of WF particles also hints manifold increment of surface area as well as aspect ratio of WF particles. It was thought, the presence of fibrils on WF particle surfaces can behave like as anchor in PP matrix. The roughness of wood particles
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surfaces or anchoring behaviour of fibrillated WF particles implies more stress transfer from PP matrix to WF particles because of more interfacial interaction between PP matrix and WF particles. Thus, use of fibrillated WF as filler into PP matrix may bring an improvement in the mechanical performances of PP/WF composites. The role of water in pulverization of wood flour
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is probably to make weak the inter fibrillar bonds through the interaction with them as well as to act as lubricant. Although, it is expected that the pulverization of wood flour with water has a
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positive effect to the physical and mechanical behaviour of PP/WF the composites, in real, pulverization of wood flour does not cause particle splitting by a single mechanism as mentioned in Fig.1 rather particle splitting is accomplished by the combination of these three mechanisms. Therefore, the resultant effect of the three mechanisms followed in pulverization of WF will dominate the physical and mechanical properties of PP/WF composites.
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Fig. 1. Scheme represents three mechanisms of wood flour particle splitting by pulverization 2. Experimental 2.1. Materials
The raw materials used to prepare composites were wood flour (WF), polypropylene (PP) and maleic anhydride graft polypropylene (PP-g-MA). Wood flour obtained from cypress tree (short and large particle) and scots pine tree were supplied from Maniwa city. The average size
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of the wood flour particles were 165.0 µm, 379.2 µm and 265.0 µm for WF obtained from cypress tree (short), cypress tree (large) and scots pine tree, respectively. Polypropylene pellet, trade name PPJ107G, melt flow rate of 30 g/10 min (230°C/2.16 kg), a density of 0.9 g cm−3, and a melting point of 150 °C was received from Prime Polymer Co., Ltd.,
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Tokyo, Japan.
Maleic anhydride modified polypropylene (PP-g-MA) powder containing 2 wt.% maleic
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anhydride (MA), average molecular weight 77000 was purchased from Kayaku Akzo Co., Ltd., Tokyo, Japan.
2.2. Pulverization of Wood Flour Wood floor (WF) obtained from cypress and scots pine trees was further pulverized by disc milling in Masscolloider MKZA 10 - 15 J, Whetstone, MKE 10 – 80, Japan. The wood flour before the disc milling process was subjected to an agitated flour, a canna refuse (Mulder scrap) by an impact pulverizer Grinding with Makino type crusher DD-3. The pulverization of WF was accomplished both in dry and wet conditions of WF.
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Pulverization of dry wood flour for different initial wood flour was done by setting the followings rotational speed of plate1800 rpm, plate gap 200 µm or 350 µm, plate size 24 cm, pulverization time 30 min. For each load, the wood flour 50 g was pulverized and the wood flour
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was passed 5 times through the pulverizer. For wet conditions, at first an aqueous slurry of wood flour was prepared with solid content of 5 wt.%. Then the slurry was passed through the pulverizer for 5 times. Machine conditions set up for dry wood flour such as rotor speed, plate gap etc. were also fixed for the pulverization of wood flour with water. After pulverization the solid wood flour was separated from the slurry by using a centrifuge machine (Micro
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Refrigerated Centrifuge 3700, Kubota Manufacturing Co., Tokyo, Japan) and dehydrated using tbutyl alcohol (2-methyl- 2-propanol, Wako Pure Chemical Industries Ltd., Osaka, Japan). The
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water from wood flour was completely removed through the centrifugation and dehydration steps and finally freeze-dried. Based on plate gap 200 µm and 350 µm as well as dry and wet conditions in pulverization, five categories of wood flour were obtained from each type of initial wood flour. An example for pine wood flour (PF) is shown in Fig. 2. Each type of wood four with their code is also reported in Table 2. The subscripts S and L in the sample code with cypress wood flour (CF) indicate small size and large size cypress wood flour particle respectively. The
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numerical number with each sample code is used to indicate the type of wood flour obtained such as 1- intial wood flour, 2-pulverized wood flour without water setting plate gap 200 µm, 3pulverized wood flour without water setting plate gap 350, 4- pulverized wood flour with water
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setting plate gap 200 µm, 5- pulverized wood flour with water setting plate gap 350 µm.
Fig. 2. The diagram shows the process and conditions how different pulverized pine wood flour (PF) obtained and named.
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2.3. Composite preparation The composites of polypropylene matrix with pulverized and nonpulverized wood flours were prepared in two step processes. In the 1st step, masterbatches of PP and PP-g-MA with 70
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wt% wood flour were obtained by a compounding machine Super Mixer SMV-20. The temperature of mixer tank was set at 200 °C and the compounding was finished at 190 °C. The rotating speed of stirring blade was 2000 rpm. Finally the compounded materials were ground. In the 2nd step the ground masterbatches were diluted in a twin-screw extruder (AS30,
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Nakatani, machinery, Co., Ltd.) with required amount of PP to make 25 wt% wood flour content in each final material. The components composition wt.% in each final composite was
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74PP/1PP-g-MA/25WF. The screw speed was 85 rpm and total throughput was 9-12 Kg/h. During processing of the composites the temperature at different zones in the extruder were maintained as following: (1) 165 °C, (2) 200 °C, (3) 215 °C, (4) 200 °C, (5) 190 °C, and (6) 190 °C. The processed composites based on different wood flour were coded as reported in the Table 1.
Table 1. List of different wood flours and composite samples with their code
Composites
Wood flour sources (tree)
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Sample code for
Wood flour
Pulverization conditions
Plate gap (µm)
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CCFS1 CFS1 Cypress CCFS2 CFS2 Cypress 200 CCFS3 CFS3 Cypress 350 CCFS4 CFS4 Cypress 200 CCFS5 CFS5 Cypress 350 CCFL1 CFL1 Cypress CCFL2 CFL2 Cypress 200 CCFL3 CFL3 Cypress 350 CCFL4 CFL4 Cypress 200 CFL5 Cypress 350 CCFL5 CPF1 PF1 Scots pine CPF2 PF2 Scots pine 200 CPF3 PF3 Scots pine 350 CPF4 PF4 Scots pine 200 CPF5 PF5 Scots pine 350 *Subscripts S and L indicate small and large particles respectively
2.4. Characterization techniques
Average wood particle size (µm)
Water No No Yes Yes No No Yes Yes No No Yes Yes
165.0165.4 161.7 135.0 167.0 379.2 313.9 323.1 245.9 304.7 265.7 201.2 213.9 168.4 198.7
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The average particle size and size distributions of the dried wood flours of different samples were determined using a laser diffraction particle size distribution analyzer (Partica LA-950V2, Horiba, Ltd., Kyoto, Japan). The instrument was equipped with two different light sources, a 650 nm red laser diode and a 405 nm blue emitting diode, and used Mie scattering theory to
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accurately measure particle sizes from 3 mm to 10 nm. Particle sizes were reported as the equivalent spherical diameters of the irregularly shaped particles. The average particle size for a sample was the average calculated from the particle size distribution. The average particle size for different wood flours particle are reported in Table 1.
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Scanning electronic microscopic (SEM) analysis of initial wood flour, pulverized wood flour with and without water was carried out on the surfaces of the dispersed wood flour on a SEM
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specimen mount by a field emission scanning electron microscope Hitachi Hightech S-4800, Hitachi, Tokyo, Japan operating at 15 kV. The surfaces of wood flour were sputter coated with gold in an Edward Sputter Coater and analysed by JEOL JSM-5600LV scanning electron microscope.
Measurements of rheological properties were performed using a Melt indexer G-01 made by Toyoseiki MCR 301 Anton Paar with parallel-plate geometry rheometer. Tests were carried out
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in steady rate mode at 180 ºC (molten state). Tests were done at shear rates 100, 149, 223, 334 and 500 s-1. From this test melt viscosity of PP/WF composites were measured. Izod impact test of unnotched specimens of the composites was carried out by a UF Impact Tester, manufactured by Ueshima Seisakusho Co., Ltd. the hammering capacity of the tester was
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2 J. The dimensions of the tested sample specimens were 3 mm x 10 mm x 60 mm. At least five specimens of the prepared composites were tested and the average values of impact energy was
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reported.
