Date palm biochar-polymer composites: An investigation of electrical, mechanical, thermal and rheological characteristics

Science of the Total Environment 619–620 (2018) 311–318

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Date palm biochar-polymer composites: An investigation of electrical, mechanical, thermal and rheological characteristics Anesh Manjaly Poulose a,⁎, Ahmed Yagoub Elnour a, Arfat Anis a, Hamid Shaikh a, S.M. Al-Zahrani a, Justin George b, Mohammad I. Al-Wabel c, Adel R. Usman c,d, Yong Sik Ok e, Daniel C.W. Tsang f, Ajit K. Sarmah g a

Chemical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia Civil Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia Soil Sciences Department, College of Food and Agricultural Sciences, King Saud University, Riyadh 11451, Saudi Arabia d Soils and Water Department, Faculty of Agriculture, Assiut University, Assiut, Egypt e O-Jeong Eco-Resilience Institute (OJERI), Division of Environmental Science and Ecological Engineering, Korea University, Seoul, Republic of Korea f Department of Civil and Environmental Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China g Civil and Environmental Engineering Department, University of Auckland, Auckland 1142, New Zealand b c

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Date palm waste derived biochar was used as filler for polymer composites' applications. • Biochar/polypropylene (BC/PP) composites' properties such as electrical, mechanical, thermal and rheological were investigated. • The BC/PP composites' surface resistivity was decreased by four orders of magnitude.

a r t i c l e

i n f o

Article history: Received 31 August 2017 Received in revised form 5 November 2017 Accepted 7 November 2017 Available online 29 November 2017 Keywords: Date palm waste Biochar Polymer composites Electrical conductivity Rheology

⁎ Corresponding author. E-mail address: [email protected] (A.M. Poulose).

https://doi.org/10.1016/j.scitotenv.2017.11.076 0048-9697/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t The application of biochar (BC) as a filler in polymers can be viewed as a sustainable approach that incorporates pyrolysed waste based value-added material and simultaneously mitigate bio-waste in a smart way. The overarching aim of this work was to investigate the electrical, mechanical, thermal and rheological properties of biocomposite developed by utilizing date palm waste-derived BC for the reinforcing of polypropylene (PP) matrix. Date palm waste derived BC prepared at (700 and 900 °C) were blended at different proportions with polypropylene and the resultant composites (BC/PP) were characterized using an array of techniques (scanning electron microscope, energy-dispersive X-ray spectroscopy and Fourier transform infra-red spectroscopy). Additionally the thermal, mechanical, electrical and rheological properties of the BC/PP composites were evaluated at different loading of BC content (from 0 to15% w/w). The mechanical properties of BC/PP composites showed an improvement in the tensile modulus while that of electrical characterization revealed an enhanced electrical conductivity with increased BC loading. Although the BC incorporation into the PP matrix has significantly reduced the total crystallinity of the resulted composites, however; a positive effect on the crystallization temperature (Tc) was observed. The rheological characterization of BC/PP composites revealed that the addition of BC had minimal effect on the storage modulus (G′) compared to the neat (PP). © 2017 Elsevier B.V. All rights reserved.

