polypropylene nanocomposites: Physical and mechanical properties

polypropylene nanocomposites: Physical and mechanical properties

Industrial Crops & Products 111 (2018) 47–54 Contents lists available at ScienceDirect Industrial Crops & Products journal homepage: www.elsevier.co...

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Industrial Crops & Products 111 (2018) 47–54

Contents lists available at ScienceDirect

Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop

Research paper

Superparamagnetic [email protected] wood flour/polypropylene nanocomposites: Physical and mechanical properties

MARK



Mahdi Mashkour , Yahya Ranjbar Laboratory of Sustainable Nanomaterials, Department of Wood Engineering and Technology, Gorgan University of Agricultural Sciences and Natural Resources, Goran, 49189-43464, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Wood-plastic nanocomposite Magnetic wood flour Polypropylene Physical properties Mechanical properties Superparamagnetic Fe3O4 nanoparticles

Given the high potential applications of magnetic green polymer nanocomposites in the construction, automotive and military industries, fabrication and characterization of superparamagnetic [email protected] wood flour/ polypropylene nanocomposites were investigated for the first time in this study. Magnetite (Fe3O4) nanoparticles were in situ synthesized on poplar wood flours. The filler loading used was 40 wt.% and the virgin wood flours (VWFs) and magnetic wood flours (MWFs) were used as filler at three levels of mixing ratios (VWFs: MWFs = 100:0, 50:50, and 0:100 wt.%). Maleic anhydride-grafted PP (MAPP) was used as the MAPP at two levels of 1 and 3 wt.%. Characterization of the MWFs confirmed the successful “in situ synthesis” of spherical superparamagnetic Fe3O4 nanoparticles on the wood flours with an average crystal size of about 8 nm at approximately 30 wt.%. The physical properties examinations indicated that the increase of MWFs up to 100 wt.% of filler phase increased the long-term water absorption and thickness swelling of specimens up to 2.6 and 2.4fold, respectively. The ultimate tensile and flexural strengths increased when MWFs increased by up to 50 wt.% and then decreased at higher weight ratios of MWFs. Increasing the MAPP from 1 to 3 wt.% improved both physical and mechanical properties, but this effect was more perceptible in improving mechanical strengths compared to the elastic modulus. The magnetization characterization showed about 10% reduction in saturation magnetization (Ms) of the final MWFs/polypropylene nanocomposites in comparison with the Ms values of the starting MWFs (≈1 emu/g). This observation was attributed to the formation of a nonmagnetic layer on the Fe3O4 nanoparticles during the melt-blending process.

1. Introduction For about three decades now, wood-plastic composites produced by mixing industrial polymers such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene with lignocellulosic fillers in various geometrical shapes and dimensions have been the focus of interest of large industries, especially the construction and automotive industries (Holbery and Houston, 2006; Klyosov, 2007; Koronis et al., 2013; La Mantia and Morreale, 2011; Martins et al., 2016; Nourbakhsh et al., 2011; Selke and Wichman, 2004; Smith and Wolcott, 2006; Taşdemır et al., 2009). Wood-plastic offers less water absorption, greater dimensional stability, and more resistance to biological degradation and weathering compared to untreated solid wood (Klyosov, 2007; Rowell et al., 1997; Selke and Wichman, 2004; Wechsler and Hiziroglu, 2007). Furthermore, it has modification capability which commensurate with the desired applications and mechanical strength, as well as the required densities. In general, compared to untreated solid wood, woodplastic composite is a greener, more cost-effective and more efficient ⁎

product mainly due to the fact that it can combine the superior features of natural fibers such as being environment-friendly, abundant, inexpensive, non-corrosive, low density, and low coefficient of thermal expansion with desirable mechanical characteristics in addition to those of the synthetic polymers, including resistance to water absorption, thickness swelling, and biological degradation, and ease of processing as well as plasticity in complex three-dimensional shapes (Hosseinaei et al., 2012; Klyosov, 2007; Mohanty et al., 2005; Rowell et al., 1997; Stark and Rowlands, 2007; Tajvidi et al., 2006; Wang et al., 2006; Wechsler and Hiziroglu, 2007; Yang et al., 2006). In addition to the warm reception of this product by large construction and automotive industries, developmental studies have boosted towards given objectives such as upgrading quality and customizing features of woodplastic composites that commensurate with their final applications. The advent of nanotechnology has influenced the surging development of industrial products and wood-plastic industry was no exception. Many researchers in this field have attempted to tackle issues such as upgrading the quality and range of the applications of their products