Tensile test of the prepared wood composites were carried out by a Material Testing Machine, EHF-F1, Shimadzu, Japan. The tensile tests were carried out using the load cell of 1 kN, gauge length of 18 mm and cross-head speed of 10 mm/min at 25 °C and 40% relative humidity. Dumbbell shaped samples were obtained from injection mold. Before testing, the samples were conditioned at 25 °C with 40% relative humidity (RH) for 5 days. Before conditioning a strain gage (Kyowa strain gage, KFGS-2N-120-C1-11, Japan) of 2 mm with a gauge factor 2.14±1.0% was inserted at about the middle point of each specimen surface. The maximum strength, the yield strength, the elongation at break and the Young´s modulus were
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determined from the stress–strain. The test was performed five times for each material and the average of the values was reported. Fatigue tests of the composite specimens were also carried by the Material Testing Machine, EHF-F1, Shimazu, Japan at 25 °C and 40% RH conditions. The average maximum tensile
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strength value, obtained by tensile test, was used as a reference maximum stress level in estimation of the applied stress. The test conditions in fatigue test were loading mode tensiontension fatigue, stress ratio 0.1, frequency 3.5 Hz as well as applied stress: 90, 80, 70, 60 and 55 % level of ultimate tensile strength. The conditions were applied until the failure occurs. At each
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loading at least two specimens of the prepared composites were tested and the average values of
3. Results and discussion 3.1. Characterization of wood flours
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fatigue cycles was reported.
Fig.3 represents the scanning electron micrographs of (a) CFL1, (b) CFL2, (c) CFL4 wood flour particles obtained from cypress tree. Fig.3a-c show the SEM images of the WF particles at lower magnification, whereas Fig.3d-f show the SEM images of the respective WF particles at higher magnification. The decrease of WF particles size in pulverized WF compared with the
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initial WF particles is obvious in the SEM images. The appearance of pulveized WF particles in the SEM images indicate that splitting of WF particles probably followed the three mechanisms of particle splitting as shown in Fig.1. In the SEM image of initial WF particle (Fig.3d), the
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compact natural deposition of fibrils which are bonded with other fibrils by lignin and or some noncellulosic materials in a single WF particle is clearly visible. On the other hand, the SEM images of pulverized WF (Fig.3e and 3f) show partial or fully separation of fibrils.
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From particle size distribution analysis, it was observed that due to pulverization wood flour with water the particles size is reduced largely. Water probably enhenced particle splitting by facilitating of breaking hydrogen bond within particle. Fully separation or isolation of fibrils may occur when the spilitting of WF particles follow the mechanism II as shown in Fig. 1. Whereas partial separation of fibrils occurs when spilitting of WF follow the mechanism III which indicate the fibrillation on the WF particles. Partial separation of fibrils or some fibrillation on the WF particles surfaces are noticed in the pulverized WF with and without water (Fig.3e and 3f). And the extent of degree of fibrillation seems higher in the case of pulverized WF with water
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compared with pulverized WF without water. It indicates that presence of water enhence fibrillation on the WF particles. The enhencing fibrillation of WF in presence of water can be atributed to the fact that water probably make weak the inter fibrillation bonds through the interaction with them and also behave as lubricant which facilitate the fibrillation process.
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Fibrillation of WF particles with water by pulverization was also reported elsewhere [37].
Fig. 3. SEM images of initial and pulverized wood flour (a,d) CFL1, (b,e) CFL2, (c,f) CFL4 at low and high magnification respectively.
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3.2. Melt-viscosity of composites
Fig. 4. Effect of shear rate to the melt-viscosity of (a) cypress (S), (b) cypress (L) and (c) scots pine wood flour reinforced PP composites. In Fig.4a-c, the melt-viscosity of PP/WF composites as a function of shear rate in the range of 100-500 1/s is plotted. The melt-viscosity values of the composites were well fitted to the classical power law expression:[38] η = ( ) (1)
where η represents the melt-viscosity of the PP/WF composites and ω represents shear rate. In equation (1) ARh is a pre-exponential factor or consistency index and nRh is the shear thinning
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exponent or flow behavior index whose values were found to be 0.6 for the processed composites. The obtained values of power law parameters are also reported in Table 2. In Fig.4, it is obvious that the melt viscosity of composites decreased with increasing shear rates. Lowering the viscosity values of composites with increasing shear rates indicates the
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pseudoplastic or shear-thinning characteristics of the composite materials. This pseudoplastic or solid like viscoelastic behavior is common in each and every type of processed composites. The pseudo-plastics or shear-thinning behavior PP/WF composites have also been reported by Haijun li et al [39]. Now if attention is focused to a single figure, as for example Fig.4a, and the curves
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are noticed, the effect of pulverized WF with water to the melt-viscosity of PP/WF composites can be easily understood.
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Table 2. Obtained values of power law parameters for the prepared composites ARh
nRh
(R2)*
CCFS1
13582
0.563
0.9995
CCFS2
13231
0.559
0.9978
CCFS3
14553
0.575
0.9993
CCFS4
13769
0.555
0.9997
CCFS5
14426
0.557
0.9967
CCFL1
9624
0.521
0.9983
CCFL2
11184
0.554
0.9981
CCFL3
14728
0.598
0.9990
CCFL4
15143
CCFL5
14441
CPF1
11805
CPF3 CPF4 CPF5
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0.9999
0.569
0.9998
0.553
0.9999
10622
0.543
0.9994
10197
0.537
0.9998
15152
0.588
0.9993
14354
0.574
0.9995
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CPF2
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Sample
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*R : correlation factor squarred
This evidence is more obvious in Fig.4b. The composites CCFS4 and CCFS5 exhibited higher values of melt-viscosity compared with CCFS1, CCFS2 and CCFS3 in the range of shear rate of
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100-500 1/s. These higher values of melt-viscosity of CCFS4 and CCFS5 indicate that the pulverized WF with water produced more impedance to flow of PP matrix. It has been reported that melt-viscosity of PP matrix are increased by the addition of WF due to increased obstruction produced to flow by the irregular-shaped WF particles and melt-viscosity of composites
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increases with filler content [21, 40]. Therefore, melt-viscosity results of the studied composites indicate that pulverization of WF with water changes the surface morphology of WF particles or make the surfaces of the wood particles more irregular. Hence, the viscosity results can be attributed as a support of the occurrence of fibrillation on the surfaces of WF particles. In Fig.4a,
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it is also noticed that the melt-viscosity values of CCFS4 and CCFS5 composites are almost similar or has no significant difference in their viscosity values. The melt-viscosity data suggests
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that plate gap difference between 200 µm and 350 µm in pulverization is not very significant. Similarly, the composites CCFL4 and CCFL5 in Fig.4b as well as composites CPF4 and CPF5 in Fig.4c also displayed the higher values of melt-viscosity compared with the composites CCFL1, CCFL2 and CCFL3 in Fig.4b as well as CPF1, CPF2 and CPF3 in Fig.4c respectively. The effect of initial wood particles type and their pulverization with water to the meltviscosity of PP/wood composites are shown in Fig.5. In Fig.5a, if the melt viscosity of the
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composites CCFS1 and CCFL1 having same type of WF but different particle size is compared, then it can be noticed that the melt viscosity of PP/WF composites are affected by the size of the WF particles. The surface area of CFS1 particles is higher due to the lower particles sized compared with the surface area of CFL1. The higher surface area of lower sized WF particles
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increased the probability of higher interfacial interaction between PP matrix and WF particles compared with that of lower surface area of larger sized WF particles. Hence, this higher
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interfacial interaction can be a cause of higher melt-viscosity of CCFS1 than that of CCFL1. Melt-viscosity values of CCFS1 are also higher compared with CPF1 because of the same reason. The increase order of melt-viscosity in these composites is CCFS1˃CPF1˃CCFL1. However, in the case of pulverised WF with water the trend of increasing melt-viscosity with decreasing particle size is not similar like as nonpulverized WF. The increase order of melt-viscosity of the PP composites of pulverized WF with water setting plate gap 200 µm is CCFS4≥CCFL4˃CPF4 and the similar trend was also found for plate gap 350 µm i.e. CCFS5˃CCFL5˃CPF5. This behaviour suggests that due to pulverization of WF with water the extent of surface roughness of WF particles become higher to large sized particles compared with that of small sized particles.