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1. Introduction Environmental pollution due to unabated disposal of organic wastes, coupled with the global climatic changes have disturbed the balance of nature to a great extent. One of the major causes of environmental pollution is due to the reckless disposal of organic waste which generates significant quantities of greenhouse gases (GHG), adversely affecting the climatic conditions. Judicious disposal of organic waste is of great prominence in today's world (Rushton, 2003). There is an urgent need to convert organic wastes into value-added products which would help not only in waste mitigation, but also enable to save and conserve energy (Xiu et al., 2017; Giusti, 2009). The organic wastes can be effectively converted into a useful renewable material called biochar through the pyrolysis of bio-wastes in a limited or oxygen free atmosphere (Balwant et al., 2010; Kookana et al., 2011). In addition to biochar, other valuable byproducts of bio-waste pyrolysis are bio-oil, hydrogen, chemicals etc. can be utilized as an alternative energy resources (Alonso et al., 2012; Huber et al., 2006; Lehmann, 2007; Zhou et al., 2011). The benefits of biochar and its applications for agricultural purposes as a soil amendment agent are well established to some extent, these applications include high crop yield, soil water retention, enriching of soil quality and storage of atmospheric carbon (Balwant et al., 2010; Creamer et al., 2014; Van Zwieten et al., 2009). Designer engineered biochar can be produced with high quantities of fixed carbon and with specific surface functional groups as well as high surface area according to the targeted niche application. In other words, the surface functionality and porosity of the biochar can be tuned for the synthesis of engineered functional materials suitable for different applications such as: catalysis (González et al., 2017; Lee et al., 2017), energy storage (Cheng et al., 2017; Xiu et al., 2017), pollutant removal (Tan et al., 2015) and CO2 capture applications (Mohd et al., 2013; Sethupathi et al., 2017). Furthermore; biochar has porous honeycomb structure with high thermal stability favorable for contaminant remediation agent applications (Balwant et al., 2010; Kilic et al., 2013; Srinivasan et al., 2015; Usman et al., 2015). The demand for reinforced plastics has grown rapidly and accordingly the carbon based filler composites as well. The carbonaceous materials such as carbon black (Manjaly Poulose et al., 2015; Zhao et al., 2014), carbon nanotubes (CNT) (Krause et al., 2009; Ma et al., 2014), carbon fiber (Kasgoz et al., 2014; Tekinalp et al., 2014) and graphene (Du and Cheng, 2012; Wang et al., 2016) have been widely used as reinforcing and conductive fillers in polymer composites. Production of petro-based carbon materials needs tedious synthetic methods and is not environmentally and economically viable. Efforts have been made to explore various renewable carbon resources as the feedstock that are environmental friendly, cost effective and are abundant in nature. In recent years; there is a scope for the successful application of biochar in thermoplastic composites due to its porous structure, large surface area, high carbon content which could facilitate the physical bonding with the polymer matrix (Das et al., 2015c). Biochar has been successfully incorporated with different types of polymer matrices for improving their mechanical, electrical and thermal properties such as polyamides (Huber et al., 2015), polyesters (Richard et al., 2016), poly (vinyl alcohol) (PVA) (Nan et al., 2015), styrene-butadiene rubber (SBR) (Peterson, 2012; Peterson et al., 2015), poly (trimethylene terephthalate) (PTT) (Myllytie et al., 2016), epoxy (Ahmetli et al., 2013), poly (trimethylene terephthalate/poly (lactic acid) (PTT/PLA) blend (Nagarajan et al., 2016) and polypropylene (Das et al., 2016a, 2016b). For instance, the impact strength of the polyester having particle loading of 2.5% w/w of biochar (45 nm particle size) increased by 77.50% and its dielectric constant increased by 7% when compared with the neat polyester resin (Richard et al., 2016). The PVA/biochar composites filled with 2 and 10%, w/w of biochar exhibited electrical conductivity values similar to carbon nanotube and graphene reinforced PVA composites. The thermal stability, tensile modulus and storage modulus of PVA/biochar composites were improved with BC addition (Nan et al.,