Corresponding author. E-mail addresses: [email protected], [email protected] (M. Mashkour).

http://dx.doi.org/10.1016/j.indcrop.2017.09.068 Received 14 May 2017; Received in revised form 29 September 2017; Accepted 30 September 2017 0926-6690/ © 2017 Elsevier B.V. All rights reserved.

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were then used to separate 60-mesh wood flours to be further used as the filler phase. Prior to the in situ synthesis of Fe3O4 nanoparticles, the wood flours were exposed to a mild hydrothermal treatment to prevent possible effect of contaminations, and extracted materials were dissolved in water. Since the temperature in the in situ synthesis of magnetite nanoparticles was presumed to be at about 75 °C, the hydrothermal pretreatment was done in two 2-h stages in distilled water at 85 °C. At the completion of the pretreatment, the wood flours clearly had a visible lighter color.

by utilizing this technology (Borah and Kim, 2016; Hosseini, 2016; Saba et al., 2016; Saha and Sutradhar, 2016). Production of wood-plastic products with much greater superiority in features of scratch, stain, fire and moisture resistance, resistance to biological agents and weathering, and of having lighter weight together with superior mechanical and engineering features are among some goals of employing nanotechnology in this industrial field (Ashori et al., 2013; Ayrilmis et al., 2014; Borah and Kim, 2016; Faruk and Matuana, 2008; Guo et al., 2007; Hosseini, 2016; Karakuş et al., 2017; Kordkheili et al., 2013; Najafi, 2013; Rasouli et al., 2016; Saba et al., 2016; Saha and Sutradhar, 2016). Reports of extensive research on the use of mineral nanoparticles such as various types of nanoclay, zinc oxide, titanium dioxide, nanosilicate and other types of nano oxides and mineral hybrids with the purpose of quality improvement and creation of special applied features in wood-plastic composites have been published in recent years (Deka and Maji, 2013; Faruk and Matuana, 2008; Guo et al., 2007; Hosseini, 2016; Nourbakhsh et al., 2011; Rasouli et al., 2016; Saba et al., 2016; Wang et al., 2015; Ye et al., 2016). Magnetic nanoparticles including iron, nickel, manganese, cobalt oxides, and their hybrids are among the most attractive members in the family of mineral nanoparticles and numerous potential applications have been predicted for them in various electronic, military, and medical industries (Frey et al., 2009; Kodama, 1999; Willard et al., 2004). During the past two decades, studies have been reported on manufacturing and evaluation of the properties of magnetic wood as an efficient material in electromagnetic shielding and indoor electromagnetic wave absorption (Gan et al., 2015; Oka and Fujita, 1999; Oka et al., 2004; Oka et al., 2002a; Oka et al., 2007; Oka et al., 2002b; Oka et al., 2012; Oka et al., 2011). Considering these features, magnetic wood can be used as a functional engineering material especially in the construction, automotive and military industries with the aim of preventing interference and leakage of electromagnetic waves. With regard to the advantages mentioned for wood-plastic composites compared to solid wood, for the first time, this research aimed to fabricate superparamagnetic [email protected] wood flour/ polypropylene nanocomposites and evaluate their physical and mechanical features. Review of literature showed that no report has hitherto been published on the manufacturing of magnetic wood-plastic composite or on evaluation of its features. Therefore, it is expected that results of the present research will be a first step in the manufacturing and development of a new generation of functional wood-plastic composites with desirable magnetic properties to be used in construction, automotive, and military industries.