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Fig. 5. Melt-viscosity Vs shear rate plot of different wood flour (a) initial WF, (b) pulverized WFwith water (plate gap 200 µm) and (c) pulverized WF with water (plate gap 350 µm) reinforced PP composites.
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3.3. Tensile properties of composites
Fig. 6. Tensile strength of neat PP and different wood flour (1) intial, (2) pulverized without
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water setting plate gap 200 µm, (3) pulverized without water setting plate gap 350, (4) pulverized with water setting plate gap 200 µm, (5) pulverized with water setting plate gap 350 µm reinforced PP composites.
The effect of wood particles type and pulverization with and without water together with effect of plate gap to the tensile strength of PP/wood composites are shown in Fig.6. Tensile properties such as elastic modulus, tensile strength and strain percentage of all the processed
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composites are also reported in Table 3. It can be noticed in Table 3 that the tensile strength and modulus of PP matrix are significantly improved by incorporating 25 wt% of WF. However strain percentage of PP is greatly reduced by WF. Comparing tensile strength of composites
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CCFS1 and CCFL1, same type of WF but different particle size, it seems that the tensile strength of PP/WF composites are affected by the size of the WF particles. CCFS1 also displayed a higher value of tensile strength compared with CPF1 because of the same reason. The increase order of
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tensile strength of these composites is CCFS1˃CPF1˃CCFL1. However, it has been reported by Stark et al [20] that tensile strength and stiffness of PP/WF composites are largely enhanced by the aspect ratio, not particle size, of WF particles. Hence, from tensile test results, it can be assumed that the increase order of aspect ratio among initial wood flours is CFS1˃PF1˃CFL1. A little lower of tensile strength values but similar trend is also obvious in pulverized WF without water i.e. the increase order of tensile strength is CCFS2 ˃CPF2 ˃CCFL2 for plate gap 200 µm and CCFS3˃ CPF3 ˃CCFL3 for plate gap 350 µm. From this result, it can be concluded that due to the pulverization of WF without water the length of WF particles is decreased by
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particle breakage as a result the aspect ratio of particles is reduced. However, in case of composites of pulverized WF with water the increase order of tensile strength of the composites
Sample
E-modulus (GPa)
Max. Strength (MPa) Strain (%)
PP
0.36 ± 0.18
33.03 ± 0.36
26.28 ± 4.25
CCFS1
3.76
43.42 ± 0.50
10.14 ± 0.6
CCFS2
3.22 ± 0.22
42.89 ± 0.43
CCFS3
-
42.32 ± 0.11
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Table 3. Mechanical properties of PP, PP/WF composites
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is not similar as mentioned above.
CCFS4
3.09 ± 0.34
41.10 ± 0.43
8.76 ± 0.33
CCFS5
3.02 ± 0.32
42.01 ± 0.38
10.85 ± 0.85
CCFL1
3.10
40.09 ± 0.21
9.16 ± 1.76
CCFL2
3.12 ± 0.19
40.14 ± 0.36
9.63 ± 1.38
CCFL3
2.90 ± 0.26
39.52 ± 0.11
9.65 ± 1.37
CCFL4
2.77 ± 0.32
40.46 ± 0.16
10.45 ± 1.08
CCFL5
3.23 ± 0.30
41.28 ± 0.33
9.24 ± 1.04
CPF1
3.53 ± 0.24
42.01 ± 0.17
9.92 ± 1.34
CPF2
3.24 ± 0.22
42.41 ± 0.16
10.03 ± 0.85
CPF3
3.45 ± 0.04
42.01 ± 0.40
10.30 ± 0.95
CPF4
3.48 ± 0.12
42.47 ± 0.29
10.43 ± 1.11
43.73 ± 0.36
9.91 ± 1.11
3.57 ± 0.08
11.20 ± 1.6
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CPF5
10.67 ± 1.73
Composites CPF4 displayed higher value of tensile strength compared with CCFS4. In addition, the tensile strength values of PP/WF composites with pulverized small sized wood flour obtained from cypress tree are decreased. Whereas, in the case of large sized wood flour the tensile values of PP/WF composites are not decreased rather little increased. It seems that pulverization of WF with water may has positive effect to the tensile strength of PP composites compared with that of pulverization of WF without water. Thus, tensile test results also
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indicated, depending on the type of initial wood flour particle, pulverization of WF with water can enhance or decrease the aspect ratio of WF. Thus, the effectiveness of pulverization of WF with water to the tensile strength of composites depend on the type of initial wood flour particles.
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3.4. Impact behaviour of composites
Fig. 7. Impact strength of neat PP and different wood flour (1) intial, (2) pulverized without water setting plate gap 200 µm, (3) pulverized without water setting plate gap 350, (4) pulverized
reinforced PP composites.
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with water setting plate gap 200 µm, (5) pulverized with water setting plate gap 350 µm
The column diagram in Fig.7 shows the impact strength of the processed composites.
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Izod impact test of unnotched specimens of the processed composites together with neat PP was carried out and it was found that incorporation of 25 wt.% WF in PP matrix dropped the impact
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strength of neat PP (50 kJ/m2) largely. This large drop of impact strength indicates that the PP/WF composites become brittle or less ductile compared with that of neat PP. However, the composites reinforced with pulverized wood flour with water setting plate gap 200 µm displayed higher values of impact energy compared with the composites of pulverized WF without water (Fig.7). Composites with smaller (165 µm) particles also displayed higher values of impact energy compared with the composites with larger (265µm and 379 µm) particles. It was also found that the composite of PP with smaller (165 µm) particles (obtained from cypress tree), which was pulverized with water displayed the highest value of impact energy (20 kJ/m2). Hence, impact test results suggest that smaller particles has better effect to the impact behaviour
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of composites compared with larger particles. The effect of particle size to the impact energy of PP/wood flour composites has also been investigated by Stark et al [20]. They reported that the unnotched impact energy of PP/WF composites is increased with decreasing particle size due to
composites can be improved by using pulverized WF with water.
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3.5. Fatigue behavior of composites
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reducing stress concentration. Hence, the impact test results indicates that the ductility of PP/WF
Fig. 8. S-N curves of neat PP and PP/WF composites with (a) initial wood flour, (b) pulverized and nonpulverized WF (small partilces), (c) pulverized and nonpulverized WF (large partilces).
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Fig. 8a represents the S-N curves of neat PP and PP/WF composites with different initial wood flour. The fatigue test data were fitted by regression technique to a logarithmic equation as σ = blog(N) c (2)
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follows:
where, σmax is applied maximum stress and N is number of frequencies to fracture. The values of b and c are constant and to depend on the type of materials. For all the processed composites, the values of regression coefficient (R2) were found to be ˃0.98. In Fig.8a, it is obvious that the
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fatigue lives of composites are higher than that of neat PP. The higher fatigue life of composites indicates the positive effect of wood flour to the fatigue life of PP/WF composites. The improved
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fatigue life of composites is attributed to the crack arresting properties of composite materials [41]. The presence of hard and strong wood flour particles arrested or diverted the cracks propagation which generated in the soft and weak PP matrix [42]. It has also been reported by Abdelhaleem et al [43] that fatigue life of composites increased with filler content. Fig.8b and 8c represent S-N curves of initial and pulverized wood flour reinforced PP composites for small and large sized cypress wood flour respectively. However, in Fig.8b and 8c, the effect of
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pulverization of wood flour to the fatigue life of composites is not easily understood as some
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experimental data are scattered and overlapped with each other.