2015). Partially replacing the carbon black (CB) with biochar in SBR matrix improved the tensile strength, toughness and elongation of the composites (Peterson, 2012). PTT/biochar composites exhibited good dimensional stability, 89% increase in heat deflection temperature, 60% increase in flexural modulus and 14% increase in flexural strength in comparison to neat PTT (Myllytie et al., 2016). Biochar is more advantageous than the natural fibers as a filler in polymer composites since the properties of biochar can be altered by modifying the pyrolysis conditions for achieving the hydrophobic nature in biochar (Das et al., 2015) and to obtain greater compatibility with the polymer matrix than the hydrophilic natural fibers (Monteiro et al., 2012). The thermal stability of the resulting biochar composites has been reported to be higher than the composites with natural fibers such as jute, sisal, flax, hemp, coir and cotton (Monteiro et al., 2012). In general, carbon fillers are incorporated into polymers to improve the mechanical, thermal, electrical and chemical corrosion resistance properties compared to metal filled composites (Das et al., 2016a; Das et al., 2016c; Das et al., 2015a; Das et al., 2015b). Improving one or more of these properties is desirable for the many applications such as for electrostatic dissipation material, electromagnetic interference shielding, semiconducting layer to prevent electrical discharge (Khushnood et al., 2015). The end properties of the resultant composites are determined by many factors such as the matrix and the filler characteristics, matrix-filler interactions and dispersion of the filler particles in polymer matrix (Manjaly Poulose et al., 2015; Alig et al., 2012). For example, the electrical conductivity of the composites is determined by the dispersion of fillers and network formation in polymer matrix above a threshold value of filler concentration (percolation threshold) (Danqi Ren et al., 2014). Date palm (Phoenix dactylifera) is highly cultivated crop in Saudi Arabia, it is estimated that N23 million date palm tree is cultivated annually which produce around 780 thousand tons of agriculture residues per year as a seasonal pruning and refinement of palms (Usman et al., 2015; Miandad et al., 2017). This huge amount of agricultural wastes produced are either burned in farms or disposed in landfills which cause a serious environmental pollution. The major constituents of date palm biomass are cellulose, hemicellulose, lignin and volatile contents which could converted into biochar. In this study, the biochar was prepared by the pyrolysis of date palm waste (Usman et al., 2015). The pyrolysis conditions such as temperature, pressure, heating rate and duration of heating are the most important factor for controlling the physical and chemical properties of the biochar prepared (Das et al., 2015b; Usman et al., 2015). Generally it has been found that biochar prepared at higher pyrolysis temperatures (N 500 °C) yields larger surface area (Usman et al., 2015; Joseph et al., 2010) due to the removal of volatile impurities clogged in its pores. Though there have been a few studies utilizing biochar in polymer composites (Das et al., 2015a, 2015b, 2015c, 2016a, 2016b, 2016c, 2016d; Ogunsona et al., 2017), to date, the application of biochar derived from date palm waste has not been investigated. Furthermore, the effect of biochar addition on the electrical conductivity of BC/PP composites has not been reported in the literature. Generally, biochar characteristics and its stability are mainly dependent upon feedstock and pyrolysis conditions. It has been reported that biochars produced at high pyrolysis temperature have high content of fixed carbon, predominately recalcitrant aromatic C structure and high thermal stability (Zhao et al., 2017; Usman et al., 2015;

Table 1 Elemental composition, moisture, ash content and BET surface area of biochar samples in percentages. Sample

Moisture

Ash

C

H

N

Oa

SSAb (m2/g)

BC700 BC900

3.48 3.06

20.57 21.35

66.70 69.38

1.01 0.65

0.19 0.21

8.05 5.35

283.62 291.11

a b

Obtained by deduction. BET specific surface area.

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Fig. 1. SEM images of biochar (BC900) (a) before grinding, (b) after grinding.

Al-Wabel et al., 2013). Several researchers reported that the biochar produced at high temperatures (≥700 °C) has no NMR peaks as a result of the occurrence of a larger number of delocalized (ᴨ electrons implying a higher conductivity) arising from the growth of aromatic planes of BC (Cao et al., 2012 and Li et al., 2013). Therefore the essential purpose of this study was to utilize biochar obtained from date palm wastes, pyrolysed at two different higher temperatures (700 °C and 900 °C), for the preparation of BC/PP composites (0–15% w/w) and to investigate their electrical, mechanical, thermal and rheological characteristics. 2. Materials and methods 2.1. Materials used The polymer matrix used in this study was PP homo-polymer (TASNEE PP H4120) supplied by TASNEE (The National Industrialization Company, Kingdom of Saudi Arabia), with melt flow rate (MFR) of 12 g/10 min (ISO 1133) and density of 0.9 g/cm3. The BC used in this study was produced through the process of slow pyrolysis of date palm tree wastes. Rachis as received was chopped into small pieces and the slow pyrolysis process was carried out in a closed stainless steel container (22 cm × 7 cm), by using an electrical muffle furnace. Two different pyrolysis temperatures of 700 °C and 900 °C were maintained for a residence time of 4 h under oxygen free environment. The BC produced at different charring temperatures (700 °C and 900 °C) were left inside the container overnight to cool down by free convection. The produced biochars were finely ground by using mortar and pestle and labelled as BC700 and BC900, where BC and numbers denote biochar and pyrolysis temperatures, respectively. 2.2. Biochar characterization Elemental analysis (C, H and N) was performed using a PerkinElmer 2400 CHNS/O series II analyzer (Norwalk, Connecticut, USA). Acetanilide was used as a standard. Approximately 2 mg of BC was used for each measurement, and each measurement was run in duplicate. The moisture and ash content in the BC samples were determined according to ASTM D1762-84. The percentage of oxygen content (O) was calculated by difference from the original dried sample and the sum of C, H, N