2.2.2. Preparation of [email protected] wood flour nanocomposite particles In situ synthesis of Fe3O4 nanoparticles was performed with the reduction of iron (II) and iron (III) solutions in the presence of the pretreated wood flour particles. The hydrothermal pretreated wood flours were immersed in twice the volume of double distilled water inside a laboratory reactor equipped with a mechanical stirrer and constant flow of the inert nitrogen gas and heated at 65 °C for one hour. The flow rate of the nitrogen gas was 10 L/min and the speed of the mechanical stirrer was 300 rpm for deoxygenation and maintenance of homogenous conditions inside the reactor until the completion of the hydrothermal synthesis. FeCl2·4H2O (0.06 mol/L) and FeCl3·6H2O (0.12 mol/L) were weighed and immediately added to the reaction mixture. In order for the dissolution to be completed and the penetration of metal cations into the wood flours to get thoroughly ended, we waited for two hours at this step. Afterwards, the wood flours were removed from the reaction environment, dehydrated immediately, and fed into another thermal chamber containing 1 M sodium hydroxide under the same hydrothermal conditions, nitrogen gas flow rate, hydrothermal duration and mechanical stirrer speed for the reduction process of the metal cations and the in situ synthesis of magnetite nanoparticles to take place. The prepared black [email protected] wood flour nanocomposite particles were removed from the reaction environment and washed several times to remove the excess nanoparticles and the chemicals that had not taken part in the reactions. The magnetic wood flours (MWFs) were first dehydrated thoroughly using the mechanical method and then dried in two 24 h stages (first at about 60 ± 2 °C and then at 103 ± 2 °C). The prepared MWFs were poured into 2-layered polymeric bags that were completely sealed to prevent moisture absorption until they were properly mixed with the matrix phase. 2.2.3. Combination of MWFs and polymer matrix The abbreviated code and weight percent ratio of the combination in each formulation are presented in Table 1. The materials in dry form and with known weight percentages in each formulation were first thoroughly mixed, and the melt mixing and granule production processes were then carried out using a Dr. Collin counter-rotating twin screw extruder/compounder machine (Dr. Collin GMBH, Germany) with four temperature zones with the maximum and minimum temperatures of 165 and 150 °C and speed of 70 rpm.

2. Materials and methods 2.1. Materials Poplar (Populus deltoides) wood flour was used as the lignocellulosic filler. Injection grade polypropylene (V30S, MFI = 18 g/ 10 min, density = 0.92 g/cm) produced by Petrochemical Company of Arak in Iran was used as the continuous phase. Furthermore, maleic anhydride coupled with polypropylene (MAPP) grade PP-G 101 (Kimya Javid Sepahan Co., Isfahan, Iran) was employed to enhance compatibility of the matrix phase with the filler. The melt-flow index of the MAPP was 100 g/10 min. Ferrous chloride tetra-hydrate (FeCl2·4H2O), ferric chloride hexahydrate (FeCl3·6H2O) and sodium hydroxide (NaOH) were purchased from Daejung Chemicals and Metals Co. (Korea). All chemicals used were of analytical grade and were used as received without further purification.

2.2.4. Injection molding and test samples preparation Prior to the injection molding process and preparation of final test samples, the granules were exposed to 103 ± 2 °C for 4 h to ensure complete removal of moisture. The final test samples were prepared Table 1 Codes of the formulations and their compositions.

2.2. Methods 2.2.1. Preparation and hydrothermal pretreatment of wood flour Poplar wood shavings were first prepared using a wood chipping machine. The shavings were dried at 103 ± 2 °C and a laboratory mill was employed to turn them into wood flour particles. Laboratory sieves 48

Formulation

PP (%)

VWF (%)

MWF (%)

MAPP (%)

PP-VWF40-MA1 PP-VWF40-MA3 PP-VWF20-MWF20-MA1 PP-VWF20-MWF20-MA3 PP-MWF40-MA1 PP-MWF40-MA3

59 57 59 57 59 57

40 40 20 20 0 0

0 0 20 20 40 40

1 3 1 3 1 3

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formulation under the ambient temperature conditions. The samples were first dried at 103 ± 2 °C for 24 h, and their weights and thicknesses were measured and recorded using a precision balance (with the accuracy of 0.001 g) and a digital micrometer (with the accuracy of 0.001 mm), respectively. The samples were immersed in double distilled water at ambient temperature, and the changes in weight and thickness were recorded after 2, 4, 6, 8, 24, 48, 72, 168, 336, 504, 672, 840, and 1008 h, hence their long-term water absorption and thickness swelling were calculated. 2.8. Static flexural and tensile tests The ASTM D 790 standard using a Cometech-00-505-B1 Universal Testing Machine (Cometech Testing Machines Co., Taiwan) with the span of 80 mm and crosshead speed of 1.2 mm/min was employed to evaluate the static flexural properties of the test samples. The ASTM D 630 standard using a Zwick/Roell Z020 Universal Testing Machine (Zwick-Roell, Ulm, Germany) was employed at the crosshead speed of 5 mm/min for the static tensile test. At least, six specimens from each formulation were examined to assess flexural and tensile properties. 3. Results and discussion