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Fig. 9. Column diagram shows 50% fatigue life at constant fatigue strength (30 MPa) of PP composites reinforced with different small (S) and large (L) size cypress wood flour (1) intial, (2) pulverized without water (plate gap 200 µm), (3) pulverized with water (plate gap 200 µm)
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To see the effect of pulverization of wood flour with and without water to the fatigue life of composites, at constant fatigue strength (30 MPa), 50% fatigue life of pulverized wood flour and nonpulverized wood flour reinforced PP composites was calculated based on equation (2). Fig.9 represents the 50% fatigue life (at fatigue strength 30 MPa) of pulverized and nonpulverized
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small (S) and large (L) size cypress wood flour reinforced PP composites. Although the changes of 50% fatigue life of the composites are not very high, in column diagram of Fig.9, now it is
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possible to see the effect of wood flour pulverization to the fatigue life of composites. In Fig.9, it is seen that in the case of small sized cypress flour the 50% fatigue life of pulverized wood flour reinforced PP composites is lower compared with that of nonpulverized WF reinforced composites. Whereas, in the case of large sized cypress flour the opposite behavior can be noticed, pulverized wood flour with water reinforced PP composites has higher 50% fatigue life than its counterparts nonpulverized and pulverized WF without water reinforced PP composites.
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The higher fatigue life of pulverized wood flour reinforced PP composites indicate the more crack-arresting behavior of fibrillated wood flour particles. In PP matrix, fibrillated wood flour have a probability to form interconnected network of wood flour particles through the sufficient extent of fibrils on their surfaces. The probability of network formation of wood particles may
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also increase with increasing degree of fibrillation on wood particle surfaces. Thus increasing the probability of network formation of WF increase crack-arresting behavior in composites.
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Pulverization of small sized WF decreased the fatigue life of composites, whereas pulverization of larger sized WF improved the fatigue life. Since fatigue life of composites depend on the particle aspect ratio like as tensile strength and fatigue life of composites increased with increasing particle aspect ratio [44] hence fatigue test results also indicated that effectiveness of pulverization of wood flour to the fatigue life of PP/WF composites also depend on the initial wood particles size. Thus, further pulverization of selective WF with water may has a positive effect to the fatigue life of PP/WF composites. 4. Conclusion
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In this study we investigated the effect of pulverization of wood flour with and without water to the melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour (WF) composites. The effects of initial wood flour particle size, type of trees (namely Cypress and scots pine trees) as well as plate gap (200 µm and 350 µm) in pulverization were also evaluated
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in terms of melt-viscosity, tensile properties, Izod impact strength and fatigue behaviour of the prepared composites.
Melt-viscosity results exhibited that PP/WF composites with pulverised wood flour with water had higher melt-viscosity than that of nonpulverized and pulverized wood flour without
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water reinforced PP composites. Higher melt-viscosity of pulverised WF reinforced PP composites indicated more roughness on particle surfaces and higher interfacial interaction
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between WF and PP matrix. Further pulverization of wood flour also affected to the tensile strength values of composites. The tensile strength of the processed composites with 25 wt% of WF were found in the range of 39-43 MPa and the tensile strength values of composites depend on the type of initial WF. Izod impact test results of unnotched specimens of the PP/WF showed that the composites reinforced with pulverized wood flour with water displayed higher values of impact energy compared with the composites of pulverized WF without water. Depending on the
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type of initial WF, fatigue test results also displayed the positive effect of further pulverization of wood flour with water to the fatigue life of the composites. Finally from this study, it can be concluded that depending on initial wood flour particles, further pulverization of wood flour with water can create more roughness on the WF particle surfaces by fibrillating and an improvement
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of mechanical properties such as tensile strength, impact strength and fatigue life of PP/WF can
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be achieved by using the wet pulverized WF.
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S2542-5048(18)30037-X
DOI:
https://doi.org/10.1016/j.aiepr.2018.11.001
Reference:
AIEPR 14
To appear in:
Advanced Industrial and Engineering Polymer Research
Received Date: 26 September 2018 Revised Date:
12 November 2018
Accepted Date: 29 November 2018
Please cite this article as: M. Minhaz-Ul Haque, K. Goda, H. Ito, S. Ogoe, M. Okamot, T. Ema, K. Kagawa, H. Nogami, Melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour composites: effect of pulverization of wood flour with and without water, Advanced Industrial and Engineering Polymer Research, https://doi.org/10.1016/j.aiepr.2018.11.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour composites: effect of pulverization of wood flour with and without water Md Minhaz-Ul Haque1a*, Koichi Goda1, Hirokazu Ito2, Shinji Ogoe3, Masaki Okamot3, Tomoyuki Ema3, Keiko Kagawa3, Hidetaka Nogami4 1
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Department of Mechanical Engineering, Yamaguchi University, Ube, Yamaguchi, 755-8611 Japan 2 National Institute of Advanced Industrial Science and Technology, Hiroshima 739-0046, Japan. 3 Technology Development Center, TOCLAS Co., Shizuoka 432-8001, Japan 4 Okayama Prefectural Research Institute for Forest and Forest Products, Okayama 717-0013 Japan
Abstract: This study analyses the effect of pulverization of wood flour with and without water
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to the melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour (WF) composites. The composites were processed in an extruder and subsequent injection moulding. The effects of initial wood flour particle size, type of trees (namely cypress and European scots pine trees) as well as plate gap (200 µm and 350 µm) in pulverization were also evaluated in terms of melt-viscosity, tensile properties, Izod impact strength and fatigue behaviour of the prepared composites. PP/WF composites with pulverised wood flour with water exhibited higher
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melt-viscosity than that of nonpulverized and pulverized wood flour without water reinforced PP composites. Tensile strength values of composites were slightly affected by the pulverization of wood flour and the enhancement of tensile strength values were depended on the type of initial WF. From Izod impact test results of composites, it was found that the composites reinforced
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with pulverized wood flour with water displayed higher values of impact energy compared with the composites of pulverized WF without water. Depending on the type of initial WF particle, the positive effect of pulverization of wood flour with water to the fatigue life of the composites
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was also observed.
*Corresponding author at: Department of Mechanical Engineering, Yamaguchi University 2-16-1 Tokiwadai, Ube, Yamaguchi, 755-8611 Japan Cel: +8150 68650354; E-mail: [email protected] a
Permanent address: Department of Applied Chemistry and Chemical Engineering, Islamic University, Kushtia, Bangladesh Keywords: composites; mechanical properties; melt-viscosity; pulverization; wood flour
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1. Introduction Generally, hard materials are used as filler in soft polymeric materials to obtain materials of desired properties. These fillers can be natural such as clay, silica, wood flour (WF), cellulose fibres, or synthetic such as carbon black, aramid fibres, carbon fibres, glass fibres etc. Presence
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of these fillers into polymer matrices can cause a diverse change in the physical and mechanical behaviour of polymeric materials. Each filler may also has several advantages and disadvantages. Among the above fillers probably the most inexpensive one is wood flour. The advantageous characters of this green filler such as biodegradability, renewability, sustainability, light weight,
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less abrasive nature to processing equipment and cheapness have drawn much attention of polymer composite researchers. Since WF is inexpensive filler, hence, from economic point of view, it can be said, addition of WF into polypropylene (PP) matrix is a simple strategy of
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making inexpensive polypropylene composites. Further, incorporation of wood flour into PP matrix not only reduce the cost of the composite materials, but it can also impart its physical and mechanical properties to composite materials. However, WF, due to its hydrophilic nature, are not compatible with hydrophobic PP matrix. Consequently, the properties of WF is not contributed into PP matrix as much as expected. Thus the prepared PP/WF composites show lower performance than that of expected from it. Solutions to incompatibility problem as well as
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to have most probable contribution of WF properties in PP/WF composites exclusive research on PP/WF composites has been running in both academic institutions and industries for last three decades.
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Many studies on the effect of interfacial adhesion between PP matrix and wood flour to the mechanical properties of PP/WF composites by using either different coupling agents as for example maleic anhydride grafted polypropylene (PP-g-MA) or treated wood flours with various
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chemicals have been reported in different periodicals. Most of the researchers used maleic anhydride grafted polypropylene [1-10] as compatibilizer in PP/WF composites. Their studies reported that PP-g-MA can improve the mechanical performance of PP/WF composites as interfacial adhesion between WF and PP matrix is improved by PP-g-MA. It has also been reported that presence of less than 1 wt% PP-g-MA can produce sufficient increases in tensile strength [2]. The improved adhesion between PP and WF is obtained due to the chemical bonding between the hydroxyl groups available in wood particles and anhydride group in maleated polyolefins [6].