and ash content. Biochar surface morphology as well as elemental analysis was carried out by using a SEM Model; JEOL JSM-6360A Japan; equipped with an EDX facility. The Brunauer Emmett Teller (BET) specific surface areas (SSA) of the biochar samples were determined by using N2 Adsorption isotherms in a Tristar-II 3020 (Micromeritics, USA) instrument at −196 °C. 2.3. Composites preparation and characterization The different formulations (5, 10 and 15% w/w) of BC/PP composites were obtained by melt blending technique in a Haake Polylab QC mixer (Thermo Fischer Scientific, Walham, USA), at (190–200 °C), for mixing time of (3 min) and at screw rotation speed of (40 rpm). The BC and PP were dried in an oven (110 °C for 4–6 h) to remove the moisture and were weighed and premixed according to the formulations mentioned. The mixture is then introduced to the (50 cm3) mixing chamber maintained at the mentioned conditions above. After specified time of mixing the BC/PP composites has been collected and subjected to a micro injection molding machine (DSM Xplore microinjection molder, IM 12cm3, Netherlands). The mold temperature of (35 °C) and pressure of (6 bar) was used to prepare the standard dumb-bell shape specimens for the tensile testing. The mechanical testing was performed by using a universal testing machine under ambient conditions according to ASTM D 790 M. The reported values are an average of three specimens. The electrical conductivity measurements were conducted using Keithley resistivity meter (Model 6517A) coupled with a resistivity chamber (Model 8009) according to ASTM D257. The differential scanning calorimetry (DSC) analyses were carried out using Shimadzu DSC-60. The heating and cooling program was from 30 °C to 220 °C at a rate of 10 °C/min and holding time of 4 min. To study the effect of BC incorporation in the PP matrix, an attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectrum analysis was carried out using a Thermo Scientific Nicolet iN10 FTIR microscope having a Germanium micro-tip accessory (400– 4000 cm−1). The rheological properties of the composites were characterized by using a stress-controlled rheometer (AR G2, TA Instruments, Ltd. USA) equipped with parallel-plate geometry (diameter 25 mm). Rheological

Table 2 The thermal properties of BC/PP composites. Material

BC700

BC900

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

Xc (%)

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

Xc (%)

PP PP + 5BC PP + 10BC PP + 15BC

120.97 121.34 121.88 121.95

85.1 69.5 60.08 62.9

165.78 164.4 165.29 164.89

89.0 65.73 63.05 61.70

43.0 33.42 33.84 35.06

120.97 123.2 123.06 123.07

85.1 69.17 68.88 55.3

165.78 165.28 165.55 166.6

89.0 63.55 63.22 60.24

43.0 32.32 33.93 34.24

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Fig. 2. ATR-FTIR spectra of BC/PP composites; (a) BC700; (b) BC900.

assessments were performed at testing temperature of 190 °C, and with fixed gap between the plates of 1000 μm. The angular frequency sweep test was conducted from 0.01 to 628.3 rad·s−1 under an oscillatory stress of 3.259 Pa in the linearity region. Strain sweep tests were performed to investigate the linear viscoelastic response region. 3. Results and discussion 3.1. Elemental composition and surface area of biochars The elemental composition and surface area of biochar samples are demonstrated in Table 1. The results showed that the carbon (C) content of BC has increased as the pyrolysis temperature is increased (66.70% and 69.38% for BC700 and BC900 respectively). These increments in the total carbon content of the BC samples were due to the intense degree of carbonization at higher pyrolysis temperatures (Al-Wabel et al., 2013). The total contents of hydrogen (H) and oxygen