Fig. 1. Tensile specimens of the different formulations made by injection molding.

3.1. MWFs characterization

according to the ASTM standards for dimensions using an injection molding instrument (Imen Machine Co., Iran) at injection temperature of 180 °C and pressure of 25 bar. All produced test samples were conditioned for at least one week under the same conditions in the testing environment before performing physical and mechanical tests. The fabricated tensile test samples for each formulation are shown in Fig. 1.

Fig. 2a and b shows digital images obtained from Populus deltoids wood flours before and after the in situ synthesis of Fe3O4 nanoparticles, respectively. The change in color of wood flours from light to dark brown following the in situ synthesis is most probably caused by the formation of Fe3O4 nanoparticles inside and on the surface of the wood flour particles. Owing to their mesoporous structures, plant fibers can be used as nanoreactors for in situ synthesis of metal oxide nanoparticles (Mashkour et al., 2011; Ovalle-Serrano et al., 2015). Fig. 2c and d, respectively shows FESEM micrographs from the surface of a virgin wood flour (VWF) and a [email protected] wood flour nanocomposite particle. As can be observed, given the applied in situ synthesis treatment, the rather smooth surface of the VWFs turned into a rough surface due to the formation of a coating of spherical Fe3O4 nanoparticles on the wood flour particles. The non-uniform dispersion of the Fe3O4 nanoparticles on the surface of wood flours was attributed to the nature of in situ syntheses and the agglomeration phenomena. Analysis of the FESEM micrographs revealed that the synthesized spherical nanoparticles had a mean size of about 38 ± 6 nm. Results of the ash gravimetric studies indicated that the weight of wood flours increased by 29.4 ± 1.2% after the in situ synthesis of Fe3O4 nanoparticles. Such an increase can be attributed to the presence of Fe3O4 nanoparticles on the surface and inside the cavities of the wood flour particles. Moreover, XRD analysis of the MWFs is shown in Fig. 2e. The position and also the relative intensity of the peaks in the XRD pattern indicated a very good match with the reference pattern (JCPDS No.01075-0033) related to magnetite. The peaks marked with asterisks are related to the Miller indices of cellulose crystals. Based on the value of full width at half maximum (FWHM) of the peak (311), the calculated mean size of the synthesized crystals of Fe3O4 nanoparticles was about 8 nm. The XRD results indicated that the Fe3O4 nanoparticles observed in the FESEM micrographs were Fe3O4 nanoclusters formed by aggregation of the Fe3O4 nanocrystals (or nanograins). This phenomenon has been previously reported in the literature for the Fe3O4 nanoparticles (Xuan et al., 2009). The size of the synthesized crystals was smaller than the average critical radius proposed for the change in behavior of Fe3O4 nanocrystal from the superparamagnetic state (Jafari et al., 2014; Phan et al., 2016). The results of the VSM test on the prepared MWFs and the initial VWFs are presented in Fig. 2f. Absence of hysteresis loop in the

2.3. Microscopic analysis A field emission scanning electron microscope (FESEM, Hitachi S4800, Japan) was used to examine the microstructure of the in situ synthesized Fe3O4 nanoparticles and [email protected] wood flour nanocomposite particles as well as fracture surfaces of the samples. Before microscopic observation, the surfaces of the samples were coated with a thin layer of gold. 2.4. X-ray diffraction (XRD) analysis A PANalytical X’Pert Pro model XRD instrument (PANalytical, Netherlands) with a CuKa radiation source and voltage and current of 40 kV and 40 mA, respectively, was employed for phase identification and for determination of the crystal size of in situ synthesized magnetic nanoparticles using Debye-Scherrer equation. 2.5. Magnetic characterization Magnetic behavior of the test samples at ambient temperature was assessed using a vibrating sample magnetometer (VSM, Megnetic Daghigh Daneshpajouh Co., Iran) in the field range of −9000 to +9000 Oe. 2.6. Gravimetric studies of in situ synthesized Fe3O4 nanoparticles The standard TAPPI method T211 om-02 was employed to determine the weight ratio of the Fe3O4 nanoparticles synthesized on wood flour particles. 2.7. Long-term water absorption and thickness swelling tests Long-term water absorption behavior and thickness swelling of the test samples were determined with at least five replications for each 49