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Chemical treatments such as sodium hydroxide and vinil-tris-(2-metoxietoxi)-silane [11], surfactant and benzyl chloride [12], acetylation [13], butyl-acrylate and methyl methacrylate [14] etc. as well as thermal treatment [15, 16] of WF have also been carried out to improve interfacial adhesion between PP and WF. It has been reported that presence of treated wood flour improved
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the mechanical properties of PP/WF composites. The presence of a third component such as glass fibres [17], nanoclay [18] etc. in PP/WF composites can also improve the properties of PP/WF composites. The properties of PP/WF composites have also been affected by wood flour content and particle size [19-21].
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PP/WF composites or wood plastic composites already has found its application in several sectors such as automotive industries, office appliances, housewares, furniture, etc. [22-24]. The
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recent published article about the development of superparamagnetic Fe3O4@ PP/WF nanocomposites by Mahdi et al [25] also indicates a future prospect of new application of this composite material. Considering the outdoor applications of PP/WF composites water absorption [26], weathering behavior [27-30], degradability [31] and flammability [32-34] of PP/WF composites are also being studied.
In the recent years, researchers of PP/WF composites have also focused their attention to
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further pulverization of WF and its effect to the mechanical properties of PP/WF composites. The properties of PP/WF composites may largely depend on how effectively the WF particles are distributed and dispersed in PP matrix. Zhihai et al [35] developed solid-state shear milling method to prepare PP/WF composites. Their study reported that after pulverization, the aspect
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ratio of the obtained WF particles did not decrease but increased instead. The prepared PP/WF composite exhibited improved mechanical performance. However, Makise et al [36] investigated
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that excessive pulverization of WF in dry condition had a negative effect to the mechanical properties of PP/WF composites. In our previous study, we found that pulverized WF with water reinforced PP composites displayed higher tensile strength than that of nonpulverized WF reinforced PP composites [37]. Hence, further pulverization of wood flour with water has a positive effect to the mechanical properties of PP/WF composites. Moreover, pulverization of WF with water, a simple mechanical treatment, is a nonhazardous and a very suitable approach compared with any other existing treatment of wood flour. Thus, the aim of this present study was to investigate the effect of pulverization of wood flour with and without water to the melt-viscosity and some mechanical behaviour particularly
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tensile strength, impact strength and fatigue behaviour of PP/WF composites. Three mechanisms of wood particles splitting shown in Fig. 1 were assumed by further pulverization of WF. The first one was the splitting of longer particles along the width, the second mechanism was the splitting of short particles along the length and the third mechanism was the making fibrillation
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on the WF particles. The pulverization of WF without water was assumed to follow the first and second mechanisms of wood particles splitting mostly. Whereas pulverization of WF with water was assumed to follow the third mechanism of wood particles splitting mostly. If splitting of wood particles is brought about by the first mechanism only, then the surface area of WF will be
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increased but aspect ratio will be decreased. On the other hand, if splitting of wood particles is brought about by the second mechanism then both the surface area of WF and aspect ratio will
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be increased. We know the physical and mechanical behaviour of composite materials largely depends on the surface area and aspect ratio of particles. However, if splitting of wood particles is brought about by the third mechanism, then the roughness of WF particles surfaces will be increased due to fibrillation. The fibrillation of WF particles also hints manifold increment of surface area as well as aspect ratio of WF particles. It was thought, the presence of fibrils on WF particle surfaces can behave like as anchor in PP matrix. The roughness of wood particles
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surfaces or anchoring behaviour of fibrillated WF particles implies more stress transfer from PP matrix to WF particles because of more interfacial interaction between PP matrix and WF particles. Thus, use of fibrillated WF as filler into PP matrix may bring an improvement in the mechanical performances of PP/WF composites. The role of water in pulverization of wood flour
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is probably to make weak the inter fibrillar bonds through the interaction with them as well as to act as lubricant. Although, it is expected that the pulverization of wood flour with water has a
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positive effect to the physical and mechanical behaviour of PP/WF the composites, in real, pulverization of wood flour does not cause particle splitting by a single mechanism as mentioned in Fig.1 rather particle splitting is accomplished by the combination of these three mechanisms. Therefore, the resultant effect of the three mechanisms followed in pulverization of WF will dominate the physical and mechanical properties of PP/WF composites.
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Fig. 1. Scheme represents three mechanisms of wood flour particle splitting by pulverization 2. Experimental 2.1. Materials
The raw materials used to prepare composites were wood flour (WF), polypropylene (PP) and maleic anhydride graft polypropylene (PP-g-MA). Wood flour obtained from cypress tree (short and large particle) and scots pine tree were supplied from Maniwa city. The average size
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of the wood flour particles were 165.0 µm, 379.2 µm and 265.0 µm for WF obtained from cypress tree (short), cypress tree (large) and scots pine tree, respectively. Polypropylene pellet, trade name PPJ107G, melt flow rate of 30 g/10 min (230°C/2.16 kg), a density of 0.9 g cm−3, and a melting point of 150 °C was received from Prime Polymer Co., Ltd.,
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Tokyo, Japan.
Maleic anhydride modified polypropylene (PP-g-MA) powder containing 2 wt.% maleic
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anhydride (MA), average molecular weight 77000 was purchased from Kayaku Akzo Co., Ltd., Tokyo, Japan.
2.2. Pulverization of Wood Flour Wood floor (WF) obtained from cypress and scots pine trees was further pulverized by disc milling in Masscolloider MKZA 10 - 15 J, Whetstone, MKE 10 – 80, Japan. The wood flour before the disc milling process was subjected to an agitated flour, a canna refuse (Mulder scrap) by an impact pulverizer Grinding with Makino type crusher DD-3. The pulverization of WF was accomplished both in dry and wet conditions of WF.
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Pulverization of dry wood flour for different initial wood flour was done by setting the followings rotational speed of plate1800 rpm, plate gap 200 µm or 350 µm, plate size 24 cm, pulverization time 30 min. For each load, the wood flour 50 g was pulverized and the wood flour
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was passed 5 times through the pulverizer. For wet conditions, at first an aqueous slurry of wood flour was prepared with solid content of 5 wt.%. Then the slurry was passed through the pulverizer for 5 times. Machine conditions set up for dry wood flour such as rotor speed, plate gap etc. were also fixed for the pulverization of wood flour with water. After pulverization the solid wood flour was separated from the slurry by using a centrifuge machine (Micro
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Refrigerated Centrifuge 3700, Kubota Manufacturing Co., Tokyo, Japan) and dehydrated using tbutyl alcohol (2-methyl- 2-propanol, Wako Pure Chemical Industries Ltd., Osaka, Japan). The
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water from wood flour was completely removed through the centrifugation and dehydration steps and finally freeze-dried. Based on plate gap 200 µm and 350 µm as well as dry and wet conditions in pulverization, five categories of wood flour were obtained from each type of initial wood flour. An example for pine wood flour (PF) is shown in Fig. 2. Each type of wood four with their code is also reported in Table 2. The subscripts S and L in the sample code with cypress wood flour (CF) indicate small size and large size cypress wood flour particle respectively. The
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numerical number with each sample code is used to indicate the type of wood flour obtained such as 1- intial wood flour, 2-pulverized wood flour without water setting plate gap 200 µm, 3pulverized wood flour without water setting plate gap 350, 4- pulverized wood flour with water
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setting plate gap 200 µm, 5- pulverized wood flour with water setting plate gap 350 µm.
Fig. 2. The diagram shows the process and conditions how different pulverized pine wood flour (PF) obtained and named.