(O) decreased with increased pyrolysis temperature. The total (H) content decreased from (1.01% to 0.65% for BC700 and BC900, respectively), while the total (O) content decreased from 8.05% for BC700 to 5.35% for BC900. This degradation in the total (H) and (O) contents of BC samples with increasing pyrolysis temperature could be attributed to the condensed dehydration process as a consequence of intensified removal of hydroxyl (OH) and carboxyl (COOH) surface functional groups at the elevated pyrolysis temperatures (Novak et al., 2009; Usman et al., 2015; Al-Wabel et al., 2013). On the other hand the ash content of BC900 was higher than BC700 which is an obvious trend due to the reduction of (O) and (H) content is compensated by an increment in the ash content, since the pyrolysis process at higher temperatures facilitates the formation of mineral compounds (Sun et al., 2014; Yuan et al., 2011). The results of the Brunauer Emmett Teller (BET) surface area analysis revealed that BC900 has higher specific surface areas (SSA) than BC700 due to the creation of more microspores on the BC surface and

Fig. 3. Mechanical properties of BC/PP composites, tensile strength (a), tensile modulus (b), elongation at yield (c) and elongation at break (d).

A.M. Poulose et al. / Science of the Total Environment 619–620 (2018) 311–318 Table 3 The surface resistivity data in BC/PP composites. Material

Surface resistivity (ohm/square)

PP PP + 5 BC PP + 10 BC PP + 15 BC

BC700

BC900

1.54 × 1014 1.90 × 1011 1.43 × 1011 5.18 × 1010

1.54 × 1014 1.99 × 1011 1.35 × 1011 3.5 × 1010

which restricts the free movement of the polymer chains, hindering the PP segments to be orderly packed into more organized crystal form (Chen et al., 2007). On the other hand, the crystallization temperature (Tc) for the composites was slightly shifted towards the higher temperature region when compared to the neat PP due to the nucleating effect of the BC particles. These particles acted as nucleating sites from which the crystal growths were initiated; causing an early onset of crystallization temperature (Das et al., 2016a).

the surface area for BC700 and BC900 was found to be 283.62 and 291.11 m2/g, respectively. 3.2. SEM analysis of biochar The SEM images for the biochar prepared at 900 °C of both, raw and ground samples are shown in Fig. 1. The honeycomb structure of raw biochar sample can be clearly seen from Fig. 1(a), while the grounded biochar shows a random particle distribution as shown in Fig. 1(b), with an approximate particle size distribution in the range of few microns (~20–50 μ), inferred from the image scale. 3.3. Differential scanning calorimetry (DSC) The thermogram of PP showed a typical melting peak at 166 °C and the crystallization process occurred at 121 °C. The biochar incorporation into PP matrix had no effect on the melting temperature (Tm) of BC/PP composites, while it has induced a negative effect on both the enthalpy of melting (ΔHm) and crystallization (ΔHc), as shown in Table 2. On increasing the biochar content from 0 to 15% w/w; the ΔHm and ΔHc values were decreased from ~89 to ~60 J/g. The biochar addition marginally reduced the crystallization of the PP matrix and the total crystallinity of the composites. The degree of crystallinity (Xc) of BC/PP composites was calculated using Eq. (1) Xc ð%Þ ¼

ΔHm  100 ð1−ΦÞΔHοm

ð1Þ

where (Φ) is weight fraction of biochar in the composites, (ΔHm) is the enthalpy at the melting point and (ΔHοm) is the enthalpy of melting of 100% crystalline PP; which is estimated to be 207 J/g (Ritva Paukkeri, 1993). On adding biochar to PP, the total crystallinity of the system decreased from ~ 43 to ~ 34 as shown in Table 2. These findings were in agreement with the previous observations reported for the carbon based composites (Manjaly Poulose et al., 2015). The reduced crystallinity in BC/PP composite is due to the agglomerating nature of biochar

Fig. 4. SEM image of 15 BC900/PP composites.