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Fig. 2. Digital photos of the VWFs (a) and MWFs (b) (The scale bar is 10 mm). FESEM micrographs from the surface of VWFs (c) and MWFs (The scale bar is 1 μm). (e) XRD pattern of the prepared MWFs. (f) Magnetization curves of the VWFs and MVFs. (g) The MWFs attracted by a permanent magnet.

filler phase and 3 wt.% of the MAPP; there was little evidence of intact VWFs having left the polymer matrix and being present on the fracture surfaces (Fig. 3a–c). On the other hand, there were many cavities on the fracture surfaces and inside the polymer matrix in the micrographs of nanocomposite specimens containing 100 wt.% MWFs and 3 wt.% MAPP. The pull-out of intact MWFs from the matrix phase led to the formation of cavities on the fracture surface and this observation can suggest the lack of suitable interaction between the matrix and the filler phases and lack of development of the interface between these two phases (Fig. 3d–f). This reduction in the interface between the matrix and the MWFs can be attributed to the presence of layers of Fe3O4 nanoparticles with weak attachments to wood flour particles. These layers can act as a physical barrier that limits access of the MAPP to the

obtained VSM curve demonstrated the superparamagnetic behavior of synthesized Fe3O4 nanoparticles and was in good agreement with the results obtained from the assessment of the crystal size of these particles in the analysis of the XRD pattern. Saturation magnetization of the prepared MWFs was about 1 emu/g. 3.2. Microscopic analysis of fracture surfaces in the test samples The visual quality of the interaction between the matrix and the filler phases in various formulations was assessed using microscopic imaging of the tensile fracture surfaces (Fig. 3a–f). FESEM micrographs indicated a desirable interaction between the matrix and the filler phases in the fabricated composites containing 100 wt.% VWFs as the

Fig. 3. SEM micrographs of tensile fracture surfaces of the PP-VWF40-MA3 samples (a, b, and c) and PP-MWF40-MA3 (d, e, and f) at different magnification levels.

50

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Fig. 4. (a) Changes in the density of different samples. (b) Magnetization curves of the MWFs (blue color), the PP-MWF40-MA1 (green color) and the PP-MWF40-MA3 (red color). Longterm water absorption (c) and thickness swelling curves (d) of specimens during 1008 h of immersion time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

greater development of the interface between the filler and matrix phases since FESEM micrographs clearly showed that the presence of the Fe3O4 nanoparticles had a negative effect on the development of this interface. These results point to the importance of the use of coupling agents in reducing the negative effect of the presence of Fe3O4 nanoparticles. In addition, the VSM test proved the stabilization of the superparamagnetic behavior of nanocomposites made of MWFs as the filler phase (Fig. 4b). As can be seen, no coercivity and remanence were observed in the VSM curves of PP-MWF40-MA1 and PP-MWF40-MA3 samples. It implies that the superparamagnetic behavior of [email protected] wood flour nanocomposite particles was not affected by the applied temperature during compounding and processing of the wood-plastic nanocomposites and no magnetic phase transition occurred. In comparison with the MWFs, saturation magnetizations of the PP-MWF40MA1 and the PP-MWF40-MA3 samples slightly declined by ≈10% and nearly reached 0.9 emu/g. This reduction was attributed to some chemical changes on the surface layer of Fe3O4 nanoparticles during the melt blending process. Indeed, formation of a nonmagnetic or “dead” surface layer on the Fe3O4 nanoparticles makes its magnetic diameter smaller than its physical diameter and thus the values of saturation magnetization slightly dropped. Similar observations have been

free hydroxyl groups on the surface of the wood material and prevents the establishment of a direct bridge between the filler and the matrix phases. Reduced development of the interface can negatively affect all physical and mechanical properties of the polymer composites.