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2.3. Composite preparation The composites of polypropylene matrix with pulverized and nonpulverized wood flours were prepared in two step processes. In the 1st step, masterbatches of PP and PP-g-MA with 70
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wt% wood flour were obtained by a compounding machine Super Mixer SMV-20. The temperature of mixer tank was set at 200 °C and the compounding was finished at 190 °C. The rotating speed of stirring blade was 2000 rpm. Finally the compounded materials were ground. In the 2nd step the ground masterbatches were diluted in a twin-screw extruder (AS30,
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Nakatani, machinery, Co., Ltd.) with required amount of PP to make 25 wt% wood flour content in each final material. The components composition wt.% in each final composite was
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74PP/1PP-g-MA/25WF. The screw speed was 85 rpm and total throughput was 9-12 Kg/h. During processing of the composites the temperature at different zones in the extruder were maintained as following: (1) 165 °C, (2) 200 °C, (3) 215 °C, (4) 200 °C, (5) 190 °C, and (6) 190 °C. The processed composites based on different wood flour were coded as reported in the Table 1.
Table 1. List of different wood flours and composite samples with their code
Composites
Wood flour sources (tree)
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Sample code for
Wood flour
Pulverization conditions
Plate gap (µm)
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CCFS1 CFS1 Cypress CCFS2 CFS2 Cypress 200 CCFS3 CFS3 Cypress 350 CCFS4 CFS4 Cypress 200 CCFS5 CFS5 Cypress 350 CCFL1 CFL1 Cypress CCFL2 CFL2 Cypress 200 CCFL3 CFL3 Cypress 350 CCFL4 CFL4 Cypress 200 CFL5 Cypress 350 CCFL5 CPF1 PF1 Scots pine CPF2 PF2 Scots pine 200 CPF3 PF3 Scots pine 350 CPF4 PF4 Scots pine 200 CPF5 PF5 Scots pine 350 *Subscripts S and L indicate small and large particles respectively
2.4. Characterization techniques
Average wood particle size (µm)
Water No No Yes Yes No No Yes Yes No No Yes Yes
165.0165.4 161.7 135.0 167.0 379.2 313.9 323.1 245.9 304.7 265.7 201.2 213.9 168.4 198.7
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The average particle size and size distributions of the dried wood flours of different samples were determined using a laser diffraction particle size distribution analyzer (Partica LA-950V2, Horiba, Ltd., Kyoto, Japan). The instrument was equipped with two different light sources, a 650 nm red laser diode and a 405 nm blue emitting diode, and used Mie scattering theory to
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accurately measure particle sizes from 3 mm to 10 nm. Particle sizes were reported as the equivalent spherical diameters of the irregularly shaped particles. The average particle size for a sample was the average calculated from the particle size distribution. The average particle size for different wood flours particle are reported in Table 1.
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Scanning electronic microscopic (SEM) analysis of initial wood flour, pulverized wood flour with and without water was carried out on the surfaces of the dispersed wood flour on a SEM
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specimen mount by a field emission scanning electron microscope Hitachi Hightech S-4800, Hitachi, Tokyo, Japan operating at 15 kV. The surfaces of wood flour were sputter coated with gold in an Edward Sputter Coater and analysed by JEOL JSM-5600LV scanning electron microscope.
Measurements of rheological properties were performed using a Melt indexer G-01 made by Toyoseiki MCR 301 Anton Paar with parallel-plate geometry rheometer. Tests were carried out
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in steady rate mode at 180 ºC (molten state). Tests were done at shear rates 100, 149, 223, 334 and 500 s-1. From this test melt viscosity of PP/WF composites were measured. Izod impact test of unnotched specimens of the composites was carried out by a UF Impact Tester, manufactured by Ueshima Seisakusho Co., Ltd. the hammering capacity of the tester was
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2 J. The dimensions of the tested sample specimens were 3 mm x 10 mm x 60 mm. At least five specimens of the prepared composites were tested and the average values of impact energy was
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reported.
Tensile test of the prepared wood composites were carried out by a Material Testing Machine, EHF-F1, Shimadzu, Japan. The tensile tests were carried out using the load cell of 1 kN, gauge length of 18 mm and cross-head speed of 10 mm/min at 25 °C and 40% relative humidity. Dumbbell shaped samples were obtained from injection mold. Before testing, the samples were conditioned at 25 °C with 40% relative humidity (RH) for 5 days. Before conditioning a strain gage (Kyowa strain gage, KFGS-2N-120-C1-11, Japan) of 2 mm with a gauge factor 2.14±1.0% was inserted at about the middle point of each specimen surface. The maximum strength, the yield strength, the elongation at break and the Young´s modulus were
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determined from the stress–strain. The test was performed five times for each material and the average of the values was reported. Fatigue tests of the composite specimens were also carried by the Material Testing Machine, EHF-F1, Shimazu, Japan at 25 °C and 40% RH conditions. The average maximum tensile
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strength value, obtained by tensile test, was used as a reference maximum stress level in estimation of the applied stress. The test conditions in fatigue test were loading mode tensiontension fatigue, stress ratio 0.1, frequency 3.5 Hz as well as applied stress: 90, 80, 70, 60 and 55 % level of ultimate tensile strength. The conditions were applied until the failure occurs. At each
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loading at least two specimens of the prepared composites were tested and the average values of
3. Results and discussion 3.1. Characterization of wood flours
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fatigue cycles was reported.
Fig.3 represents the scanning electron micrographs of (a) CFL1, (b) CFL2, (c) CFL4 wood flour particles obtained from cypress tree. Fig.3a-c show the SEM images of the WF particles at lower magnification, whereas Fig.3d-f show the SEM images of the respective WF particles at higher magnification. The decrease of WF particles size in pulverized WF compared with the
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initial WF particles is obvious in the SEM images. The appearance of pulveized WF particles in the SEM images indicate that splitting of WF particles probably followed the three mechanisms of particle splitting as shown in Fig.1. In the SEM image of initial WF particle (Fig.3d), the
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compact natural deposition of fibrils which are bonded with other fibrils by lignin and or some noncellulosic materials in a single WF particle is clearly visible. On the other hand, the SEM images of pulverized WF (Fig.3e and 3f) show partial or fully separation of fibrils.
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From particle size distribution analysis, it was observed that due to pulverization wood flour with water the particles size is reduced largely. Water probably enhenced particle splitting by facilitating of breaking hydrogen bond within particle. Fully separation or isolation of fibrils may occur when the spilitting of WF particles follow the mechanism II as shown in Fig. 1. Whereas partial separation of fibrils occurs when spilitting of WF follow the mechanism III which indicate the fibrillation on the WF particles. Partial separation of fibrils or some fibrillation on the WF particles surfaces are noticed in the pulverized WF with and without water (Fig.3e and 3f). And the extent of degree of fibrillation seems higher in the case of pulverized WF with water
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compared with pulverized WF without water. It indicates that presence of water enhence fibrillation on the WF particles. The enhencing fibrillation of WF in presence of water can be atributed to the fact that water probably make weak the inter fibrillation bonds through the interaction with them and also behave as lubricant which facilitate the fibrillation process.
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Fibrillation of WF particles with water by pulverization was also reported elsewhere [37].
Fig. 3. SEM images of initial and pulverized wood flour (a,d) CFL1, (b,e) CFL2, (c,f) CFL4 at low and high magnification respectively.
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3.2. Melt-viscosity of composites
Fig. 4. Effect of shear rate to the melt-viscosity of (a) cypress (S), (b) cypress (L) and (c) scots pine wood flour reinforced PP composites. In Fig.4a-c, the melt-viscosity of PP/WF composites as a function of shear rate in the range of 100-500 1/s is plotted. The melt-viscosity values of the composites were well fitted to the classical power law expression:[38] η = ( ) (1)
where η represents the melt-viscosity of the PP/WF composites and ω represents shear rate. In equation (1) ARh is a pre-exponential factor or consistency index and nRh is the shear thinning
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exponent or flow behavior index whose values were found to be 0.6 for the processed composites. The obtained values of power law parameters are also reported in Table 2. In Fig.4, it is obvious that the melt viscosity of composites decreased with increasing shear rates. Lowering the viscosity values of composites with increasing shear rates indicates the
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pseudoplastic or shear-thinning characteristics of the composite materials. This pseudoplastic or solid like viscoelastic behavior is common in each and every type of processed composites. The pseudo-plastics or shear-thinning behavior PP/WF composites have also been reported by Haijun li et al [39]. Now if attention is focused to a single figure, as for example Fig.4a, and the curves
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are noticed, the effect of pulverized WF with water to the melt-viscosity of PP/WF composites can be easily understood.