315

Fig. 5. (a) EDX analysis of BC700. (b) EDX analysis of BC900.

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3.4. ATR-FTIR analysis of composites The ATR-FTIR was carried out to investigate whether the biochar was chemically linked to the matrix via any co-polymerization reactions. The FTIR spectra for the neat PP and the BC/PP composites are shown in Fig. 2. The spectra showed that neither new peaks were formed nor disappeared in the composites spectra, confirming that the mixing process was purely physical. The detailed FTIR peak analysis of biochar and the biochar composites were described elsewhere (Das et al., 2016b). It can be observed from Fig. 2 that sharp peaks at 2900 cm−1, 1400 cm− 1 and 1350 cm− 1 were due to asymmetrical CH2 bend, symmetrical CH3 bend and asymmetrical CH3 bend respectively, contributed by the PP (Das et al., 2016b). It is important to note here that any compatibilizer such as maleic anhydride grafted polypropylene (MAPP) was not used in the current study, however, the use of MAPP as a fixative agent in composite manufacturing has been often reported in the literature. It is conceivable that the use of MAPP may perhaps aid better interfacial bonding between the biochar and PP as previously reported (Das et al., 2016b).

3.5. Mechanical characteristics of BC/PP composites Tensile properties of neat PP and BC/PP composites are illustrated in Fig. 3. In Fig. 3(a); there is a trend of slight decrease in the tensile strength on incorporating BC into the PP matrix. Though this drop is not significant, however BC/PP composites show a trend of more declination of tensile strength at higher BC loadings. At specific loading composites made with BC700 show a comparable tensile strength values to that made with BC900, which may be inferred by the similarity of the chemical and physical structures of both BC types, Table 1. Although both biochar samples possess high surface area that may facilities the infiltration of molten PP into the BC pores (Das et al., 2016a, 2016b, 2016c, 2016d), however the observed decrease in the tensile strength is attributed to the poor interfacial bonding between BC particles and PP matrix. The compatibilization between PP matrix segments and BC surface is required for better improvements in tensile strength (Das et al., 2016b). The tensile moduli of BC/PP composites are shown in Fig. 3(b), it is noticed that the tensile modulus of BC/PP composites increases gradually on increasing the biochar content when comparing with neat PP. At a specific loading of BC both the composites made with either BC700 or BC900 show comparable values of tensile moduli. In other words the tensile modulus for 5BC700, 10BC700, 15BC700 is 1.12, 1.23, 1.28 GPa and for 5BC900, 10BC900, 15BC900 is 1.12, 1.24, 1.36 GPa, respectively. In general, we could observe that BC/PP composites show reduced ductility as illustrated by the yield elongation percentage in Fig. 3(c)

and by the percentage elongation at break in Fig. 3(d), which is expected as result of the increased toughness due to BC inclusion into PP matrix; which leads to an increased deformation resistance of the BC/PP composites. These results are in accordance with the previously reported studies (Das et al., 2016b). 3.6. The surface resistivity measurements The surface resistivity data as a function of the biochar content is presented in Table 3. In general, there was a decline in surface resistivity with an increase in the biochar content more precisely the electrical conductivity (which is proportional to the reciprocal of surface resistivity) of BC/PP composite was enhanced by four orders of magnitudes on increasing the biochar content from 0 to 15% w/w. However; it is worth mentioning that the increase in electrical conductivity in BC/PP composites is not as drastic as reported for other types of carbon based PP composites as in CB/PP composites (Manjaly Poulose et al., 2015). One reason behind these observations may be attributed to the fact that the formation of conductive carbon network in BC/PP system is not as continuous as in CB/PP system reported (Manjaly Poulose et al., 2015), as a result of biochar agglomeration as evident from Fig. 4. The second reason for lower conductivity may be due to the presence of impurities which restricts the formation of continuous conductive network due to the high ash content of BC, Table 1 and confirmed by the elemental analysis through EDX (Fig. 5a and b). In future there is a scope for biochar purification in order to enhance the conductive properties of BC/PP composites. 3.7. Rheological characterization The angular frequency (ω) vs. storage moduli (G′) relationships of the composites containing different weight percentage of biochar are shown in Fig. 6. It is observed that the storage moduli (G′) increases proportionally with the BC content in the BC/PP composites. In general the change in storage modulus is due to the formation of polymer-filler network. As compared to the neat PP, the storage modulus of the BC/PP composites was found to be higher. At low angular frequency (0.01 rad·s) the storage modulus (G′) of neat PP is 0.84 GPa while that of 15 BC700 and 15 BC900 are 6.0 and 7.7 GPa, respectively. Although this increments is not very significant, as expected due to the impure nature of biochar (Table 1) and (Fig. 5a, b) and the weaker BCBC network formation or BC-PP interactions. A weak interaction or adhesion between fillers and polymers has been observed even in highly engineered carbon based composites system (Lee et al., 2007). This could be the plausible reason why the G′ values were not much affected even with 15% w/w of biochar.