3.3. Analysis of physical features of the test samples Individual features and also the quality of the interactions between the constituents of a composite material can determine how it responds to applied physical and mechanical stimuli. Fig. 4a shows the effects of changes in weight ratios of MWFs and the MAPP on the densities of the fabricated test samples. Increases in the weight ratio of the MWFs in the filler phase slightly increased the densities of the specimens because of the greater density of the MWFs compared to that of VWFs. Moreover, increases in the MAPP from 1 to 3 wt.% increased the density of the samples. Changes in density caused by increased share of MAPP were more noticeable in test samples in which MWFs constituted 100 wt.%. Considering the trend of changes in density, it seems that the role played by the MAPP in improving the quality of the interaction between the filler and matrix phases becomes more significant with increases in the weight ratio of MWFs. Of course, this conclusion does not mean that increases in the weight ratio of the MWFs in the filler phase lead to 51

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the MAPP from 1 to 3 wt.% improved the evaluated mechanical properties in the test samples. By and large, the MAPP was more effective in improving tensile and flexural strengths of the test samples compared to their elastic modulus. Furthermore, it was found that tensile and flexural strengths increased when the weight ratio of the MWFs was raised up to 50% of the lignocellulosic filler while the mentioned mechanical strengths declined at weight increases between 50 and 100% in the MWFs. This pattern of changes was more pronounced in the specimens containing 1 wt.% of the MAPP. It is evident that changes in flexural and tensile elastic modulus resulting from the weight percent of the wood flours do not follow any specific pattern. It has been established that increases in the filling agent in polymer composites containing particulate fillers with low aspect ratios such as wood flours generally reduce tensile and flexural strengths (Bouafif et al., 2009; Klyosov, 2007; Stark and Rowlands, 2007; Yang et al., 2006). As was previously referred to, due to the method of calculating the mixing ratio of materials and because of the higher density of MWFs compared to VWFs, increases in the weight ratio of MWFs reduce the volume ratio and abundance of filler particles per unit volume of the made specimens and increase the volume ratio of the matrix phase. Therefore, it is expected that the strengths of test samples will also improve with increases in the percentage of MWFs. However, increases in the weight ratio of MWFs decrease the interface between the filler and matrix phases and considerably increase the size of the micro-cracks inside the structure of the test samples. The mutual effects of these two factors probably had caused the advent of the observed patterns in the changes. It appears that if MWFs constitute up to 50% of the filler phase in test samples containing 1 wt.% MAPP, the positive effect of reduced abundance and volume ratio of the filler particles per unit volume of the test samples will be dominant and will lead to improved strengths. However, if the MWFs constitute 50–100 wt.% of the filler phase, the negative effect of increased abundance of the filler particles and growth of micro-cracks inside the structure of the test samples will become dominant and will result in reduced mechanical strength.