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Table 2. Obtained values of power law parameters for the prepared composites ARh
nRh
(R2)*
CCFS1
13582
0.563
0.9995
CCFS2
13231
0.559
0.9978
CCFS3
14553
0.575
0.9993
CCFS4
13769
0.555
0.9997
CCFS5
14426
0.557
0.9967
CCFL1
9624
0.521
0.9983
CCFL2
11184
0.554
0.9981
CCFL3
14728
0.598
0.9990
CCFL4
15143
CCFL5
14441
CPF1
11805
CPF3 CPF4 CPF5
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0.9999
0.569
0.9998
0.553
0.9999
10622
0.543
0.9994
10197
0.537
0.9998
15152
0.588
0.9993
14354
0.574
0.9995
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CPF2
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Sample
2
*R : correlation factor squarred
This evidence is more obvious in Fig.4b. The composites CCFS4 and CCFS5 exhibited higher values of melt-viscosity compared with CCFS1, CCFS2 and CCFS3 in the range of shear rate of
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100-500 1/s. These higher values of melt-viscosity of CCFS4 and CCFS5 indicate that the pulverized WF with water produced more impedance to flow of PP matrix. It has been reported that melt-viscosity of PP matrix are increased by the addition of WF due to increased obstruction produced to flow by the irregular-shaped WF particles and melt-viscosity of composites
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increases with filler content [21, 40]. Therefore, melt-viscosity results of the studied composites indicate that pulverization of WF with water changes the surface morphology of WF particles or make the surfaces of the wood particles more irregular. Hence, the viscosity results can be attributed as a support of the occurrence of fibrillation on the surfaces of WF particles. In Fig.4a,
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it is also noticed that the melt-viscosity values of CCFS4 and CCFS5 composites are almost similar or has no significant difference in their viscosity values. The melt-viscosity data suggests
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that plate gap difference between 200 µm and 350 µm in pulverization is not very significant. Similarly, the composites CCFL4 and CCFL5 in Fig.4b as well as composites CPF4 and CPF5 in Fig.4c also displayed the higher values of melt-viscosity compared with the composites CCFL1, CCFL2 and CCFL3 in Fig.4b as well as CPF1, CPF2 and CPF3 in Fig.4c respectively. The effect of initial wood particles type and their pulverization with water to the meltviscosity of PP/wood composites are shown in Fig.5. In Fig.5a, if the melt viscosity of the
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composites CCFS1 and CCFL1 having same type of WF but different particle size is compared, then it can be noticed that the melt viscosity of PP/WF composites are affected by the size of the WF particles. The surface area of CFS1 particles is higher due to the lower particles sized compared with the surface area of CFL1. The higher surface area of lower sized WF particles
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increased the probability of higher interfacial interaction between PP matrix and WF particles compared with that of lower surface area of larger sized WF particles. Hence, this higher
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interfacial interaction can be a cause of higher melt-viscosity of CCFS1 than that of CCFL1. Melt-viscosity values of CCFS1 are also higher compared with CPF1 because of the same reason. The increase order of melt-viscosity in these composites is CCFS1˃CPF1˃CCFL1. However, in the case of pulverised WF with water the trend of increasing melt-viscosity with decreasing particle size is not similar like as nonpulverized WF. The increase order of melt-viscosity of the PP composites of pulverized WF with water setting plate gap 200 µm is CCFS4≥CCFL4˃CPF4 and the similar trend was also found for plate gap 350 µm i.e. CCFS5˃CCFL5˃CPF5. This behaviour suggests that due to pulverization of WF with water the extent of surface roughness of WF particles become higher to large sized particles compared with that of small sized particles.
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Fig. 5. Melt-viscosity Vs shear rate plot of different wood flour (a) initial WF, (b) pulverized WFwith water (plate gap 200 µm) and (c) pulverized WF with water (plate gap 350 µm) reinforced PP composites.
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3.3. Tensile properties of composites
Fig. 6. Tensile strength of neat PP and different wood flour (1) intial, (2) pulverized without
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water setting plate gap 200 µm, (3) pulverized without water setting plate gap 350, (4) pulverized with water setting plate gap 200 µm, (5) pulverized with water setting plate gap 350 µm reinforced PP composites.
The effect of wood particles type and pulverization with and without water together with effect of plate gap to the tensile strength of PP/wood composites are shown in Fig.6. Tensile properties such as elastic modulus, tensile strength and strain percentage of all the processed
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composites are also reported in Table 3. It can be noticed in Table 3 that the tensile strength and modulus of PP matrix are significantly improved by incorporating 25 wt% of WF. However strain percentage of PP is greatly reduced by WF. Comparing tensile strength of composites
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CCFS1 and CCFL1, same type of WF but different particle size, it seems that the tensile strength of PP/WF composites are affected by the size of the WF particles. CCFS1 also displayed a higher value of tensile strength compared with CPF1 because of the same reason. The increase order of
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tensile strength of these composites is CCFS1˃CPF1˃CCFL1. However, it has been reported by Stark et al [20] that tensile strength and stiffness of PP/WF composites are largely enhanced by the aspect ratio, not particle size, of WF particles. Hence, from tensile test results, it can be assumed that the increase order of aspect ratio among initial wood flours is CFS1˃PF1˃CFL1. A little lower of tensile strength values but similar trend is also obvious in pulverized WF without water i.e. the increase order of tensile strength is CCFS2 ˃CPF2 ˃CCFL2 for plate gap 200 µm and CCFS3˃ CPF3 ˃CCFL3 for plate gap 350 µm. From this result, it can be concluded that due to the pulverization of WF without water the length of WF particles is decreased by
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particle breakage as a result the aspect ratio of particles is reduced. However, in case of composites of pulverized WF with water the increase order of tensile strength of the composites
Sample
E-modulus (GPa)
Max. Strength (MPa) Strain (%)
PP
0.36 ± 0.18
33.03 ± 0.36
26.28 ± 4.25
CCFS1
3.76
43.42 ± 0.50
10.14 ± 0.6
CCFS2
3.22 ± 0.22
42.89 ± 0.43
CCFS3
-
42.32 ± 0.11
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Table 3. Mechanical properties of PP, PP/WF composites
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is not similar as mentioned above.
CCFS4
3.09 ± 0.34
41.10 ± 0.43
8.76 ± 0.33
CCFS5
3.02 ± 0.32
42.01 ± 0.38
10.85 ± 0.85
CCFL1
3.10
40.09 ± 0.21
9.16 ± 1.76
CCFL2
3.12 ± 0.19
40.14 ± 0.36
9.63 ± 1.38
CCFL3
2.90 ± 0.26
39.52 ± 0.11
9.65 ± 1.37
CCFL4
2.77 ± 0.32
40.46 ± 0.16
10.45 ± 1.08
CCFL5
3.23 ± 0.30
41.28 ± 0.33
9.24 ± 1.04
CPF1
3.53 ± 0.24
42.01 ± 0.17
9.92 ± 1.34
CPF2
3.24 ± 0.22
42.41 ± 0.16
10.03 ± 0.85
CPF3
3.45 ± 0.04
42.01 ± 0.40
10.30 ± 0.95
CPF4
3.48 ± 0.12
42.47 ± 0.29
10.43 ± 1.11
43.73 ± 0.36
9.91 ± 1.11
3.57 ± 0.08
11.20 ± 1.6
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CPF5
10.67 ± 1.73
Composites CPF4 displayed higher value of tensile strength compared with CCFS4. In addition, the tensile strength values of PP/WF composites with pulverized small sized wood flour obtained from cypress tree are decreased. Whereas, in the case of large sized wood flour the tensile values of PP/WF composites are not decreased rather little increased. It seems that pulverization of WF with water may has positive effect to the tensile strength of PP composites compared with that of pulverization of WF without water. Thus, tensile test results also
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indicated, depending on the type of initial wood flour particle, pulverization of WF with water can enhance or decrease the aspect ratio of WF. Thus, the effectiveness of pulverization of WF with water to the tensile strength of composites depend on the type of initial wood flour particles.