Fig. 6. The angular frequency (ω) vs. storage moduli (G′) of BC/PP composites; BC varies from 0 to 15% w/w; (a) BC700 and (b) BC900.

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Fig. 7. The angular frequency (ω) vs. tan δ (G″/G′) of BC/PP composites; BC varies from 0 to 15% w/w; (a) BC700 and (b) BC900.

The tan δ which is the ratio of loss modulus (G″)/storage modulus (G′) vs. frequency data of BC/PP composites are shown in Fig. 7. At lower frequencies, the PP has a liquid-like behavior with G″ ≫ G′ and with higher tan δ value. On the other hand at higher frequencies (corresponding to shorter relaxation times), it has a solid-like behavior and both PP and BC/PP composite have similar tan δ values (Fig. 7). The general observation is that on increasing the concentration of carbon based fillers, the storage modulus keeps on increasing until the behavior of tan δ at low frequencies become similar to that at high frequencies. In other words, tan δ vs. frequency slope will decline accordingly and reach a value close to zero. This filler concentration at which the tan δ values at lower and higher frequencies are similar, has been reported as rheological threshold in polymer composites (Danqi Ren et al., 2014; Penu et al., 2012). However, such a scenario was not observed in this BC/PP system even at the highest loading studied (15% w/w) of BC due to the weak BC-PP interactions as mentioned previously. 4. Conclusions The results of this study indicated that the electrical conductivity of BC/PP composite was enhanced by four orders of magnitudes on increasing the biochar content from 0 to 15% w/w. The tensile modulus of BC/PP composites was found to be improved when comparing to the neat PP for all BC loading. The thermal studies revealed that the crystallinity of BC/PP composites was reduced compared to neat PP and the rheological studies indicated poor BC-PP interfacial interactions. Further work could be directed towards improving the BC/PP composites properties by enhancing the biochar properties such as: porosity, surface functionalization (physical and/or chemical) and purity (removal of ash). We believe that such types of enhancements might lead to an improved filler-matrix interaction and hence superior composites properties. Acknowledgements The author would like to acknowledge the support provided by the Deanship of Scientific Research at King Saud University, through the Research Centre (Grant number RC-439/20) at the College of Engineering. References Ahmetli, G., Kocaman, S., Ozaytekin, I., Bozkurt, P., 2013. Epoxy composites based on inexpensive char filler obtained from plastic waste and natural resources. Polym. Compos. 34, 500–509. Alig, I., Pötschke, P., Lellinger, D., Skipa, T., Pegel, S., Kasaliwal, G.R., Villmow, T., 2012. Establishment, morphology and properties of carbon nanotube networks in polymer melts. Polymer 53, 4–28. Alonso, D.M., Wettstein, S.G., Dumesic, J.A., 2012. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 41, 8075–8098. Al-Wabel, M.I., Al-Omran, A., El-Naggar, A.H., Nadeem, M., Usman, A.R.A., 2013. Pyrolysis temperature induced changes in characteristics and chemical composition of biochar produced from Conocarpus waste. Bioresour. Technol. 131, 374–379.

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