reported by other researchers (Unni et al., 2017). Fig. 4c and d presents results of studying the long-term water absorption and the thickness swelling of the specimens immersed for about 1008 h. Generally, three main factors can influence the extent of water absorption and thickness swelling of wood-plastic composites at a given immersion duration: the weight ratio of the lignocellulosic filler, the inclination of the lignocellulosic material to absorb water (which results from its chemical and physical structure), and the quality of the interactions between the filler and matrix phases (which affects development of the interface between these two phases) (Bouafif et al., 2009; Hosseinaei et al., 2012; Hosseinihashemi et al., 2016; Klyosov, 2007; Tajvidi et al., 2006; Yang et al., 2006). Results showed that increases in the weight ratios of the MWFs and of the MAPP caused increases and decreases in the inclination to absorb water and in thickness swelling of the test samples, respectively; therefore, the minimum volume of absorbed water (≈4.89%) and thickness swelling (≈2.77%) belonged to the PP-VWF40-MA3 specimens, and the maximum water absorption (≈12.62%) and thickness swelling (≈7.07%) belonged to the PP-MWF40-MA1 specimens. As it was previously mentioned, tests of measuring ash indicated that weight of MWFs increased by about 30% compared to VWFs. As a general rule, mixture of materials in the formulation of wood-plastic composites is based on weight ratio percentages (Klyosov, 2007). This research used a constant weight ratio percentage of the filler agent to that of the polymer matrix in all of the formulations (40 wt.%), and the density of MWFs was greater than that of VWFs (by about 30%). Therefore, it is evident that the weight ratio percentage and the volume of lignocellulosic material decreased with increases in the weight ratio of the MWFs compared to the total composition. Consequently, the volume of absorbed water and thickness swelling should also decrease with increases in the weight ratio of MWFs. However, study of water absorption and of thickness swelling curves indicated an opposite trend. To explain these results, we can point to the effect the chemical synthesis of Fe3O4 nanoparticles has on the chemical structure of wood flour particles, and also to the effect the presence of Fe3O4 nanoparticles has on lack of expansion in the interface between the matrix and filler phases. It has been established that the alkaline treatment of lignocellulosic materials causes swelling of the structure and expansion of the amorphous fractions in cellulose that are accompanied by greater access to moisture and water absorption (Mohanty et al., 2005). Moreover, the alkaline treatment of lignocellulosic materials in warm water environments, similar to what was applied in the process of in situ synthesis of magnetite nanoparticles, is probably accompanied by enhancement of the mentioned effects and by increases in the intensity of the inclination to water absorption and volume swelling of the lignocellulosic materials. Furthermore, as shown in the FESEM micrographs, presence of Fe3O4 nanoparticles on wood flours can prevent suitable interactions between the matrix and filler phases and, perhaps, can limit the complete encapsulation of the filler particles by the polymer matrix. Therefore, one can imagine a continuous network of pores and open channels that forms inside the framework of the fabricated magnetic nanocomposites (which give much enhanced accessibility of the lignocellulosic materials to water during immersion process) with increases in the weight ratio of the MWFs due to the lack of development of the interface, and hence the growth in micro-cracks between the filler and matrix phases. Thus, although increases in the weight ratio of MWFs lead to reductions in the weight ratio of the lignocellulosic materials, they lead to greater inclination to absorb water and to significant increases in thickness swelling of the specimens.

4. Conclusions In this study, for the first time, fabrication and characterization of superparamagnetic wood flour/polypropylene nanocomposites (SPMWFPPNCs) were investigated. Results revealed that in situ synthesis of Fe3O4 nanoparticles on wood flours was successful. The MWFs exhibited superparamagnetic behavior with a Ms of about 1 emu/g and an increase in weight of about 30% compared to the VWFs which was attributed to the presence of magnetite nanoparticles. FESEM micrographs indicated a weakening of the interaction between the filler and matrix phases with increases in the weight ratio of the MWFs. Magnetization measurements at room temperature showed about 10% decrease in the Ms values of SPM-WFPPNCs compared to the [email protected] wood flours. This reduction in the Ms value was attributed to the formation of a dead magnetic surface layer on Fe3O4 nanoparticles during the melt blending process. Complete substitution of the VWFs with the MWFs increased water absorption and thickness swelling of the SPMWFPPNCs specimens up to 2.6 and 2.4-fold, respectively. Compared to the mechanical properties, the physical properties of the SPMWFPPNCs were more influenced by the increases in the proportion of the MWFs. The results showed that increasing MAPP from 1 to 3 wt.% improved physical efficiency of the specimens in all of the formulations. Results of static flexural and tensile tests on the specimens displayed a relatively similar trend of changes. The trend of changes in ultimate flexural and tensile strengths of the test samples was an ascending one when MWFs constituted up to 50 wt.% of the filler, but it became a descending one when the MWFs made up a higher percentage of the weight of the filler. The results were attributed to the combined effects of the higher density of MWFs, disrupted development of the interface between the filler and the polypropylene matrix with increases in the proportion of the magnetic filler, and also to the efficient performance

3.4. Analysis of static bending and tensile features of the test samples Fig. 5 shows results of the flexural and tensile tests on the test samples. As shown in this figure, the trends of changes in the tensile and flexural features are similarly influenced by the weight ratio percentages of the MWFs and of the MAPP. Results showed that increases in 52

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