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3.4. Impact behaviour of composites
Fig. 7. Impact strength of neat PP and different wood flour (1) intial, (2) pulverized without water setting plate gap 200 µm, (3) pulverized without water setting plate gap 350, (4) pulverized
reinforced PP composites.
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with water setting plate gap 200 µm, (5) pulverized with water setting plate gap 350 µm
The column diagram in Fig.7 shows the impact strength of the processed composites.
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Izod impact test of unnotched specimens of the processed composites together with neat PP was carried out and it was found that incorporation of 25 wt.% WF in PP matrix dropped the impact
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strength of neat PP (50 kJ/m2) largely. This large drop of impact strength indicates that the PP/WF composites become brittle or less ductile compared with that of neat PP. However, the composites reinforced with pulverized wood flour with water setting plate gap 200 µm displayed higher values of impact energy compared with the composites of pulverized WF without water (Fig.7). Composites with smaller (165 µm) particles also displayed higher values of impact energy compared with the composites with larger (265µm and 379 µm) particles. It was also found that the composite of PP with smaller (165 µm) particles (obtained from cypress tree), which was pulverized with water displayed the highest value of impact energy (20 kJ/m2). Hence, impact test results suggest that smaller particles has better effect to the impact behaviour
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of composites compared with larger particles. The effect of particle size to the impact energy of PP/wood flour composites has also been investigated by Stark et al [20]. They reported that the unnotched impact energy of PP/WF composites is increased with decreasing particle size due to
composites can be improved by using pulverized WF with water.
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3.5. Fatigue behavior of composites
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reducing stress concentration. Hence, the impact test results indicates that the ductility of PP/WF
Fig. 8. S-N curves of neat PP and PP/WF composites with (a) initial wood flour, (b) pulverized and nonpulverized WF (small partilces), (c) pulverized and nonpulverized WF (large partilces).
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Fig. 8a represents the S-N curves of neat PP and PP/WF composites with different initial wood flour. The fatigue test data were fitted by regression technique to a logarithmic equation as σ = blog(N) c (2)
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follows:
where, σmax is applied maximum stress and N is number of frequencies to fracture. The values of b and c are constant and to depend on the type of materials. For all the processed composites, the values of regression coefficient (R2) were found to be ˃0.98. In Fig.8a, it is obvious that the
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fatigue lives of composites are higher than that of neat PP. The higher fatigue life of composites indicates the positive effect of wood flour to the fatigue life of PP/WF composites. The improved
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fatigue life of composites is attributed to the crack arresting properties of composite materials [41]. The presence of hard and strong wood flour particles arrested or diverted the cracks propagation which generated in the soft and weak PP matrix [42]. It has also been reported by Abdelhaleem et al [43] that fatigue life of composites increased with filler content. Fig.8b and 8c represent S-N curves of initial and pulverized wood flour reinforced PP composites for small and large sized cypress wood flour respectively. However, in Fig.8b and 8c, the effect of
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pulverization of wood flour to the fatigue life of composites is not easily understood as some
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experimental data are scattered and overlapped with each other.
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Fig. 9. Column diagram shows 50% fatigue life at constant fatigue strength (30 MPa) of PP composites reinforced with different small (S) and large (L) size cypress wood flour (1) intial, (2) pulverized without water (plate gap 200 µm), (3) pulverized with water (plate gap 200 µm)
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To see the effect of pulverization of wood flour with and without water to the fatigue life of composites, at constant fatigue strength (30 MPa), 50% fatigue life of pulverized wood flour and nonpulverized wood flour reinforced PP composites was calculated based on equation (2). Fig.9 represents the 50% fatigue life (at fatigue strength 30 MPa) of pulverized and nonpulverized
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small (S) and large (L) size cypress wood flour reinforced PP composites. Although the changes of 50% fatigue life of the composites are not very high, in column diagram of Fig.9, now it is
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possible to see the effect of wood flour pulverization to the fatigue life of composites. In Fig.9, it is seen that in the case of small sized cypress flour the 50% fatigue life of pulverized wood flour reinforced PP composites is lower compared with that of nonpulverized WF reinforced composites. Whereas, in the case of large sized cypress flour the opposite behavior can be noticed, pulverized wood flour with water reinforced PP composites has higher 50% fatigue life than its counterparts nonpulverized and pulverized WF without water reinforced PP composites.
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The higher fatigue life of pulverized wood flour reinforced PP composites indicate the more crack-arresting behavior of fibrillated wood flour particles. In PP matrix, fibrillated wood flour have a probability to form interconnected network of wood flour particles through the sufficient extent of fibrils on their surfaces. The probability of network formation of wood particles may
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also increase with increasing degree of fibrillation on wood particle surfaces. Thus increasing the probability of network formation of WF increase crack-arresting behavior in composites.
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Pulverization of small sized WF decreased the fatigue life of composites, whereas pulverization of larger sized WF improved the fatigue life. Since fatigue life of composites depend on the particle aspect ratio like as tensile strength and fatigue life of composites increased with increasing particle aspect ratio [44] hence fatigue test results also indicated that effectiveness of pulverization of wood flour to the fatigue life of PP/WF composites also depend on the initial wood particles size. Thus, further pulverization of selective WF with water may has a positive effect to the fatigue life of PP/WF composites. 4. Conclusion
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In this study we investigated the effect of pulverization of wood flour with and without water to the melt-viscosity and mechanical behaviour of polypropylene (PP)/wood flour (WF) composites. The effects of initial wood flour particle size, type of trees (namely Cypress and scots pine trees) as well as plate gap (200 µm and 350 µm) in pulverization were also evaluated
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in terms of melt-viscosity, tensile properties, Izod impact strength and fatigue behaviour of the prepared composites.
Melt-viscosity results exhibited that PP/WF composites with pulverised wood flour with water had higher melt-viscosity than that of nonpulverized and pulverized wood flour without
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water reinforced PP composites. Higher melt-viscosity of pulverised WF reinforced PP composites indicated more roughness on particle surfaces and higher interfacial interaction
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between WF and PP matrix. Further pulverization of wood flour also affected to the tensile strength values of composites. The tensile strength of the processed composites with 25 wt% of WF were found in the range of 39-43 MPa and the tensile strength values of composites depend on the type of initial WF. Izod impact test results of unnotched specimens of the PP/WF showed that the composites reinforced with pulverized wood flour with water displayed higher values of impact energy compared with the composites of pulverized WF without water. Depending on the
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type of initial WF, fatigue test results also displayed the positive effect of further pulverization of wood flour with water to the fatigue life of the composites. Finally from this study, it can be concluded that depending on initial wood flour particles, further pulverization of wood flour with water can create more roughness on the WF particle surfaces by fibrillating and an improvement
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of mechanical properties such as tensile strength, impact strength and fatigue life of PP/WF can
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References
[1] B. V. Kokta, R. G. Raj, and C. Daneault, Use of wood flour as filler in polypropylene: Studies on mechanical properties, Polym.-Plast. Technol. Eng. 28 (1989) 247-259. [2] G. E Myers, I. S. Chahyadi, C. A. Coberly, and D. S Ermer, Wood flour/polypropylene composites: influence of maleated polypropylene and process and composition variables on mechanical properties, Int. J. Polymer. Mater. 15 (1991) 21-44. [3] S. Y. Lee, H. S. Yang, H. J. Kim, C. S. Jeong, B. S. Lim, and J. N. Lee, Creep behavior and
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manufacturing parameters of wood flour filled polypropylene composites, Compos. Struct. 65 (2004) 459-469. [4] L. Danyadi, T. Janecska, Z. Szabo, G. Nagy, J. Moczo, and B. Pukanszky, Wood flour filled PP composites: compatibilization and adhesion, Compos. Sci. Technol. 67 (2007) 2838-
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2846.
[5] H. S. Kim, B. H. Lee, S. W. Choi, S. Kim, and H. J. Kim, The effect of types of maleic anhydride-grafted polypropylene (MAPP) on the interfacial adhesion properties of bio-flourfilled polypropylene composites, Composites Part A. 38 (2007) 1473-1482.
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[6] K. Carlborn and L. M. Matuana, Influence of processing conditions and material compositions on the performance of formaldehyde‐free wood‐based composites, Polym.
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Compos. 27 (2006) 599-6607.
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